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Applications of ground penetrating radar in assessing some geological hazards: examples of groundwater contamination, faults, cavities
HF LIEI ELSEVIER
Journal of Applied Geophysics 33 ( 1995 ) 177-193
Applications of ground penetrating radar in assessing some geological hazards: examples of groundwater contamination, faults, cavities Alvin K. Benson Department of Geology, Brigham Young University, Provo, Utah 84602, USA Received 30 November 1992; accepted 1 September 1993
Abstract
Ground penetrating radar (GPR) can be used in appropriate geological settings to help map subsurface geological structures and groundwater contaminants. Associated engineering and environmental applications are numerous. In particular, GPR surveys can help to identify the approximate boundaries of contaminant plumes and provide stratigraphic information at a site. Case studies from sites in Arizona and Utah show that good correlation exists between GPR signatures and hydrocarbon contamination in strategically located wells. GPR data can also be very useful in identifying shallow faulting, and the characteristics of the associated subsurface deformation can be used to gain a better understanding of the potential for surface ruptures at a site. An example from along the Wasatch Fault Zone, Utah County, Utah, shows good correlation between GPR data and trench data. Finally, examples from Provo, Utah and Eureka, Utah demonstrate the utility of using GPR for locating underground excavations and/or cavities.
I. Introduction
Ground penetrating radar (GPR) is a very useful geophysical method for use in hydrogeologic and nearsurface mapping studies. It can be used to study contaminants in groundwater, subsurface faulting, and underground cavities (natural or man-made), all of which pose potentially dangerous geological hazards. Geophysical exploration is a non-destructive, costeffective way to help locate and characterize these hazards and, at many sites, GPR is one of the better techniques for this search in the shallow subsurface. In addition to helping locate the water table, characterization of subsurface contamination produced by hazardous materials has become an important application of geophysical methods. The objectives of subsurface investigations at sites containing contamination 0926-9851/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSD10926-985 1 ( 9 4 ) 0 0 0 2 9 - N
caused by hazardous materials include: ( 1) the location of buffed materials; (2) the determination of the presence of contaminant plumes, their source(s), and geometry; and (3) the assessment of associated hydrogeologic conditions. The purpose for locating buffed hazardous materials is typically for site assessment and some kind of remedial action, usually involving excavation and safe disposal of the hazardous materials with minimal damage to the environment. Geophysical surveys can play an important role in defining the subsurface geology and the associated parameters which govern the movement of contaminant plumes (Foster et al., 1987; Benson and Mustoe, 1991; Beres and Haeni, 1991). Analysis of these data can help produce a more cost effective program in locating monitoring wells. Understanding the hydrogeologic setting is essential in defining potential contaminant
178
A.K. Benson/Jout7lal q/Applied Geophysics 33 ~ 1995) l 7 ~ I¢J3
migration pathways and assessing the fate of contaminant movement along the pathways. This understanding comes from data detailing the three-dimensional distribution of the surficial soils and fill material, the geologic strata and structure, and the groundwater conditions, such as hydraulic and chemical properties of the soils and ground water. Through early use of geophysical methods, such as seismic refraction, electrical resistivity, and/or GPR, many subsurface problems can be detected, and the sooner they are detected and evaluated, the quicker a strategic clean-up program can be implemented to minimize any further damage to the environment. GPR surveys can be very helpful and cost-effective in ( 1 ) locating strategic monitoring wells to sample subsurlace soils and fluids; (2) determining hydrogeologic gradients; and (3) monitoring the clean-up process. Case studies from northern Arizona and central Utah are presented to illustrate these applications. GPR also has the capability of mapping underground discontinuities, such as faults, concrete walls, boulders, pipes, etc., in the shallow subsurface. Planning and development along the Wasatch front in central Utah must compensate for the geological hazards associated with faulting. Studies by Gori and Hays ( 1987, 1988) show that future ruptures will probably occur along already existing zones of weakness. Although some faults can be detected by surficial geological mapping, others have no visible expression and can only be located by subsurface investigations. GPR methods can be integrated with geotechnical engineering methods, such as drilling and trenching, to obtain a better understanding of potential subsurface geological hazards at a specific site. A GPR survey conducted near Rock Canyon in eastern Provo, Utah shows subsurface faulting from which estimates were made of the magnitude and direction of throw on the fault. This area was later trenched by the Utah Geological and Mineral Survey (UGMS), which enabled a direct comparison with the GPR data. The characteristics of the subsurface deformation were analyzed to gain a better understanding of the potential for surface rupture at the site, and thereby, to plan future site development as well as devise remedial measures for mitigating the effects of potential earthquakes in populated areas. GPR can also be used to locate air-filled cavities and voids, whether natural or man-made. These caverns
could include mines, tunnels, ancient burial chambers, structures within buried cities, caves, lava vents, etc. Determining their location and geometry has diverse applications, including: ( 1 ) safe construction of buildings, roadways, and airports; (2) archaeological excavations; and (3) mineral investigations. A field study was conducted to determine the location of an underground laboratory at Brigham Young University. Good contrast exists between the dielectric constants for the overlying soil, the concrete ceiling, floors, and walls, and the large air-filled cavity. Consequently, GPR was successful in outlining this underground cavity. Likewise, GPR was helpful in locating some apparent cavities and possible associated subsidence beneath the old mining town of Eureka, Juab County, Utah.
2. Principles of ground penetrating radar The GPR technique is similar in principle to seismic reflection and sonar techniques. Pulse-mode GPR systems radiate short pulses of high frequency ( 10-1000 MHz) electromagnetic energy into the ground from a transmitting antenna. The propagation of the radar signal depends on the frequency-dependent electrical properties of the ground. Electrical conductivity of the soil or rock materials along the propagation paths introduces significant absorptive losses which limit the depth of penetration into earth formations and is primarily dependent upon the moisture content and mineralization present. When the radiated energy encounters an inhomogeneity in the electrical properties of the subsurface, part of the incident energy is reflected back to the radar antenna and part is transmitted into and possibly through the inhomogeneity. Fig. I shows the basic components and functional operation of a pulse-mode GPR system. The electrical properties of geological materials are governed primarily by the water content, dissolved minerals, and expansive clay and heavy mineral content (Topp et al., 1980; Olhoeft, 1984, 1986; Wright et al., 1984; Haeni et al., 1987; Beres and Haeni, 1991). Reflected signals are amplified, transformed to the audio-frequency range, recorded, processed, and displayed. From the recorded display, subsurface features such as soil/soil, soil/rock, and unsaturated/saturated interfaces can be identified. In addition, the presence
A.K. Benson / Journal of Applied Geophysics 33 (1995) 177-193
Ctr 4 = 2gr~-
A n t e ~ Ground Surface -,,,~, "N~ r~ ransmitted P
q
u
l
s
11
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Fig. 1. Groundpenetratingradar (GPR) schematicshowingsystem componentsand basicoperation. of floating hydrocarbons on the water table, the geometry of contaminant plumes, and the location of buried cables, pipes, drums, and tanks can be detected (Foster et al., 1987; Benson and Mustoe, 1991). The GPR data are presented as a two-dimensional depth profile along a scanned traverse line in which the vertical axis is two-way traveltime measured in nanoseconds. If the propagation velocity of the electromagnetic pulse is known, the depth to the reflector can be determined from:
Vtr d,=-2
(1)
where dr is depth to the reflector, tr is two-way traveltime, and v is velocity through the subsurface material. For low-loss media, the propagation velocity is related to the relative dielectric constant by the relationship:
c v - !/-~-E~
(2)
where c is the velocity of light in free space (30 cm/ ns) and ~ris the relative dielectric constant. The relative dielectric constant is a measure of the capacity of a material to store a charge when an electric field is applied to it relative to the same capacity in a vacuum (Sheriff, 1984). Thus, in an otherwise uniform lowloss medium of known or estimated relative dielectric constant, G, the depth to the reflector is:
179
(3)
The penetration capabilities of GPR are site specific and depend upon the frequency spectrum of the source excitation signal, the antenna radiation efficiency, and the electrical properties of the subsurface materials. Attenuation losses are caused by: (1) conversion of the radiated energy to heat through electrical conduction losses; (2) dielectric relaxation losses in water; and/or (3) chemical diffusion in clay minerals (Beres and Haeni, 1991). The effect of signal scattering by small scale heterogeneities can also increase attenuation with increasing frequency (Olhoeft, 1984). Materials with high conductivity, such as clayey soils, will rapidly reduce the depth of penetration. The radar frequency selected for a particular study is chosen to provide an acceptable compromise between deeper penetration and higher resolution. High-frequency radar signals produce greater resolution, but are more limited in depth of penetration (Davis and Annan, 1989). Ideally, the resolution is equal to a quarter of a wavelength (Sheriff, 1984), but in reality due to wave form variations and velocity uncertainties, it is typically equal to one-third to onehalf of a wavelength (Trabant, 1984). The surveys discussed in this paper were carried out using a SIR SYSTEM-3 consisting of a Model PR8304 profiling recorder with automatic gain ranging and graphic and/or magnetic tape analog data recording, and a copper-foil dipole antenna having a center operating frequency of 100 MHz.
