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A structural study of thermal tufas using ground-penetrating radar
Journal of Applied Geophysics 81 (2012) 38–47
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A structural study of thermal tufas using ground-penetrating radar John H. McBride a,⁎, W. Spencer Guthrie b, David L. Faust c, Stephen T. Nelson a a b c
Department of Geological Sciences, P. O. Box 24606, Brigham Young University, Provo, UT 84602, USA Department of Civil and Environmental Engineering, 368 Clyde Building, Brigham Young University, Provo, UT 84602, USA Kearns, UT 84118, USA
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Article history: Received 15 October 2010 Accepted 9 September 2011 Available online 19 September 2011 Keywords: Tufa Travertine Structural mapping Radar
a b s t r a c t Tufas (freshwater calcareous rocks) can provide excellent targets for ground-penetrating radar (GPR) exploration due to low clay content and low salinity. Widespread tufas occur at the surface and in the shallow subsurface of Heber Valley, an alluvium-filled basin located in the Rocky Mountains of northern Utah (USA). A set of 200-MHz GPR profiles, augmented by test profiles using higher- and lower-frequency antennas, provides a high-resolution view of the internal structure of a tufa mound and its immediately surrounding platform, including unconformities, caverns, disruptions due to voids, and “seismic” stratigraphic patterns to a depth of 5–6 m. These patterns may be used to constrain interpretations of the episodic growth of a tufa system over geologic time. Testing of different antennas (100, 200, 400 MHz) indicates that resolution of subsurface tufa features, as well as the depth of signal penetration, is sensitive to the frequency. The present study demonstrates that thermal tufas form circular, mound-shaped bodies that are lensoidal in cross section with discrete internal layering. GPR can thus be an effective tool in delineating the three-dimensional structure of subsurface tufa bodies. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Tufas (freshwater calcareous rocks) are enigmatic sedimentary rocks because of the wide range of environments in which they form and their broad spectrum of structures and textures (Pentecost, 2005). The study of tufas can benefit constraining models of climate change and paleo-hydrological reconstructions. The spatial and temporal distribution of tufa depends on groundwater recharge, which is controlled by climate variation (Evans, 1999; Miner et al., 2007; Pedley et al., 1996). From a geotechnical point of view, because any calcareous deposits may be associated with underground cavities, mapping of tufa in areas undergoing urbanization may help mitigate geologic risks (Al-fares et al., 2002; Apel and Dezelic, 2005; Pipan et al., 2002). Here we discuss the results of a ground-penetrating radar (GPR) study of the Midway tufa formations in northern Utah, USA. Previous studies have used GPR to guide interpretations of tufa deposited under ambient temperature and bio-mediated conditions (Pedley and Hill, 2003; Pedley et al., 2000). However, geologic applications of GPR have been dominated by studies of clastic sedimentary
⁎ Corresponding author. Tel. + 1 8014225219; fax + 1 8014220267. E-mail addresses: [email protected] (J.H. McBride), [email protected] (W.S. Guthrie), [email protected] (D.L. Faust), [email protected] (S.T. Nelson). 0926-9851/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jappgeo.2011.09.011
rocks (Baker and Jol, 2007), with most GPR studies of limestones focusing on rocks deposited in marine environments (e.g., Grasmueck and Weger, 2002). Because tufas actually comprise a significant portion of the stratigraphic record (Pentecost, 2005), but are proportionally less studied using GPR, a detailed study, testing specific hypotheses as to the suitability of the method, is warranted. Our study applies GPR to tufa developed in a thermal environment (thermogene travertine sensu Pentecost (2005)). We expected GPR to be successfully applied to thermal tufa because limited outcrop observations indicate a well-layered rock. We therefore tested the hypothesis that subsurface tufa, precipitated from thermal springs, has sufficient internal and boundary electrical contrasts, as well as lithologic coherency, to be successfully mapped using GPR. We also test the hypothesis that thermal tufas form laterally persistent internal and boundary layers. Since thermally deposited tufas have a high proportion of calcite, with relatively little clay, and since they tend to be porous, we expected that GPR attenuation would be relatively low. In a general sense, limestones are expected to have low attenuation compared to shales, clay-rich rocks, and siltstone (Milsom, 2003) or any materials with high electrical conductivities (e.g., high salinity soils typical of arid leaching environments). Our purpose is to map internal and boundary stratigraphy of thermal tufas so as to constrain how these rocks are deposited over time. From a geotechnical perspective, our specific objectives are to find if GPR can detect buried caverns and other voids in thermal tufas and
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can delineate their lateral subsurface extent in order to constrain planning for residential or industrial development. An advantage of our study area is that tufa is exposed at the surface so that our objective for depth penetration needed only to be about 5 m. Further, the ground surface was fairly smooth with only gentle topography, for which the profiles were corrected.
2. Tufas, and local geologic setting Pentecost (2005) has defined tufas (or travertine rocks) as chemically-precipitated non-marine limestone (calcite or aragonite) deposited around springs or in lakes and rivers. Tufa typically possesses “low to moderate intercrystalline porosity and often high moldic or framework porosity within a vadose or occasionally shallow phreatic environment” (Pentecost, 2005). The terms “meteogene” and “thermogene” have been coined (Pentecost, 2005) to classify tufas (travertine) into those for which the CO2 necessary for CaCO3 precipitation originated from the soil or from deep thermal processes, respectively. All tufa rocks have intercrystalline porosity, which will vary according to the crystal structure. Pentecost (2005) has summarized the various factors that control porosity, noting that freshly precipitated travertine can have high porosities, but that intercrystalline porosity is reduced over time by precipitation of secondary calcite in pore spaces. However, moldic (or vuggy) porosity tends to be preserved (Pentecost, 2005), which is a factor for the tufas observed in our study. Our study area is located along the western margin of Heber Valley of northern Utah (USA) within the Rocky Mountains and immediately east of the Wasatch Mountain Range (Mountain Spa, Fig. 1a). The valley is bounded by normal faults that are subsidiary to the major extensional Wasatch fault system located on the western side of the Wasatch Range (Willis and Willis, 2000). Alluvium, consisting of silt,
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sand, and well-sorted gravel, blankets Paleozoic bedrock in the central part of the valley (Bromfield et al., 1970). Quaternary-age tufas lie atop or interfinger with the alluvium, range from a few meters to 60 m thick, and represent the largest concentration of thermal tufas in Utah (Carreón-Diazconti et al., 2003; Kohler, 1979; Willis and Willis, 2000). The tufa emanates from thermal spring waters having an average temperature of 31.2–45.5 °C (Mayo and Loucks, 1995). These waters are thought to have originated from precipitation circulated along faults to depths of 2–2.5 km where the geothermal gradient is abnormally high (Baker, 1968; Mayo and Loucks, 1995). In fact, the deep reservoir temperature has been inferred to be much higher (46° to 125 °C) than surface water temperatures (Kohler, 1979). The area of our survey lies north of the village of Midway approximately in the center of a broad tufa platform (Fig. 1a). This platform is about 11.7 km 2 in area (Fig. 1a) and includes clusters of conical or hemispherical mounds of tufa (Willis and Willis, 2000). Some of these are apparently solid and others open to the surface with a circular orifice that may be filled with flowing or stagnant water (Baker, 1968). The thermal waters are nearly saturated in calcite as they reach the surface (Carreón-Diazconti et al., 2003). The heights of the mounds range from a few to 16 m, at the Homestead mound (Fig. 1a) (Willis and Willis, 2000). The platform is cut by inferred faults, including a notable intersection of north- and northeast-trending faults located about 150 m northwest of the center of the GPR survey and corresponding to a large cluster of tufa mounds and active springs (Fig. 1a).
