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Detailed gravity survey to help seismic microzonation: Mapping the thickness of unconsolidated deposits in Ottawa, Canada
Journal of Applied Geophysics 75 (2011) 444–454
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Journal of Applied Geophysics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j a p p g e o
Detailed gravity survey to help seismic microzonation: Mapping the thickness of unconsolidated deposits in Ottawa, Canada M. Lamontagne ⁎, M. Thomas, J. Silliker, D. Jobin Natural Resources Canada, 615 Booth Street, Room 216, Ottawa, ON, Canada K1A 0E9
a r t i c l e
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Article history: Received 8 September 2009 Accepted 10 June 2011 Available online 18 July 2011 Keywords: Earthquake Ottawa Canada Gravity Seismic microzonation Gravity modeling
a b s t r a c t In this study, measurements of gravity were made to map and model the thickness of Quaternary deposits (sand and clay) overlying Ordovician limestones in a suburb of Ottawa (Orléans, Ontario). Because ground motion amplification is partly related to the thickness of unconsolidated deposits, this work helps refine the assessment of the earthquake damage potential of the area. It also helps the mapping of clay basins, which can locally exceed 100 m in thickness, where ground motion amplification can occur. Previous work, including well log data and seismic methods, have yielded a wealth of information on near-surface geology in Orléans, thereby providing the necessary constraints to test the applicability of gravity modeling in other locations where other methods cannot always be used. Some 104 gravity stations were occupied in an 8 × 12 km test area in the Orléans. Stations were accurately located with differential GPS that provided centimetric accuracy in elevation. Densities of the unconsolidated Quaternary deposits (Champlain Sea clay) determined on core samples and densities determined on limestone samples from outcrops were used to constrain models of the clay layer overlying the higher density bedrock formations (limestone). The gravity anomaly map delineates areas where clay basins attain N 100 m depth. Assuming a realistic density for the Champlain Sea clays (1.9– 2.1 g/cm 3), the thickness over the higher density bedrock formations (Ordovician carbonate rocks) was modeled and compared with well logs and two seismic reflection profiles. The models match quite well with the information determined from well logs and seismic methods. It was found that gravity and the thickness of unconsolidated deposits are correlated but the uncertainties in both data sets preclude the definition of a direct correlation between the two. We propose that gravity measurements at a local scale be used as an inexpensive means of mapping the thickness of unconsolidated deposits in low-density urban areas. To obtain meaningful results, three conditions must exist. Firstly, elevations of gravity stations must be measured accurately using differential GPS; secondly, that the regional gravity field must be well defined, and thirdly, that the local geology be simple enough to be realistically represented with a two-layer model. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved.
1. Introduction This paper describes the acquisition of gravity data and their analysis that determined the thickness of unconsolidated deposits in the Ottawa region, Ontario, Canada (Fig. 1). Between 2006 and 2009, a seismic microzonation project in the Ottawa region provided a wealth of information on near-surface geology, fundamental frequency of soils and the potential for ground motion amplification (Hunter et al., 2009). This project is part of a larger eastern Canada hazard assessment effort to refine the understanding of regional earthquake hazards and, ultimately, improve the seismic provisions contained in the National Building Code of Canada. The earthquake hazard studies conducted at Natural Resources Canada map ground shaking amplification potential for
⁎ Corresponding author. Tel.: + 1 613 947 1318; fax: + 1 613 943 9285. E-mail address: [email protected] (M. Lamontagne).
hazard recognition and mitigation in two urban areas, Ottawa and Quebec City. In particular, these studies will develop methodologies for creating urban earthquake hazard maps using the soil site response categories in the National Building Code of Canada and will investigate the geotechnical response of geological materials to seismic shaking at these two locations. The Ottawa project characterizes the subsurface using 2D and 3D stratigraphy (based on boreholes, well logs, and in-situ sampling), geotechnical properties, deposit age, shallow geophysics (shallow shear-wave velocity analysis) and high resolution continuous geotechnical logging (cone penetration test). These field data are integrated in site-specific and regional analysis through GIS and database synthesis. In light of the abundant information on the subsurface of the Ottawa region, it was decided to examine if gravity data could be used to determine the thickness of surficial deposits in a low density urban environment (i.e. in a typical North American suburb). If proven reliable, detailed gravity surveys could be used as an inexpensive
0926-9851/$ – see front matter. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jappgeo.2011.06.019
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77°W
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Fig. 1. Location maps of the region of interest with the Bouguer gravity anomaly of the eastern Ontario and southwestern Quebec area. The Ottawa region is located on a regional high trending approximately NNE-SSW. (Source: Geoscience Data Repository of Natural Resources Canada).
method of evaluating the thickness of surficial deposits where other geotechnical or geophysical methods could not be used. Compared with geotechnical boreholes, gravity is not invasive and does not impact on the environment. Unlike some other geophysical methods requiring linear deployments of instruments over long distances such as in seismic profiling, gravity measurements can be made at a single point. Knowing the thickness of surficial deposits is important for studies such as seismic microzonation, geotechnical work and hydrogeology where more expensive seismic methods cannot always be used. Worldwide, other seismic microzonation projects used gravity (such as in Kingston, Jamaica: KMASHA (1999) and Adapazari, Turkey: Komazawa et al. (2002)), but it is the first time it is used in an eastern North American context where much of the near surface material is composed of glacial and post-glacial deposits. Considering the possibility of future work elsewhere is the St. Lawrence valley where conditions are similar to the Ottawa region, we decided to test the methodology as a pilot-test for future microseismic work. Gravity can map the thickness of unconsolidated deposits that lie on top of higher density bedrock. The advantage of detailed gravity surveys over other methods is that they are relatively inexpensive and logistically simple. They can complement other information sources such as well logs and seismic profiles. Its simplicity makes it a useful source of information in semi-urban and non-urban areas. We prefer to use the expression semi-urban, because the cores of urban areas may have underground infrastructures and buildings in the neighborhood of stations that would modify the natural gravity field, making correction and interpretation more difficult. The paper describes how gravity helped map the thickness of unconsolidated deposits in a suburb of Ottawa: Orléans, Ontario, Canada (Fig. 2). An overview of the geology of the area is presented, followed by descriptions of the data acquisition, elevation determination and data processing. We also present the analysis of modeling of the data and comparisons with other sources of information. Recommendations on the use of detailed gravity surveys at a local scale are also presented in the conclusions.
2. Regional Geology The Ottawa region straddles the boundary between the Precambrian Shield and Cambro-Ordovician sedimentary rocks of the St. Lawrence platform. Faults of the Ottawa-Bonnechere Graben cut through these units creating a rugged bedrock landscape made up of uplifted and subsided areas separated by E–W trending faults (Bélanger, 1998). Along a N–S cross-section near Orléans, the thickness of Paleozoic formations increases from zero north of the Ottawa River to nearly 700 m in the area of the survey. Farther south, block faulting and subsequent erosion reduces this thickness to about 200 m. Most bedrock depressions are filled from bottom to top with glacial sediments (tills of age N 12.5 kA), postglacial marine sediments (12.5 to 11 kA; called Champlain Sea (Leda) clay sediments) and younger Ottawa (or proto-Ottawa) River sediments. In Orléans, the thickness of unconsolidated deposits can be based upon water well records, engineering logs, bedrock outcrops, landstreamer (seismic reflection) profiles, and geophysical sites. Fig. 3 presents the interpolated thickness of unconsolidated deposits based on these sources of information (Crow et al., 2007; Motazedian and Hunter, 2008). 3. Data acquisition In this gravity survey, the gravity readings were taken with a Scintrex CG3 automated gravimeter (Fig. 4). Repeat measurements for 7 to 10% of the new stations were done, most differences were within 0.01 mGal of each other, the largest being 0.08 mGal. Position data were collected using two GPS receivers, a Trimble 5700 and a Trimble 5800 Real-Time Kinematic (RTK) pair. Positions for the gravity readings were achieved by three methods: static, stop and go (fast static) and RTK. Most positions were measured using RTK which records positions in 15 s with a RMS less than 2 cm in elevation relative to a base station. The GPS base station is located near the center of the survey area in a place with good sky visibility. The position of the GPS base station was determined with a high degree of accuracy by
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Fig. 2. Location map of the study are (Orléans suburb) showing the positions of the gravity stations and most geographic entities mentioned in the text.