3. Contaminated groundwater studies Fresh, clean groundwater is becoming an increasingly valuable resource around the world. In some areas, supplies have been greatly decreased by careless management and/or disposal of hazardous materials. Hydrogeologists require methods to map the soil stratigraphy at sites where contaminants are in the subsurface. The first site of investigation is in north-central Arizona near Tuba City. The general layout of this test site is illustrated in Fig. 2. Four underground gasoline tanks are located at a retail fuel outlet ("Station A " ) . At
A.K. Benson/Journal of Applied Geophysics 33 (1995) 177 193
180
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Groundwater Flow Direction Calculated gradient : 5.3 meters/lO0 meters
D ispenler Island .....................
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0
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Location
GPR T r a v e r s e
Fig. 2. U n d e r g r o u n d storage tank locations for Station A at northern Arizona Site.
least one of these tanks was known to have leaked some liquid hydrocarbons into the subsurface, causing contamination of soil and groundwater. Site studies were conducted to investigate the subsurface. These studies were accomplished in three phases. Phase 1 was a geophysical study of the area. GPR surveys were run to help outline contaminated areas and to help determine locations for monitoring wells. The second phase involved installation of monitoring wells. Soil and groundwater samples were obtained from these wells and analyzed for their hydrocarbon content. Water levels measured in the wells allowed determination of the hydrogeologic gradient and the direction of groundwater flow. The average gradient was determined to be 5.3 m/100 m toward the south (Fig. 2), consistent with regional flow patterns and groundwater discharges in seeps and springs south and southwest of the site. From the well and GPR data, the shallow subsurface geology was found to consist of three basic strata. The first layer is sand 1-2 m thick covering most of the site. This layer is underlain by sand containing gravels and some clay varying in thickness from 1.5 to 3.5 m. Near
Station A, the water table in this layer is approximately 4.5 m deep. The bedrock is the Kayenta Formation of the Glen Canyon Group, consisting of red to reddish brown interbedded sands, silts, and some clay (Chronic, 1991 ). The third phase involved integration and correlation of the GPR data with the chemical analyses of groundwater and soil samples in order to better understand the extent and severity of contamination at the site. Information on the groundwater gradient and direction of flow also helped characterize the physical extent of the contaminant plume. GPR profiles were obtained at the site as shown in Fig. 2 to help delineate the contaminant plume boundaries and to locate positions for monitoring wells. One of the more strategic profiles is shown in Fig. 3. Near Station A, this profile is oriented almost orthogonally to the general groundwater flow direction. Relative dielectric constants in the observable depth zones were between 2 and 4 as estimated using an Adek dielectric constant meter on soil samples taken from test borings at the site. The depths to reflectors were then determined from Eq. (3) using estimated dielectric con-
A.K. Benson / Journal of Applied Geophysics 33 (1995) 177-193
181
Surface Location (meters)
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.
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Interpreted greater hydrocarbon contamination Fig. 3. GPR profile near Station A in northern Arizona, showing interpreted hydrocarbon contamination.
stants for each layer. These depths were found to be in reasonable agreement with borehole samples taken from the monitoring wells. Layered soils, corresponding to the upper sand unit, can be seen just below the surface down to a depth of about 1.4 m. Below this layer, the GPR signal is wavy and/or spikey, probably because of scattering from rocks and gravel in the sand. The water table is interpreted to be the reflector at approximately 4.6 m. Measurements in the monitoring wells located along the traverse indicated the water table to be at about 4.5 m. Apparent hydrocarbon contamination is noted near the bottom of the section, especially between the surface locations of 50-80 m and in the time interval of 50-85 ns. This profile was measured approximately 50 m down-gradient from the storage tanks at Station A. The increased reflections in this part of the section
could be caused by contaminant fluids having higher resistivity than ordinary groundwater, indicative of leaking hydrocarbons. The resulting contaminated groundwater and soil would have a lower electrical conductivity than the unaffected groundwater and soil, which would reduce the attenuation of the radar signals. Some hydrocarbons appear to be floating on the water table in Fig. 3, but other effects of hydrocarbons appear to be disseminated in the saturated sediments below. The "smearing out" of the water table reflector corresponds to where hydrocarbons were found in the drilled formation. Based upon the GPR data, the surface conditions, and some existing well data, twelve monitoring wells were installed around the site. Groundwater samples were obtained from each monitoring well using a bailer. The bailer was decontaminated before and after each
182
A.K. Benson / Journal o[ Applied Geophysics 33 ( 1995 ) 177-19.3
Site Location Map: Utah County, Utah
N GPR-North
~
O MW-3 Parking
I l
Underground Storage Tank
/
GPR-South
MW-4
MW-2 /
%roping Well for Cleanup
Parking
Scale (meters) 0
I
25 I
3
50 Groundwater Flow Direction Calculated gradient: 1.1 meters/100 meters
Fig. 4. Underground storage tank location at central Utah site.
sample was obtained using a weak acid solution and distilled water. The water samples were analyzed by an independent laboratory for benzene, toluene, ethylbenzene, xylene, and total petroleum hydrocarbons (TPH). All values typically correlated well with the GPR data. Groundwater samples taken from monitoring well MW-2 located along the GPR traverse as shown in Fig. 2 contained approximately 5 ppm (parts per million) more benzene and 80 ppm more TPH than groundwater samples taken from the MW-1 and MW3 located along the same traverse. Soil samples from MW-2 contained about 170 ppm more TPH and 2.3 ppm more benzene than those taken from MW-1 and MW-3. In general, our qualitative interpretation of the GPR data is in reasonable agreement with the differences expressed by these well data and has effectively extended the horizontal extent of the borehole data. The second case study is associated with a leaking fuel tank at a site in Utah County, near Provo, Utah.
The general layout of this site is shown in Fig. 4. The underground storage tank leaked hydrocarbons into the groundwater which flowed to the southwest following the water-table gradient of about 1.1 m / 1 0 0 m. Based upon maps by Hintze (1988) and our well and GPR data, the geology at this site was determined to consist of three main layers: ( 1 ) an asphalt/sand layer ( 1.11.4 m thick) ; (2) a layer of silty sand containing some gravel ( 1.7-2.4 m thick) ; and (3) a water-bearing gravel layer. The water table at monitoring wells 1 and 2 was 3.6 alad 3.9 m, respectively. GPR data were collected across two traverses at this site. These lines are shown in Fig. 4 as " G P R - S o u t h " and "GPR-North". GPR-South is about 45 m downgradient from the leaking tank and runs across the contaminated area. GPR-North is approximately 60 m north of the tank in an uncontaminated up-gradient area.
A.K. Benson / Journal of Applied Geophysics 33 (1995) 177-193
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Fig. 5. GPR data recorded along traverse GPR-North located up-gradient from the leaking fuel tank in central Utah (surface distances are consistent with those along the GPR-South line).
The GPR-North section (Fig. 5) shows relatively "clean", continuous subsurface strata. The stronger reflection at about 3.4--3.6 m in depth is interpreted as the water table. Reflections below the water table tend to dim and fade out, characteristic of water-saturated sediments. Indications of rocks and gravel can be seen in the reflectors above the water table. On the GPR-South profile shown in Fig. 6, the water table is interpreted to be the reflectors between 3.6 and 3.8 m. Between the surface locations of 40 and 90 m, the water table is "smeared out" by reflections interpreted to come from hydrocarbon contamination. This was later confirmed by well groundwater sample data. These dispersed hydrocarbons are less conductive than water and, therefore, enhance the radar reflectivity in
this part of the profile. As with the Arizona study, some of the hydrocarbons may be floating on the water table. Some of the radar peaks between 1.2 and 3.0 m in depth seem indicative of larger rocks and gravel, which is also in agreement with the monitoring well data in the area,
Groundwater samples taken from monitoring wells 1, 2, 3, and 4 (Fig. 4) were analyzed by an independent laboratory for hydrocarbon content. For MW-3, which is near the "uncontaminated" GPR-North line, the TPH value was about 1.2 ppm, whereas for MW-1 and MW-3 which are near the "contaminated" GPR-South line, TPH values were about 44 ppm and 51 ppm, respectively. At MW-4, the TPH value was about 1.8 ppm, which is comparable to the value in MW-3 to the
A.K. Benson / Journal ¢{/'Applied Geophysics 33 (1995) 177 / 93
184
W
E
Surface Location (meters)
1.2
0 0 c~
L4
I
v
E ~.6
Interpr
1.2
nation Fig. 6. GPR data recorded along traverse GPR-SouIh located down-gradient from the leaking tuel tank in central Utah.
north. These values are in good agreement with the qualitative interpretations of the GPR data. Based upon the interpretations of the well and GPR data, a pumping well has been installed at the location indicated in Fig. 4 to clean up the contaminated groundwater by circulating it through an activated charcoal filter. These case studies illustrate that GPR can be a useful tool to help assess the location and extent of contaminant plumes in soils and groundwater. When geological conditions are appropriate, GPR surveys offer an economical, nondestructive method to select the strategic locations of monitoring wells and to provide an effective horizontal extent of borehole logs.