3. GPR survey area We chose our survey area for the following reasons: (1) the presence of an exposed tufa mound, surrounded by relatively gentle
Fig. 1. (a) Sketch map of tufa platform, Heber Valley, Utah (USA) (based in part on Willis and Willis (2000) and Kohler (1979)). (b) Map of study area with GPR profile locations (letters refer to locations along profiles). Green dots numbered 1–6 indicate locations where diffraction hyperbolae were measured for velocity estimation (see Appendix). Note that maps and cross sections display radar reflectors as depth below an elevation datum of 1712 m above sea level, as typical for geological exploration applications.
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Fig. 2. (a) Interpreted stratigraphic section measured on the inside western wall of the orifice of the main tufa mound in the study area (on the right of the diagram). The stratigraphic section is correlated with a west–east 400-MHz GPR profile (located about 8 m south of profile D–E (Fig. 1b); datum is ground surface; migrated at 0.09 m/ns) that extends to near the edge of the mound orifice where the section was measured. Note that the S reflector correlates well with the basal lithologic unit, as discussed in the text. Detailed descriptions of the lithologic units are given in Table 1. (b) Photographs of the interior of the mound where the measured section was made. Note that the basal layer of silty tufa, which was later excavated with pick and shovel, is covered by dirt in this photo.
topography, (2) a separate water-filled cavern, and (3) tufa that is exposed at the ground surface with little or no soil. Our study area is part of a broader area of tufa development that includes large tufa mounds, numerous smaller mounds, and seeps (Fig. 1a). A prominent tufa mound with a circular open orifice (at least 2.4 m deep and ~ 6.5 m in diameter, with no visible water) rises above the ground in the southeastern quadrant of the survey area (Fig. 1b). A well drilled and logged about 220 m south of the mound penetrated 17 m of tufa underlain by clay (Kohler, 1979). The inner surface of the void within the mound slopes outward from the orifice and is interrupted by multiple outcrop reentrants that penetrate up to about a meter into the mound (Fig. 2a and b). The exposed rock face within the mound was measured and described in detail (Table 1) along a vertical section (Fig. 2a) corresponding to the photograph in Fig. 2b. The reentrants mark lithology breaks involving less resistant beds, which, in one case, consist of dark, calcareous fine-grained rock that includes a vuggy zone. We suggest that the rock exposed in the reentrants represents thin ancient soil horizons upon which the overlying tufa was deposited. 4. GPR system and data processing The GPR survey used GSSI (Geophysical Survey Systems, Inc.) antennas with center frequencies of 100, 200, and 400 MHz with
Table 1 Stratigraphic description of measured section on the west inside rim of the tufa mound orifice (Fig. 1b). Layer Thickness Description (from top) (m) 1
2 3 4
5
6 7 8
9
0.91
Massive tufa with about 10% vugs and other voids. Voids range from 1 mm to solution cavities greater than 15 cm. Exhibits gross layering parallel to the surface of the mound, but shows abundant cauliform 1 mm laminations in detail. This and other tufa layers vary from dark gray to light tan on weathered surfaces. 0.18 Similar to layer 1, but lacks large solution cavities. 0.01 Gray silt layer. 0.12 High porosity tufa (30–40%). Some voids are 2–3 cm, but most are 1–5 mm vugs. Top surface forms a prominent ledge within the tufa cone. 0.24 Massive tufa with about 10% porosity. Vugs and other voids are less than 5 mm. Bottom surface of this layer forms the base of a prominent ledge. 1.22 Similar to layer 5, except porosity varies from a few to perhaps 10% within sub-layers of this unit. 0.04 Soft, reddish-brown silt layer. 0.04 Laminated tufa that separates along reddish-brown silty partings. Contains 1 mm vugs, along with horizontallyelongate cavities. Unknown Mottled white to reddish-brown to dark brown silt. Possible paleosol Damp.
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differing acquisition filter settings, sample rates, and traces per distance. The main part of the survey was performed with the 200-MHz antenna, which provided the best results, trading off penetration depth with resolution (see Appendix Fig. 1). Testing of the three antennas showed that the 200-MHz antenna was optimum (see Appendix). This antenna was pulled along the ground in continuous mode with a sample rate of 1024 samples/trace over 100 ns, about 20 traces/m, and field filters set at 50–600 MHz. The data were acquired as intersecting 2D lines (Fig. 1b) that provided a sparse, non-uniform grid, as limited by surface access. The profiles totaled about 1573 m for the survey area shown in Fig. 1b and consisted of a combination of orientations between east– west and north–south as permitted by numerous buildings, fences, open pits, dense vegetation, and other obstructions. Six profiles were oriented more or less north–south, seven approximately east–west, and two obliquely. Data processing included an exponential trace gain function, re-setting the onset of the data to time zero, low-frequency “background” noise removal, automatic gain control (AGC), predictive deconvolution (lag = 30 ns), and Kirchhoff migration (constant velocity = 0.1 m/ns) (see Appendix). The AGC helped to balance low- and high-amplitude parts of the section. We note that absolute amplitude information was not required for this study. A 9-trace mix was also applied to some of the profiles, which reduced the effects of scattering from small discontinuities, not of interest for this study. The GPR profiles were corrected for the limited topographic variation (Fig. 1b) by surveying locations along each line with sub-centimeter accuracy (the elevation datum for the survey is 1712 m above sea level). The GPR profiles were oriented around and extended away from the tufa mound discussed earlier (Fig. 1b). A second set of profiles extended away from a known natural water-filled cavern exposed to the surface (water does not presently flow to the
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ground surface; the air–water interface is roughly 1 m below the ground surface) located 53 m north of the center of the tufa mound (Fig. 1b). 5. Results The surveys reveal two prominent reflectors that can be correlated and contoured across several GPR profiles (e.g., Fig. 3). Because of the limitation of profile coverage, the contouring can only be considered accurate over and around the tufa mound (Fig. 1b). A shallow reflector (“S”) is easily recognized around the exposed mound with terminations (“pinch-outs”) near the ground surface that define an area of about 3400 m 2 (Fig. 3). The S surface is mostly continuous, but shows some disruptions (Fig. 4), as well as some diffractions, mainly from around the mound orifice. This reflector shows a slight, concave-downward shape when viewed in north– south and east–west cross sections (Figs. 3 and 4). The shape of surfaces buried beneath topography depends on the time-to-depth conversion velocity. For example, a lower velocity (e.g., by one half) may flatten some concave surfaces or even make them slightly convex. A discontinuous reflector (“D”) can be traced over part of the survey area. Unlike S, it shows considerable variability, small-scale undulations and disruptions (Fig. 4), and is marked by numerous diffractions (see Appendix). Where D approaches the area of the mound (e.g., “m” in Figs. 3 and 4), it may merge with some multiple reflection generated between the ground surface and the shallow S reflector. Although the double-time relationships may just be coincidental, we used caution interpreting the deeper parts of the records below the mound. Multiple reflections generated in this way, below topographic highs, would be expected to display a mirror-image shape with respect to topography (Bristow, 2009; Nobes et al., 2005). On profiles closest to the center of the mound, both surfaces
Fig. 3. Two GPR profiles tied together showing two main layer-bounding reflectors (S and D). See Fig. 1b for location (A, B, C refer to profile locations). The letter “m” denotes possible multiple arrival (ground surface to “S” and back to the ground surface), based on approximate double-time relations; however, in other areas, this same reflection does not appear to mimic the “S” reflector and may simply be a continuation of the deeper “D” reflector. Note that these and subsequent 200-MHz profiles are processed with an exponential gain followed by AGC. In order to compare the effects of trace mixing, the section on the left has a 9-trace mix, whereas the section on the right has none.