collecting many hours of data during the survey day and postprocessing the data with the Active Control Point at the Canadian Absolute Gravity Site (CAGS, a continuously running GPS station located in Cantley, Quebec, at the Departmental Satellite Tracking Centre). To aid the GPS post processing, precise satellite orbits were downloaded from the Crustal Dynamics Data Information System (CDDIS). The post processing provided a position for the GPS base stations relative to CAGS to better than 2 cm in elevation. To ensure that data from each day of observations were related or tied together, one particular reference point was measured during each new traverse. By doing this same measurement both an office and field check could be done on the work. It was not always possible to obtain measurements using the RTK method. When the RTK method failed, the fast static method was used. Occupation time varied depending on the number of visible satellites but was generally between 5 and 10 min. These data had to be post processed in a manner similar to static data for the GPS base station. The only difference is that it was the baseline from the base station to the measurement location that had to be processed. There was only one case where the RMS from this method exceeded 2 cm. The gravity reductions require orthometric heights rather than ellipsoid heights given by GPS. To obtain the orthometric heights, the Canadian Geoid Model HTv2 (Height Transformation version 2)
produced by the Geodetic Survey Division of Geomatics Canada was used within Trimble Geomatics Office software. The raw gravity observations were then corrected for earth tide variations, instrument scale, irregular drift, latitude and elevation effects, using a software application (PCGRAV) developed and maintained by the Geodetic Survey Division of Geomatics Canada. Free air and Bouguer corrections were applied to the data. Terrain corrections were calculated using Geosoft's Gravity Module and a digital terrain model extracted from the Shuttle Radar Topography Mission SRTM with grid cells of 90 m. Terrain corrections were calculated to an inner radius of 4 km to a maximum distance of 20 km outer radius. The main topographic feature of concern was a quarry near 2 new gravity stations, the terrain correction values were 0.18 and 0.22 mGal. All other terrain correction values were smaller, mostly in the 0.01 to 0.06 mGal range. 4. Regional gravity field The regional gravity field within and surrounding the area of interest is defined by measurements made as part of the National Gravity Mapping Program, most of which were made in the 1950s and 1960s. Vertical control was accomplished mainly by altimetry tied to available benchmarks, and wherever possible measurements were made at benchmarks themselves. Station spacing is variable within the region but generally, within the detailed study area, it varies from
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Fig. 3. Thickness of unconsolidated deposits in the Orléans suburb based on waterwell records, engineering logs, bedrock outcrops, landstreamer (seismic reflection) profiles, and geophysical sites (black dots). The data is interpolated to calculate a grid of depth values with a minimum curvature interpolation scheme. The white dots represent the positions of the gravity stations.
about 1500 m to 2300 m. To the east spacing is wider, ranging from about 3000 m to 3500 m. Even wider spacing averaging about 4000 m occurs north of the Ottawa River where it may attain 6500 m. The overall standard error would be about +/−0.98 mGal. This standard
Fig. 4. Example of a typical field station. The Scintrex CG3 automated gravimeter is the gray box leveled on a tripod. The taller tripod holds the GPS system composed of an antenna and a display. An average station would be occupied during 20 min.
error does not make any allowance for non-application of terrain correction. We know that terrain corrections are very small in our area, so one could estimate +/−1 mGal as a reasonable estimate of the accuracy of the observations defining the regional field. The gravity field defined by measurements at these stations outlines a prominent NNE-trending gravity high (Fig. 1). Typically, the high has a width of about 20 km and its amplitude attains a maximum of about 20 mGal north of the Ottawa River. Values decrease to the southwest as a saddle develops along the axis of the high and a minimum amplitude of about 13 mGal is observed immediately southwest of the study area. The amplitude then increases to about 18 mGal further to the southwest. Background values to either side of the high are similar. North of the Ottawa River where rocks of the Grenville Province are exposed, the gravity high coincides in large part with a unit of massive and foliated monzonite and syenite (Harrison, 1979). The unit contains a few significant belts of marble, lime silicate rocks, interbedded amphibolite and skarn. Spatial considerations suggest that the syenite-monzonite is the principal source of the gravity high. We interpret that the same unit buried beneath the Lower Palaeozoic sedimentary units is also the source of the high south of the Ottawa River. The magnetic data (Geoscience Data Repository of Natural Resources Canada) support our interpretation since a linear belt of relatively narrow magnetic highs (generally 1 to 3 km wide) trends more or less along the axis of the gravity high. The magnetic highs extend SSW into the area of Lower Palaeozoic sedimentary cover. Within the study area, the axis of the regional gravity high runs approximately from the NE corner to SW corner (Fig. 1). Values
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decrease by just over 8 mGal from the NE corner to the SW corner, at a rate of about 2 mGal/km. It is critical that this regional slope be removed from the gravity field defined by the detailed gravity survey completed for this study. It is presumed that short wavelength anomalies are related to the density contrast between the surficial deposits and Palaeozoic sedimentary rocks. It is further assumed that Bouguer gravity anomalies derived at stations located on the Palaeozoic rocks are representative of the background gravity field at that location. With this premise in mind, several gravity stations were located at sites where bedrock was observed at or near the surface or believed to be within a few meters of the surface.
For unconsolidated deposits, densities were measured on samples collected in a well a few kilometers south of Orléans between 3.7 m and 50.4 m depth range, i.e. within the Champlain Sea clay sequence. The mean value is 1.60 g/cm 3 (maximum 1.81 g/cm 3, minimum 1.47 g/cm 3 with a mean porosity of 64% (J. Hunter, pers. comm.)). According to Daigle and Zhao (1999), the density of Leda Clay is 1.96 g/ cm 3 with a moisture content of 40% and 1.519 g/cm 3 with an 80% moisture content which suggests that water content controls the density of these sediments. It is possible that till and postglacial sand deposits have higher average density (possibly 2.0 g/cm 3) due to the presence of pebbles, boulders in a dense sand matrix. As described later, the higher density values provide a better match in our modeling. 6. Detailed gravity field
5. Densities of rock units and unconsolidated materials
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Rock densities were determined from representative samples collected in the field. The mean density of 14 samples of limestones, including a single dolomite value, is 2.69 g/cm 3. The mean density of 10 samples excluding the dolomite and some weathered/cracked limestones is the same. This mean density value for the whole Ordovician sequence is similar to average values for limestones but remains approximate because of the irregular and very limited sampling of the numerous lithological variations. Initial modeling has proceeded using a mean value of 2.69 g/cm 3. The much deeper Precambrian rock formations have higher mean densities of the order of 2.76 g/cm 3 (as measured on samples collected to the North of the area).
A total of 104 stations were occupied in the detailed survey of the Orléans area presented in this paper (Fig. 2). Along the Boyer Road axis, stations were generally spaced 100 m apart and, for this reason, represent about one-third of the total number of stations. This axis included a portion where seismic investigations had been completed (Crow et al., 2007; Pugin et al., 2007). A few stations were also occupied along the axis of Belcourt Boulevard where seismic investigations had also been carried out. Roughly another third of the stations were distributed west of Boyer Road and south of St-Joseph Boulevard, where inter-station spacing averages about 450 m. Most of the remaining stations are located north of St-Joseph Boulevard and south of the Ottawa River, in the area between the Boyer Road line of stations and Trim Road.
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Fig. 5. Bouguer anomaly map of the study area with the gravity stations (dots).