4. Locating concealed faults The Wasatch fault zone ( shown in Fig. 7) lies along the western edge of the Middle Rocky Mountain and Colorado Plateau regions and the eastern edge of the Basin and Range region in north-central Utah. The fault zone is distinguished by extensive Late Quaternary fault scarps, truncated bedrock spurs, sag ponds, and springs (Hintze, 1988). Several strands of the fault zone have surface expression, but geophysical and trenching data show that many others are concealed (Benson and Bear, 1987; Benson and Mustoe, 1989). Since 90% of Utah's population lives near or within
185
A.K. Benson /Journal of Applied Geophysics 33 (1995) 177-193
113"
14at~l, 112"
111"
110"
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ez
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Fig. 7. Regionallocationmap showingthe WasatchFault and physiographicprovinces(afterHintze, 1988,p. 80). this active fault zone, large-magnitude earthquakes would have a significant impact (Gori and Hays, 1987, 1988). To mitigate the effects of possible earthquakes, the location and characteristics of individual fault strands need to be determined as accurately as possible in order to establish and implement building codes and zoning ordinances. In the past, a number of surfical studies have focused on the shallow structure and history of movement of the fault zone. These studies include large-scale aerial surveys and small-scale trenching along the fault zone (Cluff et al., 1973). To delineate shallow discontinuities associated with the Wasatch Fault shown in Figs.
7 and 8, a populated area northeast of Provo, Utah, near Rock Canyon was selected. The geology at the Rock Canyon site is taken from maps and reports by Hintze (1978) and Machette (1989). The surficiai sediments in the area are predominantly Late Pleistocene to Holocene alluvial deposits (Machette, 1989), deposited primarily by debris flows and low-to-high-energy intermittent streams. Bedrock in the Wasatch Range at Rock Canyon consists of Paleozoic and Proterozoic sedimentary rocks, including tillite, quartzites, limestones, dolomites and shales. These rocks were folded and thrust-faulted during the Sevier Orogeny (20-30 Ma) and subsequently cut by numer-
186
A.K. Benson / Journal of Applied Geophysics 3311995) 177-19 S
Utah Valley to the west. Older sediments within the valley are of alluvial, fluvial, and lacustrine origin. The thicknesses of sedimentary fill vary from less than I m near the margins to over 4000 m near the center ( Hintze, 1988). A GPR profile was acquired along the traverse by Rock Canyon Road as illustrated in Fig. 9. Fault scarp expressions at the surface have been culturally removed along the road, but some scarp remnants have been found a few meters north and south of the road. Such remnants exist both to the north and the south of the GPR traverse. The radar profile of Fig. 10 suggests a high-angle normal fault in the subsurface, whose surface exposure is probably the scarp to the north and south of the roadway. Some of its surface expression was probably removed when Rock Canyon Road was built. The upper layers in the GPR profile are interpreted to be road fill, soils, rocks, and gravel, Some of the rocks and gravel are probably from post-fault debris flow out of the canyon. Using an average dielectric constant of 2.5 for dry soils and rocks near Rock Canyon, Eq. (3) yields a fault: offset of approximately 3.3 m. The dip angle of the interpreted fault is estimated to be 77°W. Above the fault plane between surface locations 385-395 m, a radar reflection is seen which we interpret to come from the colluvial wedge of materials associated with the faulting.
Rock Canyon Site
km
F
Orem
Provo •
\,, % (ProvoBay)
0-~"~/e'~'~'~
°°~Z. "
o /'
~,Mapleton /
/;::"Spanish Fork
[
/
Fig. 8. Map of the Provo, Utah areashowing the trace of the Wasatch Fault and the Rock Canyon site (from US Oeological Survey 30 × 60 minute metric topographic map of Provo, Utah, 1986),
ous normal faults during Basin and Range extensions ( 10-20 Ma). The Wasatch Fault separates the bedrock of the mountains from the alluvial and lake deposits in
Rock Canyon Site: Provo, Utah
ij "
Elevation: 1,504 meters
o_
,
1,0o
N
Line
,
200
_
,
300
,
400
,__
Rock C a n y o n
s?0
,
60o
Surface Location (meters)
Fig, 9. Map of Rock CanyonRoad showing location of the GPR traverse, observed fault scarp trace, and the UGMStrench. The dashed section of the fault scarp indicates that much of it has been culturallyremovedacross Rock CanyonRoad.
A.K. Benson / Journal of Applied Geophysics 33 (1995) 177-193
W
187
E @.-
UGMS
TRENCH
surface
soils 1.5
5 v
09
E
debris flow ,gl
°~
O~ "O
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6.7 390
395
400
405
410
415
Surface Location (meters) Fig. 10. GPR profile along Rock Canyon Road. Vertical dashed lines represent a distance of about 3 m. Horizontal lines are 20 ns apart. The main fault is clearly visible.
After the geophysical work was completed, the Utah Geological and Mineral survey (UGMS) dug a 30 m long trench across the fault scarp approximately 5 m due north of our GPR line. In going north from the GPR traverse to the trench location, the north side of Rock Canyon Road slopes down a gradient of about 1.5 m, and most of this material appears to be road fill and gravel base, with some rock debris from Rock Canyon. A schematic diagram of the trench is shown in Fig. 11 (W.R. Lund, UGMS, pers. commun., 1991). The
trench indicates a single-event rupture with 3.5 m of throw, dipping at 76°W beneath the scarp. Comparing the GPR data shown in Fig. 10 with the trench crosssection suggests that the fault reflections are probably coming from the rock layer between the upper and middle soil horizons. Examination of natural exposures in the trench showed competent rock between these two horizons, consisting of fractured limestone and quartzite, as well as some debris-flow deposits from the canyon. Indications of the smaller antithetic fault
A.K. Benson / Journal o[ Applied Geophysics 33 (I 995) 177-193
188 W
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---U p ~ r Soll Horizon , Middle Soil Horizon - Lower Soil Horizon
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380
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f
390
i
395
400
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Fig. I I. Schematic diagram of the stratigraphy in the UGMS trench along Rock Canyon Road. Three mapped soil horizons are shown. Heavy black lines represent faults; arrows show directions of relative movement. The upper soil horizon is on a debris flow. the middle soil horizon is on a matrix-supported debris flow, and the lower soil horizon is on a high-energy fluvial deposit.
located farther west in the trench shown in Fig. 11 showed up in the GPR profile, but the offset of about I m was too small for adequate resolution.
5. Outlining underground cavities The effectiveness of GPR in locating and outlining the boundaries of an underground cavity was tested by running some traverses over an underground physics laboratory located on the campus of Brigham Young University, Provo, Utah. The surveys were conducted and interpretations made knowing only the general area where the lab was located. To avoid bias, comparisons with lab blueprints were not made until the geophysical work was completed. An E - W cross-section of the lab taken from the blueprints is sketched in Fig. 12. After a few GPR traverses over the area, the subsurface boundaries of the lab became rather clear. The E W profile shown in Fig. 13 outlines the top of the lab. From the blueprints, the walls of the lab are about 1 m thick and are covered with 1.2 m of soil. The ceiling is approximately 1 m thick and is covered with about 2.4 m of soil. The extra soil over the ceiling provides protection from radiation that may be given off by a large Van de Graaff generator housed in the lab. Using a lab-determined value of 4,8 for the dielectric constant of the unsaturated unconsolidated soil over the lab, Eq. (3) yields a depth of about 1.3 m to the top of the outside walls and an average depth of 2.4 m to the
top of the recessed ceiling. Similar depths to the ceiling were obtained from a N - S line over the lab shown in Fig. 14. The spikiness of the reflections from the ceiling is probably due to rebar in the ceiling, the rough texture of the concrete, and/or rocks in the soil. On both lines, multiple reflections mimic the ceiling pattern. From Fig. 13, the E - W length of the lab is estimated to be approximately 18 m. and from Fig. 14. the N - S extent is about 12.8 m. All of these values compare favorably with the actual lab dimensions shown in Fig. 12. The air cavity and floor of the lab can be clearly seen on the GPR profile in Fig. 15. This traverse was taken over the central portion of the lab ceiling. High-pass and low-pass filter settings were adjusted in the GPR recorder to retain lower frequencies in Fig. 15 than in Figs. 13 and 14. The overlying soil, concrete ceiling. and associated multiple reflections are in the upper 40 ns of this profile. Since the relative dielectric constant of air is I and the time interval between the interpreted ceiling and floor is 60 ns from Fig. 15, the average depth from the ceiling to the floor is 9.1 m. This is the same value as found on the blueprints for the lab. Between the surface positions at 6 and 9 m, the larger amplitudes along the floor of the lab are interpreted to be the approximate location of the Van de Graaff generator. This location was later confirmed by inspecting the blueprints and the lab itself. As a final study, the GPR traverse shown in Fig. 16 was recorded across US Highway 6 near the Eureka, Utah location shown earlier in Fig. 7. Eureka is an old
A.K. Benson / Journal of Applied Geophysics 33 (1995) 177-193
Surface Location (meters) 0 3 l,t 1.2
W
?