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Fig. 4. Three GPR profiles showing character of the subsurface tufa mound and two prominent reflectors (S and D) (letters along the top of the profiles refer to profile locations in Fig. 1b). “m” denotes possible multiple reflection (i.e., travel time expected for multiple generated by S and ground surface).
become more complex, exhibiting diffractions and antiformal structures (e.g., Figs. 3 and 4; see also Appendix Fig. 2). As GPR profiles approach the area of the underground cavern (Fig. 1b), D appears to merge with a distinct reflection that locally corresponds approximately with the expected air-water interface (Fig. 3; see also Appendix Fig. 1). For the area near the mound, a difference in the character of the internal horizons can be observed for the layer between the ground surface and S and the layer below S. The upper layer shows a more orderly, coherent, and higher apparent frequency reflection character, whereas the deeper record (lower layer) is less coherent and lower frequency (Fig. 4). It is possible that S may locally correspond to a water table in places. Horizon D would then exist within the saturated (or partially saturated) zone. Water will disperse the signal by attenuating high frequencies. The loss of amplitude on the flanks of the tufa mound (Figs. 3, 4, and Appendix Fig. 1) may then be due to water draining along them. The S, D, and ground surfaces were gridded and smoothed in order to compute depth and isochore (vertical thickness between two surfaces) maps and to provide a spatial context for interpretation. In the north, the D surface defines an elevated terrace that slopes down to the south and west across a gentle slope (Fig. 5), which more or less mimics the topography of the ground surface (cf. Figs. 1b and 5). Note that the cavern is situated near the end of
a localized promontory of the raised terrace and that the tufa mound is located along the down-dip edge of the gentle slope (Fig. 5). The upper layer (defined by mapping the base of S) (Figs. 1, 5, and 6a) is centered on the exposed mound orifice and laps onto the terrace. The base of the upper layer (S) is irregular with a closed-contour structural high situated beneath the mound orifice, which is itself flanked by lows (Fig. 6a). The isochore map for the upper layer (ground surface-to-S reflector) (Fig. 6b) has a roughly circular outline. The isochore reveals a crescent-shaped thickening to the southeast that partially encloses the projection of the mound orifice (Fig. 6b). 5. Geological interpretation Our results reveal two geological surfaces that can be correlated across intersecting profiles. The shallow surface (S) is more uniform, coherent, and localized, defining a discrete, moundshaped feature corresponding to the tufa exposed in the southeast quadrant of the survey area. The deep surface (D) can be confidently correlated over only a part of the survey area and is highly variable and discontinuous. In places, D apparently correlates with an unconfined water table (e.g., at the cavern entrance, Figs. 1a and 5) at the base of an approximately 1-m thick layer of tufa that crops out at the surface. The S and D surfaces are interpreted to
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Fig. 5. Perspective “3D” view of D reflector structure (Fig. 1b). The data were gridded using a 5-m cell size. Stipple represents the area of the subsurface tufa mound (D reflector cannot be everywhere reliably interpreted in this area). The area is shown in Fig. 1b. Square outline denotes area of more data coverage (survey lines shown draped over grid; see Fig. 1b); thus, gridded area beyond the box represents extrapolation only. Large circle is subsurface mound outline projected onto surface; small circle is mound orifice (Fig. 1b).
define distinct layers of tufa, with the upper layer (the mound) being more ordered and coherent and the lower being internally more complex. Although the complicated geometry of the mound makes it difficult to correlate specific outcrop horizons with specific reflectors located off the flank, lithologic breaks observed inside the orifice would be expected to be responsible for horizontal reflectors within the upper layer above the S reflector (Fig. 3). Test profiles acquired using the 400-MHz antenna (Figs. 7 and 2a) show continuous and coherent fine-scale layering between the ground surface and the S surface. Such layering is not readily apparent on the 200-MHz
profiles. The measured geologic section (Table 1) constructed from inside the mound orifice (Fig. 2a and b) indicates significant alternations in lithologic texture, degree of vugginess, and porosity, which are expressed by the rugged pattern of reentrants and promontories on the inside wall of the orifice. The vuggy zones (Fig. 2b) host the highest observable porosity. Cauliform textures in the vuggy zones (Fig. 2b) suggest some bio-mediation of calcium carbonate deposition. For most of the section, we did not observe any prominent clay or sandstone-dominated layers, which could produce strong dielectric contrasts. Therefore, we suggest that the source of reflectivity within the tufa is likely to be sharp alternations in porosity.
Fig. 6. (a) Structure map of S reflector. (b) Isochore map for ground surface to S reflector. The data were gridded using a 1-m cell size. See Fig. 1b for the location of the tufa mound outline. Large outline is limit of upper tufa layer; small outline is mound orifice. Note that the shape of a structure map depends on the speed of light used for time-to-depth conversion; however, the relative shape (but not the values) of the isochore map is independent of this speed.
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Fig. 7. A 400-MHz GPR test profile that extends from the north rim of the mound orifice to a point north of the water-filled cavern (located just east of point “B” (Fig. 1b); datum is ground surface). Arrows indicate reflectors between the ground surface and S surface that represent fine-scale layering within the tufa. Note that the section has not been corrected for topography and is shown unmigrated.
At the very base of the tufa section measured within the mound orifice (i.e., at 2.7 m below the ground, Fig. 2a) is another sharp reentrant that corresponds to an interval of silty reddish tufa, that we interpret as a paleosol. Unlike the overlying tufa layers, this basal interval consisted of mechanically weak, friable slivers of rock that are lithologically distinct from the calcite-dominated tufa. In order to correlate the results from the measured section with the radar reflections, a high-resolution (400-MHz) GPR profile was acquired right to the edge of the tufa mound orifice (Fig. 2a). After converting the section to depth using the velocity discussed in the Appendix, one can observe that the S reflector correlates to this basal unit. The inward thickening of the upper layer, defining the subsurface tufa mound (base of which is S) (Fig. 1b), is interpreted to represent vertical and lateral accretion of the mound as thermal water flowed from its orifice and precipitated calcium carbonate due to CO2 loss and the resultant increase in pH (Mayo and Loucks, 1995). This accretion is expressed by the crescent-shaped thickening in the isochores (Fig. 6b). The spatial relationship between the terrace and the subsurface tufa mound suggests that the accretion of the mound was controlled in part by an embankment effect of the terrace edge. The mapping results indicate that the subsurface extent of the mound is well-defined and has an average diameter at its base that is roughly 8–9 times that of the orifice. The anomalous antiforms seen on the gridded map (Fig. 6a) and cross sections (Fig. 3) may be related to original cavern collapse, analogous to features seen in karstified regions (Guidry et al., 2007). However, these “antiforms” may also represent incomplete migration of diffractions, which might be expected at the edges of the nearby void (e.g., Kruse et al., 2000) associated with the mound orifice (Fig. 1b). We interpret the D surface and the material directly above it as older platform tufas, probably composed of coalesced hot spring precipitates. The D surface also tracks a slightly elevated terrace or ridge, mimicking topography, cutting through the northeastern corner of the study area (Fig. 5). The origin of this terrace is unknown but could be related to pre-tufa topography or to the normal faulting that has been mapped in the area (Willis and Willis, 2000). Another tufa mound cluster appears to the northwest along this topographic slope where two normal faults intersect (Fig. 1a). We speculate that the edge of the raised tufa terrace (defined by D) was influenced by pre-existing topography. The edge of the tufa terrace may also have functioned as a conduit for thermal waters to reach the surface and precipitate calcium carbonate.