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7. Interpretation
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The Bouguer gravity field defined simply by the detailed stations (Fig. 5) is better resolved than the gravity high from the older National Mapping Program data. In essence, the newly defined western flank has a more sinuous form, related to alternating troughs and peaks superposed on the regional high. In order to focus on relatively local gravity anomalies related to variations in thickness of Quaternary sediments, it is necessary to remove longer wavelength variations of the gravity field, in this case manifested as the regional high outlined by the older data. Because the older data are so widely spaced, a regional field defined by specific stations occupied as part of the new detailed survey was used. A critical assumption for our approach of removing a regional gravity anomaly was that any residual anomalies derived by removal of a regional were related to developments of unconsolidated sediments “sitting” within a bedrock mass composed principally of limestone. According to this premise, gravity anomalies measured on the limestone itself define a regional signature. Therefore, a regional gravity grid was derived from measurements based uniquely on Bouguer anomalies for some 19 stations located on outcrops or on thin surficial deposits (Fig. 6). At these stations, bedrock was outcropping or thin drift was inferred from the surficial deposits map. Subtracting the regional field (Fig. 6) from the Bouguer anomaly (Fig. 5) produced the residual anomaly map (Fig. 7). The resulting anomaly map is a better reflection of variations in the thickness of overburden than the Bouguer anomaly map.
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The residual anomaly map (Fig. 7) is linked to depth to bedrock and shows the location of two basins, one in the center of the map area and one closer to the Ottawa River. It also shows where basins have gently sloping contacts with the bedrock (smooth gradient) and where contacts are steep (sharp gradients; possibly fault related). This map (Fig. 7) can be compared with the thickness of post-glacial materials based on combined borehole and seismic information (Fig. 3). Since the two maps provide similar information about the positions of areas with thin or thick surficial deposits, we conclude that our residual gravity map is a very good representation of the thickness of unconsolidated deposits. An added value to the residual gravity map is that it provides information in areas where the borehole and seismic information is either sparse or non-existent. One such area lies near the Ottawa River where a very deep basin exists (deep blue in Fig. 7) and another is where the central basin has a shallow arm towards the south. This feature of bedrock topography was previously poorly defined due to a lack of well and seismic information (seismic surveys could not be performed there for logistic reasons). Because a gravimeter is more portable than equipment required for a seismic layout, it was possible to access this area and show that thick sediments existed there as well. 8. Gravity modeling Gravity modeling using 2½-D software has been carried out along two profiles, located on Boyer Road and Belcourt Boulevard (Fig. 5), where seismic sections provide a critical constraint (Crow et al.,
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Fig. 6. Regional gravity map based on measurements at stations located either on bedrock or on thin surficial deposits (large dots). Other dots are gravity stations that were not used to derive the map.
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Fig. 7. Residual anomaly map representing the Bouguer anomaly grid (Fig. 5) minus the regional grid (Fig. 6). The black polygon represents the area where the residual anomaly is best constrained with our data and is discussed in text.
2007). The principal objective of this part of the study was to evaluate the gravity method as a means to provide reasonable estimates of thicknesses of superficial Quaternary sediments. The focus was, therefore, gravity modeling along two seismic sections that provided a robust constraint for the modeling. For this purpose, and given the software available to us, we proceeded to use a 2.5D profile-modeling package (GMSYS), which has the advantage of incorporating terrain. And with the ability to specify strike length either side of the profile a 3D aspect was incorporated into the model. The Boyer Road profile crosses the flanks of the central basin discussed above, starting on bedrock in the South and progressing northwards towards thick deposits (~80 m; Fig. 8). For the Boyer road profile, 2-D modeling was used (i.e. infinite lateral extent of blocks) whereas for the Belcourt model, 2¾ D modeling was used with 3 km lateral extent to the Southwest of the profile and 0.8 km to the Northeast. Our modeling matches fairly well the bedrock topography indicated by the seismic section with only minor adjustment in position of the interface between the Quaternary sediments and bedrock. This model did require, however, a rather high density for the clay deposits of 1.935 g/cm 3 (density contrast of 0.755 g/cm 3). Near the south end of the model, the sedimentary basin is bounded by a relatively steeply dipping interface (possibly fault controlled). Progressing northward, the bottom of the basin is at first relatively flat before deepening gradually towards the Ottawa River. On Belcourt Boulevard, fewer gravity stations were measured, but they outline another negative gravity anomaly that correlates quite closely with a basin of sediments outlined by the seismic section
(Fig. 9). This seismic section is parallel to that along Boyer Road and presents a similarly undulating bedrock topography. From south to north, the profiles starts on bedrock, traverses a steeply-dipping interface, a relatively shallow (~60 m) central basin, a bedrock uplift, and a much thicker basin (T N 160 m) near the Ottawa River to the north. In order to match the seismic section, the basin was modeled using a density of 2.155 g/cm 3 (density contrast of 0.535 g/cm 3) which is relatively high for clay.
9. Quantitative estimates of overburden thickness We examined if a simple relationship could be established between the residual gravity anomaly and depth to bedrock. For this exercise, we used two grids: one of overburden thickness computed using well log and seismic interpretation data (Fig. 3) and one of residual gravity anomaly (Fig. 7). The depth to bedrock was based on some 1569 point locations and the residual gravity anomalies on some 104 gravity stations. The potential correlation between residual gravity anomaly data and depth to bedrock were examined using two approaches. In the first approach, we plotted the interpolated value of the residual gravity anomaly at each point where the overburden thickness was known from wells and seismic sections (Fig. 10). A general trend emerges with a linear fit of: Residual anomaly ðmGalsÞ ¼ −0:0198 T
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Fig. 8. Gravity profile along Boyer Road. The seismic profile migrated into a depth section is shown (Crow et al., 2007; Pugin et al., 2007). Part A of the figure presents the gravity aspects whereas part B below represents the depth section. In part A, the red circles represent the gravity data obtained from the residual anomaly grid (Fig. 7), the red line is the modeled gravity field and the green line is the error between the observed and the calculated anomalies. In part B, the red circles on top represent the modeled surface topography and those below, the modeled interface between the unconsolidated deposits and the bedrock. The thin green and blue lines in the seismic sections show the interpreted positions of the interface between the glacial-lower post-glacial and the lower-postglacial and upper post-glacial respectively. The respective densities of the materials are shown.
where T is the overburden thickness in m. The density contrast that corresponds to this trend can be derived as follows. The maximum gravity variation, Δgmax resulting from an infinite horizontal slab is: Δgmax = 0:042 Δρ TðmGalÞ where Δρ is the density contrast (after Telford et al., 1990). Isolating the Δρ term gives a value of 0.47 g/cm 3, not too different from that provided by the best fit to the model on the Belcourt Road profile (0.535 g/cm 3). In the second approach, we plotted the residual anomalies at each gravity station (our “best” gravity values) as a function of the interpolated value of the thickness of unconsolidated deposits (Fig. 11). The best linear fit to the data is: Residual anomaly ðmGalsÞ ¼ −0:0229 T giving a Δρ of 0.54 g/cm 3, very close to the best fit in the modeling along the Belcourt Boulevard profile (0.535 g/cm 3). A few reasons can explain the scatter in the data points of Figs. 10 and 11. First, a number of gravity stations had no depth information (wells or seismic profiles) within hundreds of meters distance making the interpolated depth values very approximate. One example is group of Fig. 11 at around 60 m — 2.5 mGal that we interpret as a deep and relatively small basin not sampled by nearby wells. There are a few
similar areas where little well log information existed to correlate with our gravity information. To constrain our data points to the best possible information, we plotted the residual gravity anomalies at stations that had depth to bedrock information within 100 m of their position (Fig. 12). Although some scatter still exists, a better constrained relationship was found. What are the sources of uncertainty that prevent the establishment of a clear relationship between residual anomaly and depth to bedrock? In our opinion, Figs. 10 and 11 are reminders that gravity measurements are intrinsically different from depth information obtained from wells and seismic sections and its derived grid of depth to bedrock. Gravity measurements are determined by the density distribution around the station whereas depth to bedrock, on the other hand, is point information that is not influenced by other values in the surroundings. In addition, the depth to bedrock information is mainly derived from well logs that were drilled for water well purposes and were mostly made by non-geotechnical experts. Hence, it is possible that some depth values were in fact boulders that were hit by the drilling crew and was falsely interpreted as bedrock. There are, for example, spatial clusters of well log data with wide variations of interpreted depths. Geotechnical wells and seismic profiles on the other hand are more reliable sources of information. Another source of uncertainty is the actual bedrock topography of the Orléans area. The bedrock interface is made of some dramatic changes in elevation as illustrated in the seismic profiles (Figs. 8 and
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Fig. 9. Gravity profile along Belcourt Boulevard. The annotations and sources of information are the same as in Fig. 8.