9
6
12
189
15
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Soil
Ceiling
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9.1
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I Lab Floor 0
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2
3
4
I
I
I
I
I
Area of Van de~ Graaff Generator
Scale (meters) Fig, 12. Schematicdiagramof the undergroundphysicslaboratoryon the campus of BrighamYoungUniversity,Provo, Utah. mining town dating back to the 1890s having numerous mine shafts and tunnels under the town and surrounding areas. Locations of many of the older underground excavations are unknown since they were never
recorded on any maps. Most of the mines are in Cambrian and Ordovician limestone, and the area is rich in ores of gold, silver, copper, iron, and other metals. Tertiary intrusions have brought in ore-bearing solu-
(Tie
Point to N-S Line) E
W
,~
0.0
Outside Wall '
2.0
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Fig. 15. E-W GPR profile over the center of the BYU lab where the ceiling is recessed with respect to the outside walls. The underground air cavity and floor are clearly detectable. The interpreted height of the lab cavity and the location of the Van de Graaff generator are in good agreement with the structure blueprints.
A.K. Benson / Journal of Applied Geophysics 33 (1995) 177-193
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tions which have solidified in the cracks, vugs, and fissures in the limestone. In many places the ground is cracked or collapsed from the mining activity, and some mines are now just cone-shaped cave-ins (Chronic, 1990). Two areas of apparent subsidence in the upper strata can be seen in Fig. 16. Any surface expressions of this slumping were most likely filled in many years ago when the highway was resurfaced. Some nearby cracking and warping in the highway may be clues that some subsurface movement is still occurring. Almost directly below each area of subsidence is a hyperbolic diffraction pattern that is characteristic of reflections from localized objects or cavities in the subsurface. The hyperbolae in Fig. 16 most likely originate from manmade cavities, such as a mine shaft or a tunnel, but could also arise from sinkholes carved out in the limestone. Using an average relative dielectric constant of 5.5 for the soils and asphalt in the Eureka area and 7 for limestone, Eq. (3) yields an estimated depth of about 7.8 m down to the areas of subsidence and about 10.6 m down to the apparent cavities. Cavity diameters appear to be on the order of 2-4 m. Whatever the source of the apparent subsurface cavities and possible associated subsidence, the important point is that GPR can often be successful in locating
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these potential hazards. This information is very useful in planning and site development. Early detection of such cavities or the mechanism causing subsidence can result in substantial savings in time and money. Additional GPR surveys and some drilling are planned in the future in and around Eureka.
6. Conclusions In some geologic settings, GPR is a reliable, rapid, and economical method for mapping shallow subsurface sediments and assessing hydrogeologic conditions. The thickness of the sediments, the depth to the water table, and the presence of groundwater contamination can be reliably mapped at field sites where reflector geometries are not too complex and where electrical resistivities are typically high and relative dielectric constants are typically low. In particular, GPR data from sites located in Arizona and Utah proved helpful in mapping groundwater contamination produced by leaking underground fuel tanks. Based upon the estimated depths to the areas of subsurface contamination from the GPR data, the lateral extent of the contaminant plumes was outlined and monitoring wells were installed at strategic locations at
192
A.K. Benson / Journal of Applied Geophysics 33 (1995) 177-193
both sites. Analysis of the monitor well data showed that estimated water table depths and qualitative assessments of contamination location and concentrations inferred from the G P R data were in g o o d a g r e e m e n t with the depths and contamination levels found in the wells. G P R data collected in east Provo, Utah demonstrated the reliable detection o f shallow c o n c e a l e d faulting in appropriate g e o l o g i c settings. Interpretations o f the radar data coincided with m a n y features o b s e r v e d in a recently opened nearby trench. The net offset and dip o f the fault correlate quite well. D e t e r m i n i n g the characteristics and distribution o f subsurface faulting and folding can provide useful information for establishing and i m p l e m e n t i n g building codes and zoning ordinances in the Provo area. Cavity detection is another challenging exploration problem that requires g o o d resolution and g o o d depth penetration. G P R can be successful in detecting nearsurface cavities when the host rock has a relatively low electromagnetic w a v e attenuation factor. The geometrical features of an underground laboratory at B r i g h a m Y o u n g University were successfully outlined and specified in depth using G P R surveys. Interpretations o f the ceiling, floor, and air cavity were in g o o d a g r e e m e n t with the blueprints o f the laboratory. G P R data also provided useful information about the location and approximate depths for two apparent cavities and associated areas of subsidence under US H i g h w a y - 6 near Eureka, Utah. In summary, in appropriate geology, G P R surveys can provide useful cost-effective data which can help assess potentially dangerous geological hazards, including groundwater contamination, c o n c e a l e d faulting, and underground cavities. Planning and remedial measures based upon these data can then be implemented to help protect human life and produce m i n i m a l threat to the environment.
Acknowledgements Sincere appreciation is extended to T.E. O w e n for valuable c o m m e n t s on the manuscript and to graduate students in g e o l o g y at B r i g h a m Y o u n g University who helped acquire the data.
References Benson, A.K: and Bear, J.L., 1987. Close order gravity surveys: a means of fault delineation in valley fill sediments. Proc. 23rd Syrup. Engineering Geology and Soils Engineering, pp. 219240. Benson, A.K. and Mustoe, N.B., 1989. Seismic and gravity surveys as tools in delineating subsurface geology and assessing some geologic hazards. Proc. 25th Symp. Engineering Geology and Geotechnical Engineering, pp. 25-30. Benson, A.K. and Mustoe, N.B., 1991. Ground penetrating radar, electrical resistivity, and water quality studies integrated to determine the source(s) and geometry of hydrocarbon contamination at a site in north-central Arizona. Proc. 27th Syrup. Engineering Geology and Geotechnical Engineering, pp. 38.1-38.18. Beres, M. and Haeni, F.P., 1991. Application of ground-penetratingradar methods to hydrogeologic studies. Ground Water, 29: 375386: Chronic, H.. 1991. Roadside Geology of Arizona. Mountain Press, Missoula, MT, 322 pp. Chronic, H., 1990. Roadside Geology of Utah. Mountain Press, Missoula, MT, 326 pp. Cluff, L.S., Brogan, G.E. and Glass, C.E., 1973. Wasatch Fault, Southern Portion. Woodward-Lundgren and Associates, Oakland, CA (rep. prepared fi~r Utah Geol. Miner. Surv., Salt Lake City, UT). Davis, J.L. and Annan, A.P., 1989. Ground-penetrating radar for high-resolution mapping of soil and rock stratigraphy. Geophys. Prospect., 37:531-55 I. Foster, A.R., Veatch, M.D. and Baird, S.L., 1987. Hazardous waste geophysics. Geophys. Leading Edge, 6: 8-13. Gori, P.L. and Hays, W.W. (Editors), 1987. Earthquake Hazards Along the Wasatch Front, Utah. US Geol. Surv. Open-File Rep., 87-154. Gori, P.L. and Hays, W.W. ( Editors ), 1988. Assessment of Regional Earthquake Hazards and Risk Along the Wasatch Front, Utah, III. US Geol. Surv. Open-File Rep., 88-680. Haeni, F.P., McKeegan, D.K. and Capron, D.R., 1987. Ground Penetrating Radar Study of the Thickness and Extent of Sediments Beneath Silver Lake, Berlin and Meriden, Connecticut. US Geol. Surv. Water Resour. Invest, 85-4108, 19 pp. Hintze, L.F., 1978. Geologic Map of the Y Mountain Area, East o1: Provo, Utah and Cross Sections of the Y Mountain. Brigham Young Univ. Geol. Stud. Spec. PUN., 5 (1:24,000). Hintze, L.F., 1988. Geologic History of Utah. Brigham Young Univ. Geol. Stud: Spec. Publ., 7: 7-10, 29-32, 77-89. Machette, M.N., 1989. Preliminary Surficial Geologic Map of the Wasatch Fault Zone, Eastern Part of Utah Valley, Utah Country, and Parts of Salt Lake and Juab Counties, Utah. US Geol. Surv. Misc. Field Stud. Map Pamphlet, MF-2109 (1:50,1)00). Olhoeft, G.R., 1984. Applications and limitations of ground penetrating radar. Soc. Explor, Geophys., 54th Annu. Int. Meet., Atlanta, GA, pp. 147-148 t abstr. ). Olhoeft, G.R., 1986. Direct detection of hydrocarbon and organic chemicals with ground penetration radar and complex resistivity. Proc. NWWA Conf. Petroleum Hydrocarbons and Organic Chemicals Ground Water, Houston, TX, pp. 1-22.