6. Conclusion Our study in Heber Valley, Utah (USA) demonstrates that highresolution GPR can successfully map the detailed structure of
thermogene tufa (travertine). Based on the observation of coherent signal deteriorating nearer the water-filled cavern, we suggest that the GPR performance is adversely affected by changes in water versus air filling the porosity (although it is also possible that geological contrasts coincidentally vanish in the same area). Further, the presence of moist, fine-grained soils over the tufa will adversely affect signal penetration. The use of a 200-MHz antenna unit was an acceptable compromise between lower (100-MHz) and higher (400-MHz) frequency antennas. The 100-MHz antenna provided somewhat greater penetration, but with significant loss of resolution of important stratigraphic details. We recommend a 400-MHz antenna for providing better resolution of stratigraphy, although it suffers from poor penetration in lossy areas. Variations in porosity are likely a major source of reflectivity. Ancient soil horizons, whose strong electrical contrasts with the adjoining tufa layers could provide the necessary impedance differences (e.g., see outcrop-GPR profile correlation in Fig. 2a), are also a likely source of reflectivity. These horizons (e.g., S) may act like “seismic stratigraphic sequence” boundaries, which allow mapping of discrete units by correlating reflector packages across an area. We find that tufa deposited in a mound environment is welllayered with a highly coherent stratigraphy. Tufa that was developed outside of such an environment (e.g., in a “platform” setting) is much less internally coherent and is marked by diffractive structure, which may result from the coalescence over time of smaller tufa mounds or localized build-ups. This study reveals the limits of one tufa mound that formed along the margins of, and/or on top of, a pre-existing tufa platform; however, because our surveys do not cover all areas of the site, there are likely anomalies within the tufa (e.g., voids) that we have not detected. It thus shows how tufa precipitates and accretes episodically and is influenced by pre-existing tufas (or other strata) and/or topography. Future research on the tufa platform in Heber Valley would benefit from 3D GPR surveys performed using closely spaced profiles in an orthogonal grid, and detailed ground-truthing (e.g., shallow coring) in order to precisely correlate reflectors to specific horizons.
Acknowledgements The authors gratefully acknowledge the field mapping support of Mr. Dan Byer of Mountain Spa Development LLC of Midway, Utah (USA), who funded this study for the responsible development of the property corresponding to the study area. Data processing and visualization were made possible by software grants from the Landmark (Halliburton) University Grant Program (ProMAX2D™) and from Seismic Micro-Technology (The Kingdom Suite™). Appreciation is expressed to A. L. Mayo and G. C. Willis for reading an initial version of the paper and providing useful advice. Appreciation is also expressed to S. A. Arcone and an
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anonymous referee for their careful reviews, which improved the final version of the paper substantially. The authors and their employers make no warranty, expressed or implied, regarding the suitability of any report or findings arising from this article for a particular purpose. The authors and their employers shall not be liable under any circumstances for any direct, indirect, incidental, or consequential damages with respect to claims by users of any results or findings arising from this article, including claims based on allegations of errors, omissions, or negligence.
Appendix We tested three different antennas in order to determine the optimum frequency for resolving internal structure of the tufa and the effects of attenuation (Appendix Fig. 1). Each antenna provides advantages and disadvantages with regard to resolution and signal penetration. The monostatic 100-MHz antenna was expected to furnish deeper penetration; however, as can be seen from Appendix Fig. 1, only a very modest increase in signal penetration was evident before ambient noise begins to overtake coherent signal. The bistatic 400-MHz antenna was superior for resolving detailed layering within the main tufa mound (Appendix Fig. 1), but suffered in respect to resolving deeper structure away from the mound where the medium was more either “lossy” or simply lacked good reflectivity (e.g., around the water-filled cavern). Thus, the bistatic 200-MHz antenna
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seems to be a suitable compromise between the 100- and 400-MHz antennas. The optimum velocity used for time-depth conversion was determined from modeling of diffraction hyperbolae to be 0.09 m/ns (corresponding to a dielectric constant of 10.4), which lies within the range of values reported for limestones in general (e.g., Al-fares et al., 2002; Asprion et al., 2009; Grasmueck and Weger, 2002; Sigurdsson and Overgaard, 1998). For example, Milsom (2003) gives a textbook value of 0.12 m/ns for limestones in general, whereas Sigurdsson and Overgaard (1998), Grasmueck and Weger (2002), and Neal et al. (2008) give 0.07–0.08 m/ns. Naturally occurring calcite has been reported to have a dielectric constant of 7.8–8.5 for radio frequencies (Keller, 1989). For GPR studies applied in the vadose zone for rocks with a calcite matrix, the bulk dielectric constant will be reduced by air-filled porosity (Martinez and Byrnes, 2001). Our use of a dielectric constant higher than for pure calcite reflects the probability of some clay or silt component (as observed in the mound orifice outcrop (Fig. 2)) and some presence of water in the pores. The use of a single velocity is at best an approximation, and traveltime distortions are likely in places. We expected the vertical, and especially lateral, velocity structure for our study area to be quite complex, including water-saturated and dry regions, and water-filled caverns and possibly air-filled voids. Appendix Fig. 2 shows six examples of diffraction hyperbolae for which a value of 0.09 ns/m was modeled. As can be seen from the figure, there is some consistency in the curvature of the hyperbolae, although variation is also evident.
Appendix Fig. 1. Three GPR sections acquired with different antennas along the same path, approximately corresponding to a north–south section that passes just west of the mound orifice. Profiles were all acquired on the same day. Vertical arrows indicate how a lateral truncation of reflectivity, which could possibly represent void space, is variously expressed using the different antennas. Note that the sections have not been corrected for topography and are shown unmigrated.
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Appendix Fig. 2. Excerpts of unmigrated GPR sections with only an exponential gain and background removal applied showing where diffraction hyperbolae were measured with a velocity of approximately 0.09 m/ns. See Fig. 1b for locations of the hyperbolae noted in each section.
References Al-fares, W., Bakalowicz, M., Guérin, R., Dukhan, M., 2002. Analysis of the karst aquifer structure of the Lamalou area (Hérault, France) with ground penetrating radar. Journal of Applied Geophysics 51, 97–106. Apel, D.B., Dezelic, V., 2005. Evaluation of high frequency ground penetrating radar (GPR) in mapping strata of dolomite and limestone rocks for ripping technique. International Journal of Surface Mining, Reclamation, and Environment 19, 260–275. Asprion, U., Westphal, H., Nieman, M., Pomar, L., 2009. Extrapolation of depositional geometries of the Menorcan Miocene carbonate ramp with ground-penetrating radar. Facies 55, 37–46. Baker Jr., C.H., 1968. Thermal springs near Midway, Utah. U. S. Geological Survey Professional Paper 600-D, pp. 63–70. Baker, G.S., Jol, H.M. (Eds.), 2007. Stratigraphic analyses using GPR. Geological Society of America Special Paper 432 (181 pp.). Bristow, C., 2009. Ground penetrating radar in aeolian dune sands. In: Jol, H.M. (Ed.), Ground Penetrating Radar: Theory and Applications. Elsevier, Amsterdam, pp. 273–297. Bromfield, C.S., Baker, A.A., Crittenden Jr., M.D., 1970. Geologic map of the Heber quadrangle, Utah. U. S. Geological Survey Map GQ-864, scale 1:24,000. Carreón-Diazconti, C., Nelson, S.T., Mayo, A.L., Tingey, D.G., Smith, M., 2003. A mixed groundwater system at Midway, UT: discriminating superimposed local and regional discharge. Journal of Hydrology 273, 119–138. Evans, J.E., 1999. Recognition and implications of Eocene tufas and travertines in the Chadron Formation, White River Group, Badlands of South Dakota. Sedimentology 46, 771–789. Grasmueck, M., Weger, R., 2002. 3D GPR reveals complex internal structure of Pleistocene oolitic sandbar. The Leading Edge 7, 634–639. Guidry, S.A., Grasmueck, M., Carpenter, D.G., Gombos Jr., A.M., Bachtel, S.L., Viggiano, D.A., 2007. Karst and early fracture networks in carbonates, Turks and Caicos Islands, British West Indies. Journal of Sedimentary Research 77, 508–524.