9). This could bring about variations in the depth to bedrock information even over short horizontal distances, possibly creating the scatter seen in Figs. 10 to 12. The residual anomaly is also subject to some uncertainty. One limitation is the imperfect knowledge of the regional field that was determined from stations on bedrock. Where there is no bedrock nearby, the assumed regional field may diverge from the real one, increasing the uncertainty of the residual anomalies. In the discussion above, it was also assumed that the residual anomaly results only from variations in the thickness of a surface layer of low density material, and that the densities of the two layers in the model are the same beneath each station. This may be a good approximation for thick deposits assumed to be mostly Champlain Sea sediments, but where the unconsolidated surface layer is thin, there may be a larger component of glacial sediments (including till) representing a higher average density for the surface layer. Finally, it should be kept in mind that in contrast to well log information, gravity does not provide point data but rather a summation of mass distributions at varying distances. Consequently, the residual gravity anomaly constitutes a representation of the overall variations of depth to bedrock. 10. Conclusions and recommendations Our detailed gravity experiment provides useful knowledge on the applicability of this technique to mapping the depth to bedrock. In the data acquisition stage, it is essential to obtain accurate height information for each station. In our experiment, this requirement was
met by determining positions with RTK that provided centimetric precision for elevation. This level of accuracy is essential for a detailed gravity survey and cannot be obtained with other methods such as barometric heights. Using a slow drift gravimeter is also essential to obtaining good results. A detailed gravity survey was successful in approximating the depth of unconsolidated deposits in the Orléans suburb of Ottawa. The density contrast between bedrock and unconsolidated deposits was sufficiently high to make gravity applicable there. Our results generally agree with the depth to bedrock information defined by well logs and seismic investigations and add information where the latter could not be made. The residual gravity anomaly map is a good indicator of the dramatic changes in bedrock topography where bedrock elevation changes from the surface to depth exceeding 100 m over the course of a few hundred meters. Gravity surveys can be used as a preliminary tool to obtain bedrock topography and delineate areas with thick unconsolidated deposits where a clear density contrast exists. Gravity could also help map areas where borehole and seismic information are lacking or are incomplete. The capacity of gravity to map bedrock topography is highly dependent on defining a good regional anomaly map. In the case of Orléans, this was accomplished using gravity measurements made directly on bedrock or on thin Quaternary sediments (T b 5 m). The data acquired in the 1950s and 1960s to delineate did not have the resolution (1500 to 2300 m spacing or more) and the precision (±1 mGal ) necessary to clearly delineate the local field that was needed in our study. In addition, most of those stations were not located where bedrock outcrops or is near the
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Fig. 10. Residual gravity anomaly from the grid of Fig. 3 at each point where depth is known from well logs and seismic profile (1569 points). In addition, the gravity anomaly corresponding to density contrasts of 0.25, 0.5 and 1.0 g/cm3 are shown.
surface. For these reasons, it was necessary to better define the regional gravity field at the local level with new gravity stations. Gravity modeling can only provide absolute depth information where other sources of information can constrain the thickness of
surficial deposits. It is well known that an infinite number of solutions can explain a potential field such as gravity. Consequently, when the objective is to define the thickness of surficial deposits, it is beneficial to include borehole and seismic information as a constraint on depth to
Fig. 11. This graph shows the residual anomaly at each gravity station as a function of thickness of overburden interpolated from a grid based on well logs and seismic profiles. In addition, the gravity anomaly corresponding to density contrasts of 0.25, 0.5 and 1.0 g/cm3 are shown.
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Fig. 12. This graph shows the residual anomaly at each gravity station where well or seismic profile information existed within 100 m of the station. In addition, the gravity anomaly corresponding to density contrasts of 0.25, 0.5 and 1.0 g/cm3 are shown.
bedrock and on the stratigraphy of the surficial deposits. In addition, good estimates of densities of rocks and unconsolidated materials (which can be quite variable) are essential. The Orléans survey was conducted over an area where the Paleozoic rock units had relatively similar densities. Hence, changes of density of bedrock were not a major concern. A gravity survey would produce the most accurate depth-to-bedrock information where bedrock topography varies gently with a slowly-varying regional gravity field. In conclusion, we propose that detailed gravity measurements be used as an inexpensive means of mapping the thickness of unconsolidated deposits in geological environments where there is a strong density contrast between these deposits and bedrock. Its simplicity makes one of the best tools in semi-urban and non-urban areas. As explained above, we prefer to use the term semi-urban, because the cores of urban areas may have underground infrastructures that would modify the natural gravity field, making the interpretation more difficult. To obtain meaningful results, four conditions must exist. Firstly, that the elevation of gravity stations be accurately measured (using differential GPS for example); secondly, that the regional gravity field be determined with precision; thirdly, that the local geology be simple enough to be realistically modeled with a two-layer geology. Finally, in the case where depth-to-bedrock estimates are required, realistic density values must be available together with good depth control from borehole and/or other information. 11. Data sources The gravity and magnetic data are archived on the Canadian Gravity and Magnetic Databases (Natural Resources Canada; http:// gdr.nrcan.gc.ca/gravity/index_e.php). Acknowledgments The authors thank their colleagues of NRCan: Carey Gagnon for the technical support of the gravimeter and data conversion, Dr James Hunter and Dr Jacques Liard for supporting this project and Drs Mark
Pilkington and Sue Pullan of NRCan for their reviews of a draft of this paper. Earth Sciences Sector contribution number ***.
References Bélanger, J.R., 1998. Urban geology of Canada's national capital area. In: Karrow, P.F., White, O.L. (Eds.), Special Paper, 42. Geological Association of Canada, Urban Geology of Canadian Cities, pp. 365–384. On line at: http://gsc.nrcan.gc.ca/urbgeo/ natcap/his_quaternary_e.php. Crow, H., Pyne, M., Hunter, J.A., Pullan, S.E., Motazedian, D., Pugin, A., 2007. Shear wave measurements for earthquake response evaluation in Orleans, Ontario. Geological Survey of Canada. Open File 5579, 26 (+app.) pages 1 CD-ROM. On line: http:// geopub.nrcan.gc.ca/moreinfo_e.php?id=223897. Daigle, L., Zhao, J.Q., 1999. Effectiveness of rigid insulation for thermal protection of buried water pipes in rock trenches. 27th Annual CSCE Conference, Cold Regions Engineering Specialty Conference (Regina, SK., Canada, June 03, 1999), pp. 389–398. June 01, 1999 (NRCC-43093). URL: http://irc.nrc-cnrc.gc.ca/pubs/ fulltext/nrcc43093/ (last accessed Jun 16, 2009). Harrison, J.E., 1979. Generalized bedrock geology, Ottawa - Hull, Ontario and Quebec. Geological Survey of Canada, Map 1508A, Scale 1:125 000. Hunter, J.A., Crow, H., Brooks, G.R., Pyne, M., Lamontagne, M., Pugin, A., Pullan, S.E., Cartwright, T., Douma, M., Burns, R.A., Good, R.L., Motazedian, D., Kaheshi-Banab, K., Caron, R., Kolaj, M., Muir, D.,Jones, A., Dixon, L., Plastow, G., Dion, K., Duxbury, A., Landriault, A., Ter-Emmanuil, V., Folahan, I., 2009. City of Ottawa Seismic Site Classification Map From Combined Geological/Geophysical Data; Geological Survey of Canada Open File Report 6191, 1:80 000 scale map. KMASHA (Kingston Metropolitan Area Seismic Hazard Assessment), 1999. Final Report, 1999. Caribbean Disaster Mitigation Project Implemented by the Organization of American States Unit of Sustainable Development and Environment for the USAID Office of Foreign Disaster Assistance and the Caribbean Regional Program. http:// www.oas.org/cdera/document/kma/seismic/kmac.htm. (last accessed 9 Dec 2008). Komazawa, M., Morikawa, H., Nakamura, K., Akamatsu, J., Nishimura, K., Sawada, S., Erken, A., Onalp, A., 2002. Bedrock structure in Adapazari, Turkey; a possible cause of severe damage by the 1999 Kocaeli earthquake. Soil Dynamics and Earthquake Engineering 22, 829–836 (1984). Motazedian, D., Hunter, J., 2008. Development of an NEHRP map for the Orleans suburb of Ottawa, Ontario. Canadian Geotechnical Journal 45, 1180–1188. Pugin, A.J.-M., Hunter, J.A., Motazedian, D., Brooks, G.R., Khaheshi-Banab, K., 2007. An application of shear wave reflection landstreamer technology to soil response evaluation of earthquake shaking in an Urban Area, Ottawa, Ontario. Proceedings, SAGEEP'07 (Symposium on the Application of Geophysics to Engineering and Environmental Problems), Denver, CO, April 1–5, 2007. 11 pp. Telford, W.M., Geldart, L.P., Sheriff, R.E., 1990. Applied Geophysics, 2nd ed. London, Cambridge Univ, Press. 770 p.