A,K. Benson / Journal of Applied Geophysics 33 (1995) 177-193 Sheriff, R.E., 1984. Encyclopedic Dictionary of Exploration Geophysics. Soc. Explor. Geophys., Tulsa, OK, 2nd ed., 323 pp. Topp, G.C., Davis, J.L. and Annan, A.P., 1980. Electromagnetic determination of soil water content: measurements in coaxial transmission lines. Water Resour. Res., 16: 574-582.
193
Trabant, P.K., 1984. Applied high-resolution geophysical methods. Int. Human Resour. Dev. Co., Boston, MA, 365 pp. Wright, D.C., Olhoeft, G.R. and Watts, R.D., 1984. Ground penetrating radar studies on Cape Cod. Proc. NWWA/EPA Conf. Surface and Borehole Geophysical Methods in Ground Water Investigations, San Antonio, TX, pp. 666--680.
Journal of Applied Geophysics 33 ( 1995 ) 177-193
Applications of ground penetrating radar in assessing some geological hazards: examples of groundwater contamination, faults, cavities Alvin K. Benson Department of Geology, Brigham Young University, Provo, Utah 84602, USA Received 30 November 1992; accepted 1 September 1993
Abstract
Ground penetrating radar (GPR) can be used in appropriate geological settings to help map subsurface geological structures and groundwater contaminants. Associated engineering and environmental applications are numerous. In particular, GPR surveys can help to identify the approximate boundaries of contaminant plumes and provide stratigraphic information at a site. Case studies from sites in Arizona and Utah show that good correlation exists between GPR signatures and hydrocarbon contamination in strategically located wells. GPR data can also be very useful in identifying shallow faulting, and the characteristics of the associated subsurface deformation can be used to gain a better understanding of the potential for surface ruptures at a site. An example from along the Wasatch Fault Zone, Utah County, Utah, shows good correlation between GPR data and trench data. Finally, examples from Provo, Utah and Eureka, Utah demonstrate the utility of using GPR for locating underground excavations and/or cavities.
I. Introduction
Ground penetrating radar (GPR) is a very useful geophysical method for use in hydrogeologic and nearsurface mapping studies. It can be used to study contaminants in groundwater, subsurface faulting, and underground cavities (natural or man-made), all of which pose potentially dangerous geological hazards. Geophysical exploration is a non-destructive, costeffective way to help locate and characterize these hazards and, at many sites, GPR is one of the better techniques for this search in the shallow subsurface. In addition to helping locate the water table, characterization of subsurface contamination produced by hazardous materials has become an important application of geophysical methods. The objectives of subsurface investigations at sites containing contamination 0926-9851/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSD10926-985 1 ( 9 4 ) 0 0 0 2 9 - N
caused by hazardous materials include: ( 1) the location of buffed materials; (2) the determination of the presence of contaminant plumes, their source(s), and geometry; and (3) the assessment of associated hydrogeologic conditions. The purpose for locating buffed hazardous materials is typically for site assessment and some kind of remedial action, usually involving excavation and safe disposal of the hazardous materials with minimal damage to the environment. Geophysical surveys can play an important role in defining the subsurface geology and the associated parameters which govern the movement of contaminant plumes (Foster et al., 1987; Benson and Mustoe, 1991; Beres and Haeni, 1991). Analysis of these data can help produce a more cost effective program in locating monitoring wells. Understanding the hydrogeologic setting is essential in defining potential contaminant
178
A.K. Benson/Jout7lal q/Applied Geophysics 33 ~ 1995) l 7 ~ I¢J3
migration pathways and assessing the fate of contaminant movement along the pathways. This understanding comes from data detailing the three-dimensional distribution of the surficial soils and fill material, the geologic strata and structure, and the groundwater conditions, such as hydraulic and chemical properties of the soils and ground water. Through early use of geophysical methods, such as seismic refraction, electrical resistivity, and/or GPR, many subsurface problems can be detected, and the sooner they are detected and evaluated, the quicker a strategic clean-up program can be implemented to minimize any further damage to the environment. GPR surveys can be very helpful and cost-effective in ( 1 ) locating strategic monitoring wells to sample subsurlace soils and fluids; (2) determining hydrogeologic gradients; and (3) monitoring the clean-up process. Case studies from northern Arizona and central Utah are presented to illustrate these applications. GPR also has the capability of mapping underground discontinuities, such as faults, concrete walls, boulders, pipes, etc., in the shallow subsurface. Planning and development along the Wasatch front in central Utah must compensate for the geological hazards associated with faulting. Studies by Gori and Hays ( 1987, 1988) show that future ruptures will probably occur along already existing zones of weakness. Although some faults can be detected by surficial geological mapping, others have no visible expression and can only be located by subsurface investigations. GPR methods can be integrated with geotechnical engineering methods, such as drilling and trenching, to obtain a better understanding of potential subsurface geological hazards at a specific site. A GPR survey conducted near Rock Canyon in eastern Provo, Utah shows subsurface faulting from which estimates were made of the magnitude and direction of throw on the fault. This area was later trenched by the Utah Geological and Mineral Survey (UGMS), which enabled a direct comparison with the GPR data. The characteristics of the subsurface deformation were analyzed to gain a better understanding of the potential for surface rupture at the site, and thereby, to plan future site development as well as devise remedial measures for mitigating the effects of potential earthquakes in populated areas. GPR can also be used to locate air-filled cavities and voids, whether natural or man-made. These caverns
could include mines, tunnels, ancient burial chambers, structures within buried cities, caves, lava vents, etc. Determining their location and geometry has diverse applications, including: ( 1 ) safe construction of buildings, roadways, and airports; (2) archaeological excavations; and (3) mineral investigations. A field study was conducted to determine the location of an underground laboratory at Brigham Young University. Good contrast exists between the dielectric constants for the overlying soil, the concrete ceiling, floors, and walls, and the large air-filled cavity. Consequently, GPR was successful in outlining this underground cavity. Likewise, GPR was helpful in locating some apparent cavities and possible associated subsidence beneath the old mining town of Eureka, Juab County, Utah.
2. Principles of ground penetrating radar The GPR technique is similar in principle to seismic reflection and sonar techniques. Pulse-mode GPR systems radiate short pulses of high frequency ( 10-1000 MHz) electromagnetic energy into the ground from a transmitting antenna. The propagation of the radar signal depends on the frequency-dependent electrical properties of the ground. Electrical conductivity of the soil or rock materials along the propagation paths introduces significant absorptive losses which limit the depth of penetration into earth formations and is primarily dependent upon the moisture content and mineralization present. When the radiated energy encounters an inhomogeneity in the electrical properties of the subsurface, part of the incident energy is reflected back to the radar antenna and part is transmitted into and possibly through the inhomogeneity. Fig. I shows the basic components and functional operation of a pulse-mode GPR system. The electrical properties of geological materials are governed primarily by the water content, dissolved minerals, and expansive clay and heavy mineral content (Topp et al., 1980; Olhoeft, 1984, 1986; Wright et al., 1984; Haeni et al., 1987; Beres and Haeni, 1991). Reflected signals are amplified, transformed to the audio-frequency range, recorded, processed, and displayed. From the recorded display, subsurface features such as soil/soil, soil/rock, and unsaturated/saturated interfaces can be identified. In addition, the presence
A.K. Benson / Journal of Applied Geophysics 33 (1995) 177-193
Ctr 4 = 2gr~-
A n t e ~ Ground Surface -,,,~, "N~ r~ ransmitted P
q
u
l
s
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Vtr d,=-2
(1)
where dr is depth to the reflector, tr is two-way traveltime, and v is velocity through the subsurface material. For low-loss media, the propagation velocity is related to the relative dielectric constant by the relationship:
c v - !/-~-E~
(2)
where c is the velocity of light in free space (30 cm/ ns) and ~ris the relative dielectric constant. The relative dielectric constant is a measure of the capacity of a material to store a charge when an electric field is applied to it relative to the same capacity in a vacuum (Sheriff, 1984). Thus, in an otherwise uniform lowloss medium of known or estimated relative dielectric constant, G, the depth to the reflector is:
179
(3)
The penetration capabilities of GPR are site specific and depend upon the frequency spectrum of the source excitation signal, the antenna radiation efficiency, and the electrical properties of the subsurface materials. Attenuation losses are caused by: (1) conversion of the radiated energy to heat through electrical conduction losses; (2) dielectric relaxation losses in water; and/or (3) chemical diffusion in clay minerals (Beres and Haeni, 1991). The effect of signal scattering by small scale heterogeneities can also increase attenuation with increasing frequency (Olhoeft, 1984). Materials with high conductivity, such as clayey soils, will rapidly reduce the depth of penetration. The radar frequency selected for a particular study is chosen to provide an acceptable compromise between deeper penetration and higher resolution. High-frequency radar signals produce greater resolution, but are more limited in depth of penetration (Davis and Annan, 1989). Ideally, the resolution is equal to a quarter of a wavelength (Sheriff, 1984), but in reality due to wave form variations and velocity uncertainties, it is typically equal to one-third to onehalf of a wavelength (Trabant, 1984). The surveys discussed in this paper were carried out using a SIR SYSTEM-3 consisting of a Model PR8304 profiling recorder with automatic gain ranging and graphic and/or magnetic tape analog data recording, and a copper-foil dipole antenna having a center operating frequency of 100 MHz.