Keller, G.V., 1989. Electrical properties. In: Carmichael, R.S. (Ed.), Practical Handbook of Physical Properties of Rocks and Minerals. CRC Press, Boca Raton, FL, pp. 359–428. Kohler, J.F., 1979. Geology, characteristics, and resource potential of the lowtemperature geothermal system near Midway, Wasatch County, Utah. Utah Geological and Mineral Survey Report of Investigation, 142, p. 142 (45 pp.). Kruse, S.E., Schneider, J.C., Campagna, D.J., Inman, J.A., Hickey, T.D., 2000. Ground penetrating radar imaging of cap rock, caliche and carbonate strata. Journal of Applied Geophysics 43, 239–249. Martinez, A., Byrnes, A.P., 2001. Modeling dielectric-constant values of geologic materials: an aid to ground-penetrating radar data collection and interpretation. Kansas Geological Survey, Current Research in Earth Sciences, Bulletin 247 (part 1, 16 pp.). Mayo, A.L., Loucks, M.D., 1995. Solute and isotopic geochemistry and ground water flow in the central Wasatch Range, Utah. Journal of Hydrology 172, 31–59. Milsom, J., 2003. Field Geophysics, Third ed. Wiley, New York. (232 pp.). Miner, R.E., Nelson, S.T., Tingey, D.G., Murrell, M.T., 2007. Using fossil spring deposits in the Death Valley region, USA to evaluate palaeoflowpaths. Journal of Quaternary Science 22, 373–386. Neal, A., Grasmueck, M., McNeill, D.F., Viggiano, D.A., Eberli, G.P., 2008. Full-resolution 3D radar stratigraphy of complex oolitic sedimentary architecture: Miami Limestone, Florida, U.S.A. Journal of Sedimentary Research 78, 638–653. doi:10.2110/ jsr.2008.070. Nobes, D.C., Davis, E.F., Arcone, S.A., 2005. “Mirror-image” multiples in groundpenetrating radar. Geophysics 70, 20–22. Pedley, M., Hill, I., 2003. The recognition of barrage and paludal tufa systems by GPR: case studies in the geometry and correlation of Quaternary freshwater carbonates. In: Bristow, C.S., Jol, H.M. (Eds.), Ground penetrating radar in sediments: Geological Society, London, Special Publication, 211, pp. 207–223. Pedley, M., Andrews, J., Ordonez, S., Garcia del Cura, M.A., Gonzales Martin, J.-A., Taylor, D., 1996. Does climate control the morphological fabric of freshwater carbonates?
J.H. McBride et al. / Journal of Applied Geophysics 81 (2012) 38–47 A comparative study of Holocene barrage tufas from Spain and Britain. Palaeogeography, Palaeoclimatology, Palaeoecology 121, 239–257. Pedley, H.M., Hill, I., Denton, P., Brasington, J., 2000. Three-dimensional modelling of a Holocene tufa system in the Lathkill Valley, north Derbyshire, using groundpenetrating radar. Sedimentology 47, 721–737. Pentecost, A., 2005. Travertine. Springer-Verlag, Berlin, pp. 12–14. Pipan, M., Forte, E., Sugan, M., Finetti, I., 2002. Application of GPR to image bedding planes, fractures and cavities in limestone. Memoir, Società Geologica Italiana 57, 605–612.
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Sigurdsson, T., Overgaard, T., 1998. Application of GPR for 3-D visualization of geological and structural variation in a limestone formation. Journal of Applied Geophysics 40, 29–36. Willis, J.B., Willis, G.C., 2000. Geology of Wasatch Mountain State Park, Utah. In: Sprinkel, D.A., Chidsey Jr., T.C., Anderson, P.B. (Eds.), Geology of Utah's parks and monuments: Utah Geological Association Publication, 28, pp. 495–516.
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Journal of Applied Geophysics journal homepage: www.elsevier.com/locate/jappgeo
A structural study of thermal tufas using ground-penetrating radar John H. McBride a,⁎, W. Spencer Guthrie b, David L. Faust c, Stephen T. Nelson a a b c
Department of Geological Sciences, P. O. Box 24606, Brigham Young University, Provo, UT 84602, USA Department of Civil and Environmental Engineering, 368 Clyde Building, Brigham Young University, Provo, UT 84602, USA Kearns, UT 84118, USA
a r t i c l e
i n f o
Article history: Received 15 October 2010 Accepted 9 September 2011 Available online 19 September 2011 Keywords: Tufa Travertine Structural mapping Radar
a b s t r a c t Tufas (freshwater calcareous rocks) can provide excellent targets for ground-penetrating radar (GPR) exploration due to low clay content and low salinity. Widespread tufas occur at the surface and in the shallow subsurface of Heber Valley, an alluvium-filled basin located in the Rocky Mountains of northern Utah (USA). A set of 200-MHz GPR profiles, augmented by test profiles using higher- and lower-frequency antennas, provides a high-resolution view of the internal structure of a tufa mound and its immediately surrounding platform, including unconformities, caverns, disruptions due to voids, and “seismic” stratigraphic patterns to a depth of 5–6 m. These patterns may be used to constrain interpretations of the episodic growth of a tufa system over geologic time. Testing of different antennas (100, 200, 400 MHz) indicates that resolution of subsurface tufa features, as well as the depth of signal penetration, is sensitive to the frequency. The present study demonstrates that thermal tufas form circular, mound-shaped bodies that are lensoidal in cross section with discrete internal layering. GPR can thus be an effective tool in delineating the three-dimensional structure of subsurface tufa bodies. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Tufas (freshwater calcareous rocks) are enigmatic sedimentary rocks because of the wide range of environments in which they form and their broad spectrum of structures and textures (Pentecost, 2005). The study of tufas can benefit constraining models of climate change and paleo-hydrological reconstructions. The spatial and temporal distribution of tufa depends on groundwater recharge, which is controlled by climate variation (Evans, 1999; Miner et al., 2007; Pedley et al., 1996). From a geotechnical point of view, because any calcareous deposits may be associated with underground cavities, mapping of tufa in areas undergoing urbanization may help mitigate geologic risks (Al-fares et al., 2002; Apel and Dezelic, 2005; Pipan et al., 2002). Here we discuss the results of a ground-penetrating radar (GPR) study of the Midway tufa formations in northern Utah, USA. Previous studies have used GPR to guide interpretations of tufa deposited under ambient temperature and bio-mediated conditions (Pedley and Hill, 2003; Pedley et al., 2000). However, geologic applications of GPR have been dominated by studies of clastic sedimentary
⁎ Corresponding author. Tel. + 1 8014225219; fax + 1 8014220267. E-mail addresses: [email protected] (J.H. McBride), [email protected] (W.S. Guthrie), [email protected] (D.L. Faust), [email protected] (S.T. Nelson). 0926-9851/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jappgeo.2011.09.011
rocks (Baker and Jol, 2007), with most GPR studies of limestones focusing on rocks deposited in marine environments (e.g., Grasmueck and Weger, 2002). Because tufas actually comprise a significant portion of the stratigraphic record (Pentecost, 2005), but are proportionally less studied using GPR, a detailed study, testing specific hypotheses as to the suitability of the method, is warranted. Our study applies GPR to tufa developed in a thermal environment (thermogene travertine sensu Pentecost (2005)). We expected GPR to be successfully applied to thermal tufa because limited outcrop observations indicate a well-layered rock. We therefore tested the hypothesis that subsurface tufa, precipitated from thermal springs, has sufficient internal and boundary electrical contrasts, as well as lithologic coherency, to be successfully mapped using GPR. We also test the hypothesis that thermal tufas form laterally persistent internal and boundary layers. Since thermally deposited tufas have a high proportion of calcite, with relatively little clay, and since they tend to be porous, we expected that GPR attenuation would be relatively low. In a general sense, limestones are expected to have low attenuation compared to shales, clay-rich rocks, and siltstone (Milsom, 2003) or any materials with high electrical conductivities (e.g., high salinity soils typical of arid leaching environments). Our purpose is to map internal and boundary stratigraphy of thermal tufas so as to constrain how these rocks are deposited over time. From a geotechnical perspective, our specific objectives are to find if GPR can detect buried caverns and other voids in thermal tufas and
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can delineate their lateral subsurface extent in order to constrain planning for residential or industrial development. An advantage of our study area is that tufa is exposed at the surface so that our objective for depth penetration needed only to be about 5 m. Further, the ground surface was fairly smooth with only gentle topography, for which the profiles were corrected.