Contents lists available at ScienceDirect
Journal of Applied Geophysics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j a p p g e o
Detailed gravity survey to help seismic microzonation: Mapping the thickness of unconsolidated deposits in Ottawa, Canada M. Lamontagne ⁎, M. Thomas, J. Silliker, D. Jobin Natural Resources Canada, 615 Booth Street, Room 216, Ottawa, ON, Canada K1A 0E9
a r t i c l e
i n f o
Article history: Received 8 September 2009 Accepted 10 June 2011 Available online 18 July 2011 Keywords: Earthquake Ottawa Canada Gravity Seismic microzonation Gravity modeling
a b s t r a c t In this study, measurements of gravity were made to map and model the thickness of Quaternary deposits (sand and clay) overlying Ordovician limestones in a suburb of Ottawa (Orléans, Ontario). Because ground motion amplification is partly related to the thickness of unconsolidated deposits, this work helps refine the assessment of the earthquake damage potential of the area. It also helps the mapping of clay basins, which can locally exceed 100 m in thickness, where ground motion amplification can occur. Previous work, including well log data and seismic methods, have yielded a wealth of information on near-surface geology in Orléans, thereby providing the necessary constraints to test the applicability of gravity modeling in other locations where other methods cannot always be used. Some 104 gravity stations were occupied in an 8 × 12 km test area in the Orléans. Stations were accurately located with differential GPS that provided centimetric accuracy in elevation. Densities of the unconsolidated Quaternary deposits (Champlain Sea clay) determined on core samples and densities determined on limestone samples from outcrops were used to constrain models of the clay layer overlying the higher density bedrock formations (limestone). The gravity anomaly map delineates areas where clay basins attain N 100 m depth. Assuming a realistic density for the Champlain Sea clays (1.9– 2.1 g/cm 3), the thickness over the higher density bedrock formations (Ordovician carbonate rocks) was modeled and compared with well logs and two seismic reflection profiles. The models match quite well with the information determined from well logs and seismic methods. It was found that gravity and the thickness of unconsolidated deposits are correlated but the uncertainties in both data sets preclude the definition of a direct correlation between the two. We propose that gravity measurements at a local scale be used as an inexpensive means of mapping the thickness of unconsolidated deposits in low-density urban areas. To obtain meaningful results, three conditions must exist. Firstly, elevations of gravity stations must be measured accurately using differential GPS; secondly, that the regional gravity field must be well defined, and thirdly, that the local geology be simple enough to be realistically represented with a two-layer model. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved.
1. Introduction This paper describes the acquisition of gravity data and their analysis that determined the thickness of unconsolidated deposits in the Ottawa region, Ontario, Canada (Fig. 1). Between 2006 and 2009, a seismic microzonation project in the Ottawa region provided a wealth of information on near-surface geology, fundamental frequency of soils and the potential for ground motion amplification (Hunter et al., 2009). This project is part of a larger eastern Canada hazard assessment effort to refine the understanding of regional earthquake hazards and, ultimately, improve the seismic provisions contained in the National Building Code of Canada. The earthquake hazard studies conducted at Natural Resources Canada map ground shaking amplification potential for
⁎ Corresponding author. Tel.: + 1 613 947 1318; fax: + 1 613 943 9285. E-mail address: [email protected] (M. Lamontagne).
hazard recognition and mitigation in two urban areas, Ottawa and Quebec City. In particular, these studies will develop methodologies for creating urban earthquake hazard maps using the soil site response categories in the National Building Code of Canada and will investigate the geotechnical response of geological materials to seismic shaking at these two locations. The Ottawa project characterizes the subsurface using 2D and 3D stratigraphy (based on boreholes, well logs, and in-situ sampling), geotechnical properties, deposit age, shallow geophysics (shallow shear-wave velocity analysis) and high resolution continuous geotechnical logging (cone penetration test). These field data are integrated in site-specific and regional analysis through GIS and database synthesis. In light of the abundant information on the subsurface of the Ottawa region, it was decided to examine if gravity data could be used to determine the thickness of surficial deposits in a low density urban environment (i.e. in a typical North American suburb). If proven reliable, detailed gravity surveys could be used as an inexpensive
0926-9851/$ – see front matter. Crown Copyright © 2011 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jappgeo.2011.06.019
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77°W
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Fig. 1. Location maps of the region of interest with the Bouguer gravity anomaly of the eastern Ontario and southwestern Quebec area. The Ottawa region is located on a regional high trending approximately NNE-SSW. (Source: Geoscience Data Repository of Natural Resources Canada).
method of evaluating the thickness of surficial deposits where other geotechnical or geophysical methods could not be used. Compared with geotechnical boreholes, gravity is not invasive and does not impact on the environment. Unlike some other geophysical methods requiring linear deployments of instruments over long distances such as in seismic profiling, gravity measurements can be made at a single point. Knowing the thickness of surficial deposits is important for studies such as seismic microzonation, geotechnical work and hydrogeology where more expensive seismic methods cannot always be used. Worldwide, other seismic microzonation projects used gravity (such as in Kingston, Jamaica: KMASHA (1999) and Adapazari, Turkey: Komazawa et al. (2002)), but it is the first time it is used in an eastern North American context where much of the near surface material is composed of glacial and post-glacial deposits. Considering the possibility of future work elsewhere is the St. Lawrence valley where conditions are similar to the Ottawa region, we decided to test the methodology as a pilot-test for future microseismic work. Gravity can map the thickness of unconsolidated deposits that lie on top of higher density bedrock. The advantage of detailed gravity surveys over other methods is that they are relatively inexpensive and logistically simple. They can complement other information sources such as well logs and seismic profiles. Its simplicity makes it a useful source of information in semi-urban and non-urban areas. We prefer to use the expression semi-urban, because the cores of urban areas may have underground infrastructures and buildings in the neighborhood of stations that would modify the natural gravity field, making correction and interpretation more difficult. The paper describes how gravity helped map the thickness of unconsolidated deposits in a suburb of Ottawa: Orléans, Ontario, Canada (Fig. 2). An overview of the geology of the area is presented, followed by descriptions of the data acquisition, elevation determination and data processing. We also present the analysis of modeling of the data and comparisons with other sources of information. Recommendations on the use of detailed gravity surveys at a local scale are also presented in the conclusions.