3. Contaminated groundwater studies Fresh, clean groundwater is becoming an increasingly valuable resource around the world. In some areas, supplies have been greatly decreased by careless management and/or disposal of hazardous materials. Hydrogeologists require methods to map the soil stratigraphy at sites where contaminants are in the subsurface. The first site of investigation is in north-central Arizona near Tuba City. The general layout of this test site is illustrated in Fig. 2. Four underground gasoline tanks are located at a retail fuel outlet ("Station A " ) . At
A.K. Benson/Journal of Applied Geophysics 33 (1995) 177 193
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Fig. 2. U n d e r g r o u n d storage tank locations for Station A at northern Arizona Site.
least one of these tanks was known to have leaked some liquid hydrocarbons into the subsurface, causing contamination of soil and groundwater. Site studies were conducted to investigate the subsurface. These studies were accomplished in three phases. Phase 1 was a geophysical study of the area. GPR surveys were run to help outline contaminated areas and to help determine locations for monitoring wells. The second phase involved installation of monitoring wells. Soil and groundwater samples were obtained from these wells and analyzed for their hydrocarbon content. Water levels measured in the wells allowed determination of the hydrogeologic gradient and the direction of groundwater flow. The average gradient was determined to be 5.3 m/100 m toward the south (Fig. 2), consistent with regional flow patterns and groundwater discharges in seeps and springs south and southwest of the site. From the well and GPR data, the shallow subsurface geology was found to consist of three basic strata. The first layer is sand 1-2 m thick covering most of the site. This layer is underlain by sand containing gravels and some clay varying in thickness from 1.5 to 3.5 m. Near
Station A, the water table in this layer is approximately 4.5 m deep. The bedrock is the Kayenta Formation of the Glen Canyon Group, consisting of red to reddish brown interbedded sands, silts, and some clay (Chronic, 1991 ). The third phase involved integration and correlation of the GPR data with the chemical analyses of groundwater and soil samples in order to better understand the extent and severity of contamination at the site. Information on the groundwater gradient and direction of flow also helped characterize the physical extent of the contaminant plume. GPR profiles were obtained at the site as shown in Fig. 2 to help delineate the contaminant plume boundaries and to locate positions for monitoring wells. One of the more strategic profiles is shown in Fig. 3. Near Station A, this profile is oriented almost orthogonally to the general groundwater flow direction. Relative dielectric constants in the observable depth zones were between 2 and 4 as estimated using an Adek dielectric constant meter on soil samples taken from test borings at the site. The depths to reflectors were then determined from Eq. (3) using estimated dielectric con-
A.K. Benson / Journal of Applied Geophysics 33 (1995) 177-193
181
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stants for each layer. These depths were found to be in reasonable agreement with borehole samples taken from the monitoring wells. Layered soils, corresponding to the upper sand unit, can be seen just below the surface down to a depth of about 1.4 m. Below this layer, the GPR signal is wavy and/or spikey, probably because of scattering from rocks and gravel in the sand. The water table is interpreted to be the reflector at approximately 4.6 m. Measurements in the monitoring wells located along the traverse indicated the water table to be at about 4.5 m. Apparent hydrocarbon contamination is noted near the bottom of the section, especially between the surface locations of 50-80 m and in the time interval of 50-85 ns. This profile was measured approximately 50 m down-gradient from the storage tanks at Station A. The increased reflections in this part of the section
could be caused by contaminant fluids having higher resistivity than ordinary groundwater, indicative of leaking hydrocarbons. The resulting contaminated groundwater and soil would have a lower electrical conductivity than the unaffected groundwater and soil, which would reduce the attenuation of the radar signals. Some hydrocarbons appear to be floating on the water table in Fig. 3, but other effects of hydrocarbons appear to be disseminated in the saturated sediments below. The "smearing out" of the water table reflector corresponds to where hydrocarbons were found in the drilled formation. Based upon the GPR data, the surface conditions, and some existing well data, twelve monitoring wells were installed around the site. Groundwater samples were obtained from each monitoring well using a bailer. The bailer was decontaminated before and after each
182
A.K. Benson / Journal o[ Applied Geophysics 33 ( 1995 ) 177-19.3
Site Location Map: Utah County, Utah
N GPR-North
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/
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Fig. 4. Underground storage tank location at central Utah site.
sample was obtained using a weak acid solution and distilled water. The water samples were analyzed by an independent laboratory for benzene, toluene, ethylbenzene, xylene, and total petroleum hydrocarbons (TPH). All values typically correlated well with the GPR data. Groundwater samples taken from monitoring well MW-2 located along the GPR traverse as shown in Fig. 2 contained approximately 5 ppm (parts per million) more benzene and 80 ppm more TPH than groundwater samples taken from the MW-1 and MW3 located along the same traverse. Soil samples from MW-2 contained about 170 ppm more TPH and 2.3 ppm more benzene than those taken from MW-1 and MW-3. In general, our qualitative interpretation of the GPR data is in reasonable agreement with the differences expressed by these well data and has effectively extended the horizontal extent of the borehole data. The second case study is associated with a leaking fuel tank at a site in Utah County, near Provo, Utah.
The general layout of this site is shown in Fig. 4. The underground storage tank leaked hydrocarbons into the groundwater which flowed to the southwest following the water-table gradient of about 1.1 m / 1 0 0 m. Based upon maps by Hintze (1988) and our well and GPR data, the geology at this site was determined to consist of three main layers: ( 1 ) an asphalt/sand layer ( 1.11.4 m thick) ; (2) a layer of silty sand containing some gravel ( 1.7-2.4 m thick) ; and (3) a water-bearing gravel layer. The water table at monitoring wells 1 and 2 was 3.6 alad 3.9 m, respectively. GPR data were collected across two traverses at this site. These lines are shown in Fig. 4 as " G P R - S o u t h " and "GPR-North". GPR-South is about 45 m downgradient from the leaking tank and runs across the contaminated area. GPR-North is approximately 60 m north of the tank in an uncontaminated up-gradient area.
A.K. Benson / Journal of Applied Geophysics 33 (1995) 177-193
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Fig. 5. GPR data recorded along traverse GPR-North located up-gradient from the leaking fuel tank in central Utah (surface distances are consistent with those along the GPR-South line).