2. Tufas, and local geologic setting Pentecost (2005) has defined tufas (or travertine rocks) as chemically-precipitated non-marine limestone (calcite or aragonite) deposited around springs or in lakes and rivers. Tufa typically possesses “low to moderate intercrystalline porosity and often high moldic or framework porosity within a vadose or occasionally shallow phreatic environment” (Pentecost, 2005). The terms “meteogene” and “thermogene” have been coined (Pentecost, 2005) to classify tufas (travertine) into those for which the CO2 necessary for CaCO3 precipitation originated from the soil or from deep thermal processes, respectively. All tufa rocks have intercrystalline porosity, which will vary according to the crystal structure. Pentecost (2005) has summarized the various factors that control porosity, noting that freshly precipitated travertine can have high porosities, but that intercrystalline porosity is reduced over time by precipitation of secondary calcite in pore spaces. However, moldic (or vuggy) porosity tends to be preserved (Pentecost, 2005), which is a factor for the tufas observed in our study. Our study area is located along the western margin of Heber Valley of northern Utah (USA) within the Rocky Mountains and immediately east of the Wasatch Mountain Range (Mountain Spa, Fig. 1a). The valley is bounded by normal faults that are subsidiary to the major extensional Wasatch fault system located on the western side of the Wasatch Range (Willis and Willis, 2000). Alluvium, consisting of silt,
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sand, and well-sorted gravel, blankets Paleozoic bedrock in the central part of the valley (Bromfield et al., 1970). Quaternary-age tufas lie atop or interfinger with the alluvium, range from a few meters to 60 m thick, and represent the largest concentration of thermal tufas in Utah (Carreón-Diazconti et al., 2003; Kohler, 1979; Willis and Willis, 2000). The tufa emanates from thermal spring waters having an average temperature of 31.2–45.5 °C (Mayo and Loucks, 1995). These waters are thought to have originated from precipitation circulated along faults to depths of 2–2.5 km where the geothermal gradient is abnormally high (Baker, 1968; Mayo and Loucks, 1995). In fact, the deep reservoir temperature has been inferred to be much higher (46° to 125 °C) than surface water temperatures (Kohler, 1979). The area of our survey lies north of the village of Midway approximately in the center of a broad tufa platform (Fig. 1a). This platform is about 11.7 km 2 in area (Fig. 1a) and includes clusters of conical or hemispherical mounds of tufa (Willis and Willis, 2000). Some of these are apparently solid and others open to the surface with a circular orifice that may be filled with flowing or stagnant water (Baker, 1968). The thermal waters are nearly saturated in calcite as they reach the surface (Carreón-Diazconti et al., 2003). The heights of the mounds range from a few to 16 m, at the Homestead mound (Fig. 1a) (Willis and Willis, 2000). The platform is cut by inferred faults, including a notable intersection of north- and northeast-trending faults located about 150 m northwest of the center of the GPR survey and corresponding to a large cluster of tufa mounds and active springs (Fig. 1a).
3. GPR survey area We chose our survey area for the following reasons: (1) the presence of an exposed tufa mound, surrounded by relatively gentle
Fig. 1. (a) Sketch map of tufa platform, Heber Valley, Utah (USA) (based in part on Willis and Willis (2000) and Kohler (1979)). (b) Map of study area with GPR profile locations (letters refer to locations along profiles). Green dots numbered 1–6 indicate locations where diffraction hyperbolae were measured for velocity estimation (see Appendix). Note that maps and cross sections display radar reflectors as depth below an elevation datum of 1712 m above sea level, as typical for geological exploration applications.
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Fig. 2. (a) Interpreted stratigraphic section measured on the inside western wall of the orifice of the main tufa mound in the study area (on the right of the diagram). The stratigraphic section is correlated with a west–east 400-MHz GPR profile (located about 8 m south of profile D–E (Fig. 1b); datum is ground surface; migrated at 0.09 m/ns) that extends to near the edge of the mound orifice where the section was measured. Note that the S reflector correlates well with the basal lithologic unit, as discussed in the text. Detailed descriptions of the lithologic units are given in Table 1. (b) Photographs of the interior of the mound where the measured section was made. Note that the basal layer of silty tufa, which was later excavated with pick and shovel, is covered by dirt in this photo.
topography, (2) a separate water-filled cavern, and (3) tufa that is exposed at the ground surface with little or no soil. Our study area is part of a broader area of tufa development that includes large tufa mounds, numerous smaller mounds, and seeps (Fig. 1a). A prominent tufa mound with a circular open orifice (at least 2.4 m deep and ~ 6.5 m in diameter, with no visible water) rises above the ground in the southeastern quadrant of the survey area (Fig. 1b). A well drilled and logged about 220 m south of the mound penetrated 17 m of tufa underlain by clay (Kohler, 1979). The inner surface of the void within the mound slopes outward from the orifice and is interrupted by multiple outcrop reentrants that penetrate up to about a meter into the mound (Fig. 2a and b). The exposed rock face within the mound was measured and described in detail (Table 1) along a vertical section (Fig. 2a) corresponding to the photograph in Fig. 2b. The reentrants mark lithology breaks involving less resistant beds, which, in one case, consist of dark, calcareous fine-grained rock that includes a vuggy zone. We suggest that the rock exposed in the reentrants represents thin ancient soil horizons upon which the overlying tufa was deposited. 4. GPR system and data processing The GPR survey used GSSI (Geophysical Survey Systems, Inc.) antennas with center frequencies of 100, 200, and 400 MHz with
Table 1 Stratigraphic description of measured section on the west inside rim of the tufa mound orifice (Fig. 1b). Layer Thickness Description (from top) (m) 1
2 3 4
5
6 7 8
9
0.91
Massive tufa with about 10% vugs and other voids. Voids range from 1 mm to solution cavities greater than 15 cm. Exhibits gross layering parallel to the surface of the mound, but shows abundant cauliform 1 mm laminations in detail. This and other tufa layers vary from dark gray to light tan on weathered surfaces. 0.18 Similar to layer 1, but lacks large solution cavities. 0.01 Gray silt layer. 0.12 High porosity tufa (30–40%). Some voids are 2–3 cm, but most are 1–5 mm vugs. Top surface forms a prominent ledge within the tufa cone. 0.24 Massive tufa with about 10% porosity. Vugs and other voids are less than 5 mm. Bottom surface of this layer forms the base of a prominent ledge. 1.22 Similar to layer 5, except porosity varies from a few to perhaps 10% within sub-layers of this unit. 0.04 Soft, reddish-brown silt layer. 0.04 Laminated tufa that separates along reddish-brown silty partings. Contains 1 mm vugs, along with horizontallyelongate cavities. Unknown Mottled white to reddish-brown to dark brown silt. Possible paleosol Damp.