2. Regional Geology The Ottawa region straddles the boundary between the Precambrian Shield and Cambro-Ordovician sedimentary rocks of the St. Lawrence platform. Faults of the Ottawa-Bonnechere Graben cut through these units creating a rugged bedrock landscape made up of uplifted and subsided areas separated by E–W trending faults (Bélanger, 1998). Along a N–S cross-section near Orléans, the thickness of Paleozoic formations increases from zero north of the Ottawa River to nearly 700 m in the area of the survey. Farther south, block faulting and subsequent erosion reduces this thickness to about 200 m. Most bedrock depressions are filled from bottom to top with glacial sediments (tills of age N 12.5 kA), postglacial marine sediments (12.5 to 11 kA; called Champlain Sea (Leda) clay sediments) and younger Ottawa (or proto-Ottawa) River sediments. In Orléans, the thickness of unconsolidated deposits can be based upon water well records, engineering logs, bedrock outcrops, landstreamer (seismic reflection) profiles, and geophysical sites. Fig. 3 presents the interpolated thickness of unconsolidated deposits based on these sources of information (Crow et al., 2007; Motazedian and Hunter, 2008). 3. Data acquisition In this gravity survey, the gravity readings were taken with a Scintrex CG3 automated gravimeter (Fig. 4). Repeat measurements for 7 to 10% of the new stations were done, most differences were within 0.01 mGal of each other, the largest being 0.08 mGal. Position data were collected using two GPS receivers, a Trimble 5700 and a Trimble 5800 Real-Time Kinematic (RTK) pair. Positions for the gravity readings were achieved by three methods: static, stop and go (fast static) and RTK. Most positions were measured using RTK which records positions in 15 s with a RMS less than 2 cm in elevation relative to a base station. The GPS base station is located near the center of the survey area in a place with good sky visibility. The position of the GPS base station was determined with a high degree of accuracy by
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Fig. 2. Location map of the study are (Orléans suburb) showing the positions of the gravity stations and most geographic entities mentioned in the text.
collecting many hours of data during the survey day and postprocessing the data with the Active Control Point at the Canadian Absolute Gravity Site (CAGS, a continuously running GPS station located in Cantley, Quebec, at the Departmental Satellite Tracking Centre). To aid the GPS post processing, precise satellite orbits were downloaded from the Crustal Dynamics Data Information System (CDDIS). The post processing provided a position for the GPS base stations relative to CAGS to better than 2 cm in elevation. To ensure that data from each day of observations were related or tied together, one particular reference point was measured during each new traverse. By doing this same measurement both an office and field check could be done on the work. It was not always possible to obtain measurements using the RTK method. When the RTK method failed, the fast static method was used. Occupation time varied depending on the number of visible satellites but was generally between 5 and 10 min. These data had to be post processed in a manner similar to static data for the GPS base station. The only difference is that it was the baseline from the base station to the measurement location that had to be processed. There was only one case where the RMS from this method exceeded 2 cm. The gravity reductions require orthometric heights rather than ellipsoid heights given by GPS. To obtain the orthometric heights, the Canadian Geoid Model HTv2 (Height Transformation version 2)
produced by the Geodetic Survey Division of Geomatics Canada was used within Trimble Geomatics Office software. The raw gravity observations were then corrected for earth tide variations, instrument scale, irregular drift, latitude and elevation effects, using a software application (PCGRAV) developed and maintained by the Geodetic Survey Division of Geomatics Canada. Free air and Bouguer corrections were applied to the data. Terrain corrections were calculated using Geosoft's Gravity Module and a digital terrain model extracted from the Shuttle Radar Topography Mission SRTM with grid cells of 90 m. Terrain corrections were calculated to an inner radius of 4 km to a maximum distance of 20 km outer radius. The main topographic feature of concern was a quarry near 2 new gravity stations, the terrain correction values were 0.18 and 0.22 mGal. All other terrain correction values were smaller, mostly in the 0.01 to 0.06 mGal range. 4. Regional gravity field The regional gravity field within and surrounding the area of interest is defined by measurements made as part of the National Gravity Mapping Program, most of which were made in the 1950s and 1960s. Vertical control was accomplished mainly by altimetry tied to available benchmarks, and wherever possible measurements were made at benchmarks themselves. Station spacing is variable within the region but generally, within the detailed study area, it varies from
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Fig. 3. Thickness of unconsolidated deposits in the Orléans suburb based on waterwell records, engineering logs, bedrock outcrops, landstreamer (seismic reflection) profiles, and geophysical sites (black dots). The data is interpolated to calculate a grid of depth values with a minimum curvature interpolation scheme. The white dots represent the positions of the gravity stations.
about 1500 m to 2300 m. To the east spacing is wider, ranging from about 3000 m to 3500 m. Even wider spacing averaging about 4000 m occurs north of the Ottawa River where it may attain 6500 m. The overall standard error would be about +/−0.98 mGal. This standard
Fig. 4. Example of a typical field station. The Scintrex CG3 automated gravimeter is the gray box leveled on a tripod. The taller tripod holds the GPS system composed of an antenna and a display. An average station would be occupied during 20 min.
error does not make any allowance for non-application of terrain correction. We know that terrain corrections are very small in our area, so one could estimate +/−1 mGal as a reasonable estimate of the accuracy of the observations defining the regional field. The gravity field defined by measurements at these stations outlines a prominent NNE-trending gravity high (Fig. 1). Typically, the high has a width of about 20 km and its amplitude attains a maximum of about 20 mGal north of the Ottawa River. Values decrease to the southwest as a saddle develops along the axis of the high and a minimum amplitude of about 13 mGal is observed immediately southwest of the study area. The amplitude then increases to about 18 mGal further to the southwest. Background values to either side of the high are similar. North of the Ottawa River where rocks of the Grenville Province are exposed, the gravity high coincides in large part with a unit of massive and foliated monzonite and syenite (Harrison, 1979). The unit contains a few significant belts of marble, lime silicate rocks, interbedded amphibolite and skarn. Spatial considerations suggest that the syenite-monzonite is the principal source of the gravity high. We interpret that the same unit buried beneath the Lower Palaeozoic sedimentary units is also the source of the high south of the Ottawa River. The magnetic data (Geoscience Data Repository of Natural Resources Canada) support our interpretation since a linear belt of relatively narrow magnetic highs (generally 1 to 3 km wide) trends more or less along the axis of the gravity high. The magnetic highs extend SSW into the area of Lower Palaeozoic sedimentary cover. Within the study area, the axis of the regional gravity high runs approximately from the NE corner to SW corner (Fig. 1). Values
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decrease by just over 8 mGal from the NE corner to the SW corner, at a rate of about 2 mGal/km. It is critical that this regional slope be removed from the gravity field defined by the detailed gravity survey completed for this study. It is presumed that short wavelength anomalies are related to the density contrast between the surficial deposits and Palaeozoic sedimentary rocks. It is further assumed that Bouguer gravity anomalies derived at stations located on the Palaeozoic rocks are representative of the background gravity field at that location. With this premise in mind, several gravity stations were located at sites where bedrock was observed at or near the surface or believed to be within a few meters of the surface.
For unconsolidated deposits, densities were measured on samples collected in a well a few kilometers south of Orléans between 3.7 m and 50.4 m depth range, i.e. within the Champlain Sea clay sequence. The mean value is 1.60 g/cm 3 (maximum 1.81 g/cm 3, minimum 1.47 g/cm 3 with a mean porosity of 64% (J. Hunter, pers. comm.)). According to Daigle and Zhao (1999), the density of Leda Clay is 1.96 g/ cm 3 with a moisture content of 40% and 1.519 g/cm 3 with an 80% moisture content which suggests that water content controls the density of these sediments. It is possible that till and postglacial sand deposits have higher average density (possibly 2.0 g/cm 3) due to the presence of pebbles, boulders in a dense sand matrix. As described later, the higher density values provide a better match in our modeling. 6. Detailed gravity field
5. Densities of rock units and unconsolidated materials
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Rock densities were determined from representative samples collected in the field. The mean density of 14 samples of limestones, including a single dolomite value, is 2.69 g/cm 3. The mean density of 10 samples excluding the dolomite and some weathered/cracked limestones is the same. This mean density value for the whole Ordovician sequence is similar to average values for limestones but remains approximate because of the irregular and very limited sampling of the numerous lithological variations. Initial modeling has proceeded using a mean value of 2.69 g/cm 3. The much deeper Precambrian rock formations have higher mean densities of the order of 2.76 g/cm 3 (as measured on samples collected to the North of the area).