The GPR-North section (Fig. 5) shows relatively "clean", continuous subsurface strata. The stronger reflection at about 3.4--3.6 m in depth is interpreted as the water table. Reflections below the water table tend to dim and fade out, characteristic of water-saturated sediments. Indications of rocks and gravel can be seen in the reflectors above the water table. On the GPR-South profile shown in Fig. 6, the water table is interpreted to be the reflectors between 3.6 and 3.8 m. Between the surface locations of 40 and 90 m, the water table is "smeared out" by reflections interpreted to come from hydrocarbon contamination. This was later confirmed by well groundwater sample data. These dispersed hydrocarbons are less conductive than water and, therefore, enhance the radar reflectivity in
this part of the profile. As with the Arizona study, some of the hydrocarbons may be floating on the water table. Some of the radar peaks between 1.2 and 3.0 m in depth seem indicative of larger rocks and gravel, which is also in agreement with the monitoring well data in the area,
Groundwater samples taken from monitoring wells 1, 2, 3, and 4 (Fig. 4) were analyzed by an independent laboratory for hydrocarbon content. For MW-3, which is near the "uncontaminated" GPR-North line, the TPH value was about 1.2 ppm, whereas for MW-1 and MW-3 which are near the "contaminated" GPR-South line, TPH values were about 44 ppm and 51 ppm, respectively. At MW-4, the TPH value was about 1.8 ppm, which is comparable to the value in MW-3 to the
A.K. Benson / Journal ¢{/'Applied Geophysics 33 (1995) 177 / 93
184
W
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1.2
0 0 c~
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v
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nation Fig. 6. GPR data recorded along traverse GPR-SouIh located down-gradient from the leaking tuel tank in central Utah.
north. These values are in good agreement with the qualitative interpretations of the GPR data. Based upon the interpretations of the well and GPR data, a pumping well has been installed at the location indicated in Fig. 4 to clean up the contaminated groundwater by circulating it through an activated charcoal filter. These case studies illustrate that GPR can be a useful tool to help assess the location and extent of contaminant plumes in soils and groundwater. When geological conditions are appropriate, GPR surveys offer an economical, nondestructive method to select the strategic locations of monitoring wells and to provide an effective horizontal extent of borehole logs.
4. Locating concealed faults The Wasatch fault zone ( shown in Fig. 7) lies along the western edge of the Middle Rocky Mountain and Colorado Plateau regions and the eastern edge of the Basin and Range region in north-central Utah. The fault zone is distinguished by extensive Late Quaternary fault scarps, truncated bedrock spurs, sag ponds, and springs (Hintze, 1988). Several strands of the fault zone have surface expression, but geophysical and trenching data show that many others are concealed (Benson and Bear, 1987; Benson and Mustoe, 1989). Since 90% of Utah's population lives near or within
185
A.K. Benson /Journal of Applied Geophysics 33 (1995) 177-193
113"
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111"
110"
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Fig. 7. Regionallocationmap showingthe WasatchFault and physiographicprovinces(afterHintze, 1988,p. 80). this active fault zone, large-magnitude earthquakes would have a significant impact (Gori and Hays, 1987, 1988). To mitigate the effects of possible earthquakes, the location and characteristics of individual fault strands need to be determined as accurately as possible in order to establish and implement building codes and zoning ordinances. In the past, a number of surfical studies have focused on the shallow structure and history of movement of the fault zone. These studies include large-scale aerial surveys and small-scale trenching along the fault zone (Cluff et al., 1973). To delineate shallow discontinuities associated with the Wasatch Fault shown in Figs.
7 and 8, a populated area northeast of Provo, Utah, near Rock Canyon was selected. The geology at the Rock Canyon site is taken from maps and reports by Hintze (1978) and Machette (1989). The surficiai sediments in the area are predominantly Late Pleistocene to Holocene alluvial deposits (Machette, 1989), deposited primarily by debris flows and low-to-high-energy intermittent streams. Bedrock in the Wasatch Range at Rock Canyon consists of Paleozoic and Proterozoic sedimentary rocks, including tillite, quartzites, limestones, dolomites and shales. These rocks were folded and thrust-faulted during the Sevier Orogeny (20-30 Ma) and subsequently cut by numer-
186
A.K. Benson / Journal of Applied Geophysics 3311995) 177-19 S
Utah Valley to the west. Older sediments within the valley are of alluvial, fluvial, and lacustrine origin. The thicknesses of sedimentary fill vary from less than I m near the margins to over 4000 m near the center ( Hintze, 1988). A GPR profile was acquired along the traverse by Rock Canyon Road as illustrated in Fig. 9. Fault scarp expressions at the surface have been culturally removed along the road, but some scarp remnants have been found a few meters north and south of the road. Such remnants exist both to the north and the south of the GPR traverse. The radar profile of Fig. 10 suggests a high-angle normal fault in the subsurface, whose surface exposure is probably the scarp to the north and south of the roadway. Some of its surface expression was probably removed when Rock Canyon Road was built. The upper layers in the GPR profile are interpreted to be road fill, soils, rocks, and gravel, Some of the rocks and gravel are probably from post-fault debris flow out of the canyon. Using an average dielectric constant of 2.5 for dry soils and rocks near Rock Canyon, Eq. (3) yields a fault: offset of approximately 3.3 m. The dip angle of the interpreted fault is estimated to be 77°W. Above the fault plane between surface locations 385-395 m, a radar reflection is seen which we interpret to come from the colluvial wedge of materials associated with the faulting.
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ous normal faults during Basin and Range extensions ( 10-20 Ma). The Wasatch Fault separates the bedrock of the mountains from the alluvial and lake deposits in
Rock Canyon Site: Provo, Utah
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A.K. Benson / Journal of Applied Geophysics 33 (1995) 177-193
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After the geophysical work was completed, the Utah Geological and Mineral survey (UGMS) dug a 30 m long trench across the fault scarp approximately 5 m due north of our GPR line. In going north from the GPR traverse to the trench location, the north side of Rock Canyon Road slopes down a gradient of about 1.5 m, and most of this material appears to be road fill and gravel base, with some rock debris from Rock Canyon. A schematic diagram of the trench is shown in Fig. 11 (W.R. Lund, UGMS, pers. commun., 1991). The
trench indicates a single-event rupture with 3.5 m of throw, dipping at 76°W beneath the scarp. Comparing the GPR data shown in Fig. 10 with the trench crosssection suggests that the fault reflections are probably coming from the rock layer between the upper and middle soil horizons. Examination of natural exposures in the trench showed competent rock between these two horizons, consisting of fractured limestone and quartzite, as well as some debris-flow deposits from the canyon. Indications of the smaller antithetic fault
A.K. Benson / Journal o[ Applied Geophysics 33 (I 995) 177-193
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located farther west in the trench shown in Fig. 11 showed up in the GPR profile, but the offset of about I m was too small for adequate resolution.
5. Outlining underground cavities The effectiveness of GPR in locating and outlining the boundaries of an underground cavity was tested by running some traverses over an underground physics laboratory located on the campus of Brigham Young University, Provo, Utah. The surveys were conducted and interpretations made knowing only the general area where the lab was located. To avoid bias, comparisons with lab blueprints were not made until the geophysical work was completed. An E - W cross-section of the lab taken from the blueprints is sketched in Fig. 12. After a few GPR traverses over the area, the subsurface boundaries of the lab became rather clear. The E W profile shown in Fig. 13 outlines the top of the lab. From the blueprints, the walls of the lab are about 1 m thick and are covered with 1.2 m of soil. The ceiling is approximately 1 m thick and is covered with about 2.4 m of soil. The extra soil over the ceiling provides protection from radiation that may be given off by a large Van de Graaff generator housed in the lab. Using a lab-determined value of 4,8 for the dielectric constant of the unsaturated unconsolidated soil over the lab, Eq. (3) yields a depth of about 1.3 m to the top of the outside walls and an average depth of 2.4 m to the
top of the recessed ceiling. Similar depths to the ceiling were obtained from a N - S line over the lab shown in Fig. 14. The spikiness of the reflections from the ceiling is probably due to rebar in the ceiling, the rough texture of the concrete, and/or rocks in the soil. On both lines, multiple reflections mimic the ceiling pattern. From Fig. 13, the E - W length of the lab is estimated to be approximately 18 m. and from Fig. 14. the N - S extent is about 12.8 m. All of these values compare favorably with the actual lab dimensions shown in Fig. 12. The air cavity and floor of the lab can be clearly seen on the GPR profile in Fig. 15. This traverse was taken over the central portion of the lab ceiling. High-pass and low-pass filter settings were adjusted in the GPR recorder to retain lower frequencies in Fig. 15 than in Figs. 13 and 14. The overlying soil, concrete ceiling. and associated multiple reflections are in the upper 40 ns of this profile. Since the relative dielectric constant of air is I and the time interval between the interpreted ceiling and floor is 60 ns from Fig. 15, the average depth from the ceiling to the floor is 9.1 m. This is the same value as found on the blueprints for the lab. Between the surface positions at 6 and 9 m, the larger amplitudes along the floor of the lab are interpreted to be the approximate location of the Van de Graaff generator. This location was later confirmed by inspecting the blueprints and the lab itself. As a final study, the GPR traverse shown in Fig. 16 was recorded across US Highway 6 near the Eureka, Utah location shown earlier in Fig. 7. Eureka is an old
A.K. Benson / Journal of Applied Geophysics 33 (1995) 177-193
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Scale (meters) Fig, 12. Schematicdiagramof the undergroundphysicslaboratoryon the campus of BrighamYoungUniversity,Provo, Utah. mining town dating back to the 1890s having numerous mine shafts and tunnels under the town and surrounding areas. Locations of many of the older underground excavations are unknown since they were never
recorded on any maps. Most of the mines are in Cambrian and Ordovician limestone, and the area is rich in ores of gold, silver, copper, iron, and other metals. Tertiary intrusions have brought in ore-bearing solu-
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A.K. Benson / Journal of Applied Geophysics 33 (1995) 177-19 J
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A.K. Benson / Journal of Applied Geophysics 33 (1995) 177-193
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tions which have solidified in the cracks, vugs, and fissures in the limestone. In many places the ground is cracked or collapsed from the mining activity, and some mines are now just cone-shaped cave-ins (Chronic, 1990). Two areas of apparent subsidence in the upper strata can be seen in Fig. 16. Any surface expressions of this slumping were most likely filled in many years ago when the highway was resurfaced. Some nearby cracking and warping in the highway may be clues that some subsurface movement is still occurring. Almost directly below each area of subsidence is a hyperbolic diffraction pattern that is characteristic of reflections from localized objects or cavities in the subsurface. The hyperbolae in Fig. 16 most likely originate from manmade cavities, such as a mine shaft or a tunnel, but could also arise from sinkholes carved out in the limestone. Using an average relative dielectric constant of 5.5 for the soils and asphalt in the Eureka area and 7 for limestone, Eq. (3) yields an estimated depth of about 7.8 m down to the areas of subsidence and about 10.6 m down to the apparent cavities. Cavity diameters appear to be on the order of 2-4 m. Whatever the source of the apparent subsurface cavities and possible associated subsidence, the important point is that GPR can often be successful in locating
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these potential hazards. This information is very useful in planning and site development. Early detection of such cavities or the mechanism causing subsidence can result in substantial savings in time and money. Additional GPR surveys and some drilling are planned in the future in and around Eureka.