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differing acquisition filter settings, sample rates, and traces per distance. The main part of the survey was performed with the 200-MHz antenna, which provided the best results, trading off penetration depth with resolution (see Appendix Fig. 1). Testing of the three antennas showed that the 200-MHz antenna was optimum (see Appendix). This antenna was pulled along the ground in continuous mode with a sample rate of 1024 samples/trace over 100 ns, about 20 traces/m, and field filters set at 50–600 MHz. The data were acquired as intersecting 2D lines (Fig. 1b) that provided a sparse, non-uniform grid, as limited by surface access. The profiles totaled about 1573 m for the survey area shown in Fig. 1b and consisted of a combination of orientations between east– west and north–south as permitted by numerous buildings, fences, open pits, dense vegetation, and other obstructions. Six profiles were oriented more or less north–south, seven approximately east–west, and two obliquely. Data processing included an exponential trace gain function, re-setting the onset of the data to time zero, low-frequency “background” noise removal, automatic gain control (AGC), predictive deconvolution (lag = 30 ns), and Kirchhoff migration (constant velocity = 0.1 m/ns) (see Appendix). The AGC helped to balance low- and high-amplitude parts of the section. We note that absolute amplitude information was not required for this study. A 9-trace mix was also applied to some of the profiles, which reduced the effects of scattering from small discontinuities, not of interest for this study. The GPR profiles were corrected for the limited topographic variation (Fig. 1b) by surveying locations along each line with sub-centimeter accuracy (the elevation datum for the survey is 1712 m above sea level). The GPR profiles were oriented around and extended away from the tufa mound discussed earlier (Fig. 1b). A second set of profiles extended away from a known natural water-filled cavern exposed to the surface (water does not presently flow to the
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ground surface; the air–water interface is roughly 1 m below the ground surface) located 53 m north of the center of the tufa mound (Fig. 1b). 5. Results The surveys reveal two prominent reflectors that can be correlated and contoured across several GPR profiles (e.g., Fig. 3). Because of the limitation of profile coverage, the contouring can only be considered accurate over and around the tufa mound (Fig. 1b). A shallow reflector (“S”) is easily recognized around the exposed mound with terminations (“pinch-outs”) near the ground surface that define an area of about 3400 m 2 (Fig. 3). The S surface is mostly continuous, but shows some disruptions (Fig. 4), as well as some diffractions, mainly from around the mound orifice. This reflector shows a slight, concave-downward shape when viewed in north– south and east–west cross sections (Figs. 3 and 4). The shape of surfaces buried beneath topography depends on the time-to-depth conversion velocity. For example, a lower velocity (e.g., by one half) may flatten some concave surfaces or even make them slightly convex. A discontinuous reflector (“D”) can be traced over part of the survey area. Unlike S, it shows considerable variability, small-scale undulations and disruptions (Fig. 4), and is marked by numerous diffractions (see Appendix). Where D approaches the area of the mound (e.g., “m” in Figs. 3 and 4), it may merge with some multiple reflection generated between the ground surface and the shallow S reflector. Although the double-time relationships may just be coincidental, we used caution interpreting the deeper parts of the records below the mound. Multiple reflections generated in this way, below topographic highs, would be expected to display a mirror-image shape with respect to topography (Bristow, 2009; Nobes et al., 2005). On profiles closest to the center of the mound, both surfaces
Fig. 3. Two GPR profiles tied together showing two main layer-bounding reflectors (S and D). See Fig. 1b for location (A, B, C refer to profile locations). The letter “m” denotes possible multiple arrival (ground surface to “S” and back to the ground surface), based on approximate double-time relations; however, in other areas, this same reflection does not appear to mimic the “S” reflector and may simply be a continuation of the deeper “D” reflector. Note that these and subsequent 200-MHz profiles are processed with an exponential gain followed by AGC. In order to compare the effects of trace mixing, the section on the left has a 9-trace mix, whereas the section on the right has none.
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Fig. 4. Three GPR profiles showing character of the subsurface tufa mound and two prominent reflectors (S and D) (letters along the top of the profiles refer to profile locations in Fig. 1b). “m” denotes possible multiple reflection (i.e., travel time expected for multiple generated by S and ground surface).
become more complex, exhibiting diffractions and antiformal structures (e.g., Figs. 3 and 4; see also Appendix Fig. 2). As GPR profiles approach the area of the underground cavern (Fig. 1b), D appears to merge with a distinct reflection that locally corresponds approximately with the expected air-water interface (Fig. 3; see also Appendix Fig. 1). For the area near the mound, a difference in the character of the internal horizons can be observed for the layer between the ground surface and S and the layer below S. The upper layer shows a more orderly, coherent, and higher apparent frequency reflection character, whereas the deeper record (lower layer) is less coherent and lower frequency (Fig. 4). It is possible that S may locally correspond to a water table in places. Horizon D would then exist within the saturated (or partially saturated) zone. Water will disperse the signal by attenuating high frequencies. The loss of amplitude on the flanks of the tufa mound (Figs. 3, 4, and Appendix Fig. 1) may then be due to water draining along them. The S, D, and ground surfaces were gridded and smoothed in order to compute depth and isochore (vertical thickness between two surfaces) maps and to provide a spatial context for interpretation. In the north, the D surface defines an elevated terrace that slopes down to the south and west across a gentle slope (Fig. 5), which more or less mimics the topography of the ground surface (cf. Figs. 1b and 5). Note that the cavern is situated near the end of
a localized promontory of the raised terrace and that the tufa mound is located along the down-dip edge of the gentle slope (Fig. 5). The upper layer (defined by mapping the base of S) (Figs. 1, 5, and 6a) is centered on the exposed mound orifice and laps onto the terrace. The base of the upper layer (S) is irregular with a closed-contour structural high situated beneath the mound orifice, which is itself flanked by lows (Fig. 6a). The isochore map for the upper layer (ground surface-to-S reflector) (Fig. 6b) has a roughly circular outline. The isochore reveals a crescent-shaped thickening to the southeast that partially encloses the projection of the mound orifice (Fig. 6b). 5. Geological interpretation Our results reveal two geological surfaces that can be correlated across intersecting profiles. The shallow surface (S) is more uniform, coherent, and localized, defining a discrete, moundshaped feature corresponding to the tufa exposed in the southeast quadrant of the survey area. The deep surface (D) can be confidently correlated over only a part of the survey area and is highly variable and discontinuous. In places, D apparently correlates with an unconfined water table (e.g., at the cavern entrance, Figs. 1a and 5) at the base of an approximately 1-m thick layer of tufa that crops out at the surface. The S and D surfaces are interpreted to
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Fig. 5. Perspective “3D” view of D reflector structure (Fig. 1b). The data were gridded using a 5-m cell size. Stipple represents the area of the subsurface tufa mound (D reflector cannot be everywhere reliably interpreted in this area). The area is shown in Fig. 1b. Square outline denotes area of more data coverage (survey lines shown draped over grid; see Fig. 1b); thus, gridded area beyond the box represents extrapolation only. Large circle is subsurface mound outline projected onto surface; small circle is mound orifice (Fig. 1b).
define distinct layers of tufa, with the upper layer (the mound) being more ordered and coherent and the lower being internally more complex. Although the complicated geometry of the mound makes it difficult to correlate specific outcrop horizons with specific reflectors located off the flank, lithologic breaks observed inside the orifice would be expected to be responsible for horizontal reflectors within the upper layer above the S reflector (Fig. 3). Test profiles acquired using the 400-MHz antenna (Figs. 7 and 2a) show continuous and coherent fine-scale layering between the ground surface and the S surface. Such layering is not readily apparent on the 200-MHz
profiles. The measured geologic section (Table 1) constructed from inside the mound orifice (Fig. 2a and b) indicates significant alternations in lithologic texture, degree of vugginess, and porosity, which are expressed by the rugged pattern of reentrants and promontories on the inside wall of the orifice. The vuggy zones (Fig. 2b) host the highest observable porosity. Cauliform textures in the vuggy zones (Fig. 2b) suggest some bio-mediation of calcium carbonate deposition. For most of the section, we did not observe any prominent clay or sandstone-dominated layers, which could produce strong dielectric contrasts. Therefore, we suggest that the source of reflectivity within the tufa is likely to be sharp alternations in porosity.
Fig. 6. (a) Structure map of S reflector. (b) Isochore map for ground surface to S reflector. The data were gridded using a 1-m cell size. See Fig. 1b for the location of the tufa mound outline. Large outline is limit of upper tufa layer; small outline is mound orifice. Note that the shape of a structure map depends on the speed of light used for time-to-depth conversion; however, the relative shape (but not the values) of the isochore map is independent of this speed.
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Fig. 7. A 400-MHz GPR test profile that extends from the north rim of the mound orifice to a point north of the water-filled cavern (located just east of point “B” (Fig. 1b); datum is ground surface). Arrows indicate reflectors between the ground surface and S surface that represent fine-scale layering within the tufa. Note that the section has not been corrected for topography and is shown unmigrated.