A total of 104 stations were occupied in the detailed survey of the Orléans area presented in this paper (Fig. 2). Along the Boyer Road axis, stations were generally spaced 100 m apart and, for this reason, represent about one-third of the total number of stations. This axis included a portion where seismic investigations had been completed (Crow et al., 2007; Pugin et al., 2007). A few stations were also occupied along the axis of Belcourt Boulevard where seismic investigations had also been carried out. Roughly another third of the stations were distributed west of Boyer Road and south of St-Joseph Boulevard, where inter-station spacing averages about 450 m. Most of the remaining stations are located north of St-Joseph Boulevard and south of the Ottawa River, in the area between the Boyer Road line of stations and Trim Road.
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Fig. 5. Bouguer anomaly map of the study area with the gravity stations (dots).
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7. Interpretation
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The Bouguer gravity field defined simply by the detailed stations (Fig. 5) is better resolved than the gravity high from the older National Mapping Program data. In essence, the newly defined western flank has a more sinuous form, related to alternating troughs and peaks superposed on the regional high. In order to focus on relatively local gravity anomalies related to variations in thickness of Quaternary sediments, it is necessary to remove longer wavelength variations of the gravity field, in this case manifested as the regional high outlined by the older data. Because the older data are so widely spaced, a regional field defined by specific stations occupied as part of the new detailed survey was used. A critical assumption for our approach of removing a regional gravity anomaly was that any residual anomalies derived by removal of a regional were related to developments of unconsolidated sediments “sitting” within a bedrock mass composed principally of limestone. According to this premise, gravity anomalies measured on the limestone itself define a regional signature. Therefore, a regional gravity grid was derived from measurements based uniquely on Bouguer anomalies for some 19 stations located on outcrops or on thin surficial deposits (Fig. 6). At these stations, bedrock was outcropping or thin drift was inferred from the surficial deposits map. Subtracting the regional field (Fig. 6) from the Bouguer anomaly (Fig. 5) produced the residual anomaly map (Fig. 7). The resulting anomaly map is a better reflection of variations in the thickness of overburden than the Bouguer anomaly map.
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The residual anomaly map (Fig. 7) is linked to depth to bedrock and shows the location of two basins, one in the center of the map area and one closer to the Ottawa River. It also shows where basins have gently sloping contacts with the bedrock (smooth gradient) and where contacts are steep (sharp gradients; possibly fault related). This map (Fig. 7) can be compared with the thickness of post-glacial materials based on combined borehole and seismic information (Fig. 3). Since the two maps provide similar information about the positions of areas with thin or thick surficial deposits, we conclude that our residual gravity map is a very good representation of the thickness of unconsolidated deposits. An added value to the residual gravity map is that it provides information in areas where the borehole and seismic information is either sparse or non-existent. One such area lies near the Ottawa River where a very deep basin exists (deep blue in Fig. 7) and another is where the central basin has a shallow arm towards the south. This feature of bedrock topography was previously poorly defined due to a lack of well and seismic information (seismic surveys could not be performed there for logistic reasons). Because a gravimeter is more portable than equipment required for a seismic layout, it was possible to access this area and show that thick sediments existed there as well. 8. Gravity modeling Gravity modeling using 2½-D software has been carried out along two profiles, located on Boyer Road and Belcourt Boulevard (Fig. 5), where seismic sections provide a critical constraint (Crow et al.,
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Fig. 6. Regional gravity map based on measurements at stations located either on bedrock or on thin surficial deposits (large dots). Other dots are gravity stations that were not used to derive the map.
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Fig. 7. Residual anomaly map representing the Bouguer anomaly grid (Fig. 5) minus the regional grid (Fig. 6). The black polygon represents the area where the residual anomaly is best constrained with our data and is discussed in text.
2007). The principal objective of this part of the study was to evaluate the gravity method as a means to provide reasonable estimates of thicknesses of superficial Quaternary sediments. The focus was, therefore, gravity modeling along two seismic sections that provided a robust constraint for the modeling. For this purpose, and given the software available to us, we proceeded to use a 2.5D profile-modeling package (GMSYS), which has the advantage of incorporating terrain. And with the ability to specify strike length either side of the profile a 3D aspect was incorporated into the model. The Boyer Road profile crosses the flanks of the central basin discussed above, starting on bedrock in the South and progressing northwards towards thick deposits (~80 m; Fig. 8). For the Boyer road profile, 2-D modeling was used (i.e. infinite lateral extent of blocks) whereas for the Belcourt model, 2¾ D modeling was used with 3 km lateral extent to the Southwest of the profile and 0.8 km to the Northeast. Our modeling matches fairly well the bedrock topography indicated by the seismic section with only minor adjustment in position of the interface between the Quaternary sediments and bedrock. This model did require, however, a rather high density for the clay deposits of 1.935 g/cm 3 (density contrast of 0.755 g/cm 3). Near the south end of the model, the sedimentary basin is bounded by a relatively steeply dipping interface (possibly fault controlled). Progressing northward, the bottom of the basin is at first relatively flat before deepening gradually towards the Ottawa River. On Belcourt Boulevard, fewer gravity stations were measured, but they outline another negative gravity anomaly that correlates quite closely with a basin of sediments outlined by the seismic section
(Fig. 9). This seismic section is parallel to that along Boyer Road and presents a similarly undulating bedrock topography. From south to north, the profiles starts on bedrock, traverses a steeply-dipping interface, a relatively shallow (~60 m) central basin, a bedrock uplift, and a much thicker basin (T N 160 m) near the Ottawa River to the north. In order to match the seismic section, the basin was modeled using a density of 2.155 g/cm 3 (density contrast of 0.535 g/cm 3) which is relatively high for clay.
9. Quantitative estimates of overburden thickness We examined if a simple relationship could be established between the residual gravity anomaly and depth to bedrock. For this exercise, we used two grids: one of overburden thickness computed using well log and seismic interpretation data (Fig. 3) and one of residual gravity anomaly (Fig. 7). The depth to bedrock was based on some 1569 point locations and the residual gravity anomalies on some 104 gravity stations. The potential correlation between residual gravity anomaly data and depth to bedrock were examined using two approaches. In the first approach, we plotted the interpolated value of the residual gravity anomaly at each point where the overburden thickness was known from wells and seismic sections (Fig. 10). A general trend emerges with a linear fit of: Residual anomaly ðmGalsÞ ¼ −0:0198 T
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Fig. 8. Gravity profile along Boyer Road. The seismic profile migrated into a depth section is shown (Crow et al., 2007; Pugin et al., 2007). Part A of the figure presents the gravity aspects whereas part B below represents the depth section. In part A, the red circles represent the gravity data obtained from the residual anomaly grid (Fig. 7), the red line is the modeled gravity field and the green line is the error between the observed and the calculated anomalies. In part B, the red circles on top represent the modeled surface topography and those below, the modeled interface between the unconsolidated deposits and the bedrock. The thin green and blue lines in the seismic sections show the interpreted positions of the interface between the glacial-lower post-glacial and the lower-postglacial and upper post-glacial respectively. The respective densities of the materials are shown.