6. Conclusions In some geologic settings, GPR is a reliable, rapid, and economical method for mapping shallow subsurface sediments and assessing hydrogeologic conditions. The thickness of the sediments, the depth to the water table, and the presence of groundwater contamination can be reliably mapped at field sites where reflector geometries are not too complex and where electrical resistivities are typically high and relative dielectric constants are typically low. In particular, GPR data from sites located in Arizona and Utah proved helpful in mapping groundwater contamination produced by leaking underground fuel tanks. Based upon the estimated depths to the areas of subsurface contamination from the GPR data, the lateral extent of the contaminant plumes was outlined and monitoring wells were installed at strategic locations at
192
A.K. Benson / Journal of Applied Geophysics 33 (1995) 177-193
both sites. Analysis of the monitor well data showed that estimated water table depths and qualitative assessments of contamination location and concentrations inferred from the G P R data were in g o o d a g r e e m e n t with the depths and contamination levels found in the wells. G P R data collected in east Provo, Utah demonstrated the reliable detection o f shallow c o n c e a l e d faulting in appropriate g e o l o g i c settings. Interpretations o f the radar data coincided with m a n y features o b s e r v e d in a recently opened nearby trench. The net offset and dip o f the fault correlate quite well. D e t e r m i n i n g the characteristics and distribution o f subsurface faulting and folding can provide useful information for establishing and i m p l e m e n t i n g building codes and zoning ordinances in the Provo area. Cavity detection is another challenging exploration problem that requires g o o d resolution and g o o d depth penetration. G P R can be successful in detecting nearsurface cavities when the host rock has a relatively low electromagnetic w a v e attenuation factor. The geometrical features of an underground laboratory at B r i g h a m Y o u n g University were successfully outlined and specified in depth using G P R surveys. Interpretations o f the ceiling, floor, and air cavity were in g o o d a g r e e m e n t with the blueprints o f the laboratory. G P R data also provided useful information about the location and approximate depths for two apparent cavities and associated areas of subsidence under US H i g h w a y - 6 near Eureka, Utah. In summary, in appropriate geology, G P R surveys can provide useful cost-effective data which can help assess potentially dangerous geological hazards, including groundwater contamination, c o n c e a l e d faulting, and underground cavities. Planning and remedial measures based upon these data can then be implemented to help protect human life and produce m i n i m a l threat to the environment.
Acknowledgements Sincere appreciation is extended to T.E. O w e n for valuable c o m m e n t s on the manuscript and to graduate students in g e o l o g y at B r i g h a m Y o u n g University who helped acquire the data.
References Benson, A.K: and Bear, J.L., 1987. Close order gravity surveys: a means of fault delineation in valley fill sediments. Proc. 23rd Syrup. Engineering Geology and Soils Engineering, pp. 219240. Benson, A.K. and Mustoe, N.B., 1989. Seismic and gravity surveys as tools in delineating subsurface geology and assessing some geologic hazards. Proc. 25th Symp. Engineering Geology and Geotechnical Engineering, pp. 25-30. Benson, A.K. and Mustoe, N.B., 1991. Ground penetrating radar, electrical resistivity, and water quality studies integrated to determine the source(s) and geometry of hydrocarbon contamination at a site in north-central Arizona. Proc. 27th Syrup. Engineering Geology and Geotechnical Engineering, pp. 38.1-38.18. Beres, M. and Haeni, F.P., 1991. Application of ground-penetratingradar methods to hydrogeologic studies. Ground Water, 29: 375386: Chronic, H.. 1991. Roadside Geology of Arizona. Mountain Press, Missoula, MT, 322 pp. Chronic, H., 1990. Roadside Geology of Utah. Mountain Press, Missoula, MT, 326 pp. Cluff, L.S., Brogan, G.E. and Glass, C.E., 1973. Wasatch Fault, Southern Portion. Woodward-Lundgren and Associates, Oakland, CA (rep. prepared fi~r Utah Geol. Miner. Surv., Salt Lake City, UT). Davis, J.L. and Annan, A.P., 1989. Ground-penetrating radar for high-resolution mapping of soil and rock stratigraphy. Geophys. Prospect., 37:531-55 I. Foster, A.R., Veatch, M.D. and Baird, S.L., 1987. Hazardous waste geophysics. Geophys. Leading Edge, 6: 8-13. Gori, P.L. and Hays, W.W. (Editors), 1987. Earthquake Hazards Along the Wasatch Front, Utah. US Geol. Surv. Open-File Rep., 87-154. Gori, P.L. and Hays, W.W. ( Editors ), 1988. Assessment of Regional Earthquake Hazards and Risk Along the Wasatch Front, Utah, III. US Geol. Surv. Open-File Rep., 88-680. Haeni, F.P., McKeegan, D.K. and Capron, D.R., 1987. Ground Penetrating Radar Study of the Thickness and Extent of Sediments Beneath Silver Lake, Berlin and Meriden, Connecticut. US Geol. Surv. Water Resour. Invest, 85-4108, 19 pp. Hintze, L.F., 1978. Geologic Map of the Y Mountain Area, East o1: Provo, Utah and Cross Sections of the Y Mountain. Brigham Young Univ. Geol. Stud. Spec. PUN., 5 (1:24,000). Hintze, L.F., 1988. Geologic History of Utah. Brigham Young Univ. Geol. Stud: Spec. Publ., 7: 7-10, 29-32, 77-89. Machette, M.N., 1989. Preliminary Surficial Geologic Map of the Wasatch Fault Zone, Eastern Part of Utah Valley, Utah Country, and Parts of Salt Lake and Juab Counties, Utah. US Geol. Surv. Misc. Field Stud. Map Pamphlet, MF-2109 (1:50,1)00). Olhoeft, G.R., 1984. Applications and limitations of ground penetrating radar. Soc. Explor, Geophys., 54th Annu. Int. Meet., Atlanta, GA, pp. 147-148 t abstr. ). Olhoeft, G.R., 1986. Direct detection of hydrocarbon and organic chemicals with ground penetration radar and complex resistivity. Proc. NWWA Conf. Petroleum Hydrocarbons and Organic Chemicals Ground Water, Houston, TX, pp. 1-22.
A,K. Benson / Journal of Applied Geophysics 33 (1995) 177-193 Sheriff, R.E., 1984. Encyclopedic Dictionary of Exploration Geophysics. Soc. Explor. Geophys., Tulsa, OK, 2nd ed., 323 pp. Topp, G.C., Davis, J.L. and Annan, A.P., 1980. Electromagnetic determination of soil water content: measurements in coaxial transmission lines. Water Resour. Res., 16: 574-582.
193
Trabant, P.K., 1984. Applied high-resolution geophysical methods. Int. Human Resour. Dev. Co., Boston, MA, 365 pp. Wright, D.C., Olhoeft, G.R. and Watts, R.D., 1984. Ground penetrating radar studies on Cape Cod. Proc. NWWA/EPA Conf. Surface and Borehole Geophysical Methods in Ground Water Investigations, San Antonio, TX, pp. 666--680.