At the very base of the tufa section measured within the mound orifice (i.e., at 2.7 m below the ground, Fig. 2a) is another sharp reentrant that corresponds to an interval of silty reddish tufa, that we interpret as a paleosol. Unlike the overlying tufa layers, this basal interval consisted of mechanically weak, friable slivers of rock that are lithologically distinct from the calcite-dominated tufa. In order to correlate the results from the measured section with the radar reflections, a high-resolution (400-MHz) GPR profile was acquired right to the edge of the tufa mound orifice (Fig. 2a). After converting the section to depth using the velocity discussed in the Appendix, one can observe that the S reflector correlates to this basal unit. The inward thickening of the upper layer, defining the subsurface tufa mound (base of which is S) (Fig. 1b), is interpreted to represent vertical and lateral accretion of the mound as thermal water flowed from its orifice and precipitated calcium carbonate due to CO2 loss and the resultant increase in pH (Mayo and Loucks, 1995). This accretion is expressed by the crescent-shaped thickening in the isochores (Fig. 6b). The spatial relationship between the terrace and the subsurface tufa mound suggests that the accretion of the mound was controlled in part by an embankment effect of the terrace edge. The mapping results indicate that the subsurface extent of the mound is well-defined and has an average diameter at its base that is roughly 8–9 times that of the orifice. The anomalous antiforms seen on the gridded map (Fig. 6a) and cross sections (Fig. 3) may be related to original cavern collapse, analogous to features seen in karstified regions (Guidry et al., 2007). However, these “antiforms” may also represent incomplete migration of diffractions, which might be expected at the edges of the nearby void (e.g., Kruse et al., 2000) associated with the mound orifice (Fig. 1b). We interpret the D surface and the material directly above it as older platform tufas, probably composed of coalesced hot spring precipitates. The D surface also tracks a slightly elevated terrace or ridge, mimicking topography, cutting through the northeastern corner of the study area (Fig. 5). The origin of this terrace is unknown but could be related to pre-tufa topography or to the normal faulting that has been mapped in the area (Willis and Willis, 2000). Another tufa mound cluster appears to the northwest along this topographic slope where two normal faults intersect (Fig. 1a). We speculate that the edge of the raised tufa terrace (defined by D) was influenced by pre-existing topography. The edge of the tufa terrace may also have functioned as a conduit for thermal waters to reach the surface and precipitate calcium carbonate.
6. Conclusion Our study in Heber Valley, Utah (USA) demonstrates that highresolution GPR can successfully map the detailed structure of
thermogene tufa (travertine). Based on the observation of coherent signal deteriorating nearer the water-filled cavern, we suggest that the GPR performance is adversely affected by changes in water versus air filling the porosity (although it is also possible that geological contrasts coincidentally vanish in the same area). Further, the presence of moist, fine-grained soils over the tufa will adversely affect signal penetration. The use of a 200-MHz antenna unit was an acceptable compromise between lower (100-MHz) and higher (400-MHz) frequency antennas. The 100-MHz antenna provided somewhat greater penetration, but with significant loss of resolution of important stratigraphic details. We recommend a 400-MHz antenna for providing better resolution of stratigraphy, although it suffers from poor penetration in lossy areas. Variations in porosity are likely a major source of reflectivity. Ancient soil horizons, whose strong electrical contrasts with the adjoining tufa layers could provide the necessary impedance differences (e.g., see outcrop-GPR profile correlation in Fig. 2a), are also a likely source of reflectivity. These horizons (e.g., S) may act like “seismic stratigraphic sequence” boundaries, which allow mapping of discrete units by correlating reflector packages across an area. We find that tufa deposited in a mound environment is welllayered with a highly coherent stratigraphy. Tufa that was developed outside of such an environment (e.g., in a “platform” setting) is much less internally coherent and is marked by diffractive structure, which may result from the coalescence over time of smaller tufa mounds or localized build-ups. This study reveals the limits of one tufa mound that formed along the margins of, and/or on top of, a pre-existing tufa platform; however, because our surveys do not cover all areas of the site, there are likely anomalies within the tufa (e.g., voids) that we have not detected. It thus shows how tufa precipitates and accretes episodically and is influenced by pre-existing tufas (or other strata) and/or topography. Future research on the tufa platform in Heber Valley would benefit from 3D GPR surveys performed using closely spaced profiles in an orthogonal grid, and detailed ground-truthing (e.g., shallow coring) in order to precisely correlate reflectors to specific horizons.
Acknowledgements The authors gratefully acknowledge the field mapping support of Mr. Dan Byer of Mountain Spa Development LLC of Midway, Utah (USA), who funded this study for the responsible development of the property corresponding to the study area. Data processing and visualization were made possible by software grants from the Landmark (Halliburton) University Grant Program (ProMAX2D™) and from Seismic Micro-Technology (The Kingdom Suite™). Appreciation is expressed to A. L. Mayo and G. C. Willis for reading an initial version of the paper and providing useful advice. Appreciation is also expressed to S. A. Arcone and an
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anonymous referee for their careful reviews, which improved the final version of the paper substantially. The authors and their employers make no warranty, expressed or implied, regarding the suitability of any report or findings arising from this article for a particular purpose. The authors and their employers shall not be liable under any circumstances for any direct, indirect, incidental, or consequential damages with respect to claims by users of any results or findings arising from this article, including claims based on allegations of errors, omissions, or negligence.
Appendix We tested three different antennas in order to determine the optimum frequency for resolving internal structure of the tufa and the effects of attenuation (Appendix Fig. 1). Each antenna provides advantages and disadvantages with regard to resolution and signal penetration. The monostatic 100-MHz antenna was expected to furnish deeper penetration; however, as can be seen from Appendix Fig. 1, only a very modest increase in signal penetration was evident before ambient noise begins to overtake coherent signal. The bistatic 400-MHz antenna was superior for resolving detailed layering within the main tufa mound (Appendix Fig. 1), but suffered in respect to resolving deeper structure away from the mound where the medium was more either “lossy” or simply lacked good reflectivity (e.g., around the water-filled cavern). Thus, the bistatic 200-MHz antenna
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seems to be a suitable compromise between the 100- and 400-MHz antennas. The optimum velocity used for time-depth conversion was determined from modeling of diffraction hyperbolae to be 0.09 m/ns (corresponding to a dielectric constant of 10.4), which lies within the range of values reported for limestones in general (e.g., Al-fares et al., 2002; Asprion et al., 2009; Grasmueck and Weger, 2002; Sigurdsson and Overgaard, 1998). For example, Milsom (2003) gives a textbook value of 0.12 m/ns for limestones in general, whereas Sigurdsson and Overgaard (1998), Grasmueck and Weger (2002), and Neal et al. (2008) give 0.07–0.08 m/ns. Naturally occurring calcite has been reported to have a dielectric constant of 7.8–8.5 for radio frequencies (Keller, 1989). For GPR studies applied in the vadose zone for rocks with a calcite matrix, the bulk dielectric constant will be reduced by air-filled porosity (Martinez and Byrnes, 2001). Our use of a dielectric constant higher than for pure calcite reflects the probability of some clay or silt component (as observed in the mound orifice outcrop (Fig. 2)) and some presence of water in the pores. The use of a single velocity is at best an approximation, and traveltime distortions are likely in places. We expected the vertical, and especially lateral, velocity structure for our study area to be quite complex, including water-saturated and dry regions, and water-filled caverns and possibly air-filled voids. Appendix Fig. 2 shows six examples of diffraction hyperbolae for which a value of 0.09 ns/m was modeled. As can be seen from the figure, there is some consistency in the curvature of the hyperbolae, although variation is also evident.
Appendix Fig. 1. Three GPR sections acquired with different antennas along the same path, approximately corresponding to a north–south section that passes just west of the mound orifice. Profiles were all acquired on the same day. Vertical arrows indicate how a lateral truncation of reflectivity, which could possibly represent void space, is variously expressed using the different antennas. Note that the sections have not been corrected for topography and are shown unmigrated.
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Appendix Fig. 2. Excerpts of unmigrated GPR sections with only an exponential gain and background removal applied showing where diffraction hyperbolae were measured with a velocity of approximately 0.09 m/ns. See Fig. 1b for locations of the hyperbolae noted in each section.
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