where T is the overburden thickness in m. The density contrast that corresponds to this trend can be derived as follows. The maximum gravity variation, Δgmax resulting from an infinite horizontal slab is: Δgmax = 0:042 Δρ TðmGalÞ where Δρ is the density contrast (after Telford et al., 1990). Isolating the Δρ term gives a value of 0.47 g/cm 3, not too different from that provided by the best fit to the model on the Belcourt Road profile (0.535 g/cm 3). In the second approach, we plotted the residual anomalies at each gravity station (our “best” gravity values) as a function of the interpolated value of the thickness of unconsolidated deposits (Fig. 11). The best linear fit to the data is: Residual anomaly ðmGalsÞ ¼ −0:0229 T giving a Δρ of 0.54 g/cm 3, very close to the best fit in the modeling along the Belcourt Boulevard profile (0.535 g/cm 3). A few reasons can explain the scatter in the data points of Figs. 10 and 11. First, a number of gravity stations had no depth information (wells or seismic profiles) within hundreds of meters distance making the interpolated depth values very approximate. One example is group of Fig. 11 at around 60 m — 2.5 mGal that we interpret as a deep and relatively small basin not sampled by nearby wells. There are a few
similar areas where little well log information existed to correlate with our gravity information. To constrain our data points to the best possible information, we plotted the residual gravity anomalies at stations that had depth to bedrock information within 100 m of their position (Fig. 12). Although some scatter still exists, a better constrained relationship was found. What are the sources of uncertainty that prevent the establishment of a clear relationship between residual anomaly and depth to bedrock? In our opinion, Figs. 10 and 11 are reminders that gravity measurements are intrinsically different from depth information obtained from wells and seismic sections and its derived grid of depth to bedrock. Gravity measurements are determined by the density distribution around the station whereas depth to bedrock, on the other hand, is point information that is not influenced by other values in the surroundings. In addition, the depth to bedrock information is mainly derived from well logs that were drilled for water well purposes and were mostly made by non-geotechnical experts. Hence, it is possible that some depth values were in fact boulders that were hit by the drilling crew and was falsely interpreted as bedrock. There are, for example, spatial clusters of well log data with wide variations of interpreted depths. Geotechnical wells and seismic profiles on the other hand are more reliable sources of information. Another source of uncertainty is the actual bedrock topography of the Orléans area. The bedrock interface is made of some dramatic changes in elevation as illustrated in the seismic profiles (Figs. 8 and
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S
N 0.0
Gravity(mGals)
Observed Calculated
-1.0
Error
-2.0
-3.0
-100
D=2670
air
Elevation(meters)
clay
D=2155
limestone
D=2690
ls
0
100 0
1000
2000
3000
4000
5000
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Fig. 9. Gravity profile along Belcourt Boulevard. The annotations and sources of information are the same as in Fig. 8.
9). This could bring about variations in the depth to bedrock information even over short horizontal distances, possibly creating the scatter seen in Figs. 10 to 12. The residual anomaly is also subject to some uncertainty. One limitation is the imperfect knowledge of the regional field that was determined from stations on bedrock. Where there is no bedrock nearby, the assumed regional field may diverge from the real one, increasing the uncertainty of the residual anomalies. In the discussion above, it was also assumed that the residual anomaly results only from variations in the thickness of a surface layer of low density material, and that the densities of the two layers in the model are the same beneath each station. This may be a good approximation for thick deposits assumed to be mostly Champlain Sea sediments, but where the unconsolidated surface layer is thin, there may be a larger component of glacial sediments (including till) representing a higher average density for the surface layer. Finally, it should be kept in mind that in contrast to well log information, gravity does not provide point data but rather a summation of mass distributions at varying distances. Consequently, the residual gravity anomaly constitutes a representation of the overall variations of depth to bedrock. 10. Conclusions and recommendations Our detailed gravity experiment provides useful knowledge on the applicability of this technique to mapping the depth to bedrock. In the data acquisition stage, it is essential to obtain accurate height information for each station. In our experiment, this requirement was
met by determining positions with RTK that provided centimetric precision for elevation. This level of accuracy is essential for a detailed gravity survey and cannot be obtained with other methods such as barometric heights. Using a slow drift gravimeter is also essential to obtaining good results. A detailed gravity survey was successful in approximating the depth of unconsolidated deposits in the Orléans suburb of Ottawa. The density contrast between bedrock and unconsolidated deposits was sufficiently high to make gravity applicable there. Our results generally agree with the depth to bedrock information defined by well logs and seismic investigations and add information where the latter could not be made. The residual gravity anomaly map is a good indicator of the dramatic changes in bedrock topography where bedrock elevation changes from the surface to depth exceeding 100 m over the course of a few hundred meters. Gravity surveys can be used as a preliminary tool to obtain bedrock topography and delineate areas with thick unconsolidated deposits where a clear density contrast exists. Gravity could also help map areas where borehole and seismic information are lacking or are incomplete. The capacity of gravity to map bedrock topography is highly dependent on defining a good regional anomaly map. In the case of Orléans, this was accomplished using gravity measurements made directly on bedrock or on thin Quaternary sediments (T b 5 m). The data acquired in the 1950s and 1960s to delineate did not have the resolution (1500 to 2300 m spacing or more) and the precision (±1 mGal ) necessary to clearly delineate the local field that was needed in our study. In addition, most of those stations were not located where bedrock outcrops or is near the
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Fig. 10. Residual gravity anomaly from the grid of Fig. 3 at each point where depth is known from well logs and seismic profile (1569 points). In addition, the gravity anomaly corresponding to density contrasts of 0.25, 0.5 and 1.0 g/cm3 are shown.
surface. For these reasons, it was necessary to better define the regional gravity field at the local level with new gravity stations. Gravity modeling can only provide absolute depth information where other sources of information can constrain the thickness of
surficial deposits. It is well known that an infinite number of solutions can explain a potential field such as gravity. Consequently, when the objective is to define the thickness of surficial deposits, it is beneficial to include borehole and seismic information as a constraint on depth to
Fig. 11. This graph shows the residual anomaly at each gravity station as a function of thickness of overburden interpolated from a grid based on well logs and seismic profiles. In addition, the gravity anomaly corresponding to density contrasts of 0.25, 0.5 and 1.0 g/cm3 are shown.
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Fig. 12. This graph shows the residual anomaly at each gravity station where well or seismic profile information existed within 100 m of the station. In addition, the gravity anomaly corresponding to density contrasts of 0.25, 0.5 and 1.0 g/cm3 are shown.
bedrock and on the stratigraphy of the surficial deposits. In addition, good estimates of densities of rocks and unconsolidated materials (which can be quite variable) are essential. The Orléans survey was conducted over an area where the Paleozoic rock units had relatively similar densities. Hence, changes of density of bedrock were not a major concern. A gravity survey would produce the most accurate depth-to-bedrock information where bedrock topography varies gently with a slowly-varying regional gravity field. In conclusion, we propose that detailed gravity measurements be used as an inexpensive means of mapping the thickness of unconsolidated deposits in geological environments where there is a strong density contrast between these deposits and bedrock. Its simplicity makes one of the best tools in semi-urban and non-urban areas. As explained above, we prefer to use the term semi-urban, because the cores of urban areas may have underground infrastructures that would modify the natural gravity field, making the interpretation more difficult. To obtain meaningful results, four conditions must exist. Firstly, that the elevation of gravity stations be accurately measured (using differential GPS for example); secondly, that the regional gravity field be determined with precision; thirdly, that the local geology be simple enough to be realistically modeled with a two-layer geology. Finally, in the case where depth-to-bedrock estimates are required, realistic density values must be available together with good depth control from borehole and/or other information. 11. Data sources The gravity and magnetic data are archived on the Canadian Gravity and Magnetic Databases (Natural Resources Canada; http:// gdr.nrcan.gc.ca/gravity/index_e.php). Acknowledgments The authors thank their colleagues of NRCan: Carey Gagnon for the technical support of the gravimeter and data conversion, Dr James Hunter and Dr Jacques Liard for supporting this project and Drs Mark
Pilkington and Sue Pullan of NRCan for their reviews of a draft of this paper. Earth Sciences Sector contribution number ***.
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