Gravity interpretation of bedrock topography: the case of the Oak Ridges Moraine, southern Ontario, Canada

Abstract

Gravity data collected by the Geological Survey of Canada (GSC) over the Oak Ridges Moraine, southern Ontario, Canada, are processed and interpreted to obtain images of bedrock topography. The moraine has been the subject of a regional hydrogeological study conducted by the GSC to determine the geological framework and to study groundwater flow in the Greater Toronto area.

Upward continuation is used to extract a residual gravity anomaly approximating the gravity signal of the bedrock topography. The residual gravity data is then inverted using two techniques. The first inverts for the topography of a slab of infinite lateral extent having a flat bottom and a uniform density contrast with the material above. The calculation is done in the Fourier domain. The second technique is a 3D inversion for the density distribution using an algorithm implementing compactness and smoothness as global constraints.

Forward calculations are done on the borehole model of bedrock topography to assess the suitability of the residual gravity anomaly. Profiles on the order of 25 km in length are modelled using a 2.5D modelling program. The results indicate that the quality of interpretation of the gravity signal is highly subject to the choice of residual anomaly, the noise caused by the heterogeneity and surface topography of the region, and the degree of accuracy in the data.

Inverted bedrock topographies obtained from both inversion techniques are similar in structure. The results of inversions for density distribution provide more realistic bedrock relief. The gravity interpretation indicates the presence of 10–30-km wide channels trending northwest in the western part of the moraine and northeast east of the Laurentian Channel.

Introduction

Since April 1993, the Oak Ridges Moraine area has been the subject of a regional hydrogeological study conducted by the Geological Survey of Canada (GSC) (Sharpe et al., 1996). An interdisciplinary approach was adopted to determine the geological framework and to study groundwater flow in the Greater Toronto area Sharpe et al., 1996, Russell et al., 1996, Pullan et al., 1994. In particular, the aim of the study is to map major Quaternary aquifers and aquitards in the region and to understand their influence on groundwater flow. Various factors may control groundwater flow within the moraine and across the moraine area. One factor, which may influence groundwater flow across the moraine on a regional scale, is bedrock topography. Coarse sediments deposited in deep channels in the bedrock form important aquifers that are often exploited for municipal water supply. The largest and deepest channel in the Oak Ridges Moraine area is the Laurentian Channel (Fig. 1(b)), which extends from Georgian Bay (Fig. 2(a)) to Lake Ontario and is estimated to be 30-km wide Spencer, 1881, Spencer, 1907, Brennand et al., 1998. An economical method of obtaining a regional image of bedrock topography is sought. The gravity method in applied geophysics is the most promising means to this end.

Traditionally, mapping bedrock channels has not been a customary application of the gravity method. Some isolated cases exist where the gravity method was used to map buried valleys Thomsen et al., 1999, Lennox and Carlson, 1967, Hall and Hajnal, 1962. Leão et al. (1996) developed an algorithm to constrain inverted basement relief using a few known depths. The ease of data acquisition on a regional scale and 2D coverage in an urban setting have been its main advantage over other geophysical methods employed by the GSC in its investigation of the moraine Pullan et al., 1994, Todd and Lewis, 1993, Todd et al., 1993, Pilon et al., 1994, Pugin et al., 1999. Seismic profiles in the region are at most 8 km in length. With its high acoustic impedance, the dense layer of Newmarket till contained in the moraine is a limitation in the seismic method in imaging the bedrock/sediment contact below. Electromagnetic soundings were limited by the low conductivities of the limestone bedrock north and east of the moraine (Pullan et al., 1994). In western areas, the shale/sediment contact was poorly resolved.

The Oak Ridges Moraine (ORM) is situated in southern Ontario, Canada, north of Toronto and east of the Niagara Escarpment (Fig. 1). It covers an area roughly 160×20 km and is composed of discontinuous layers of glaciogenic Pleistocene sediments Sharpe et al., 1994, Sharpe et al., 1996. The sediments include clays, tills, sands and gravel. The moraine is underlain by Ordovician shale and limestone bedrock (Johnson et al., 1992). Beneath Ordovician rocks are Precambrian rocks of the Grenville Province. A literature review of ORM-type sediments and bedrock provided estimates of moraine and bedrock densities Berman et al., 1942, Jakosky, 1950, Daly et al., 1966. The average sediment density (saturated) is estimated to be within a range of 1.8–2.0 g/cm³. Average bedrock density (dry) is estimated to be between 2.5 and 2.6 g/cm³. The sediment/bedrock density contrast is, thus, estimated to be between 0.5 and 0.8 g/cm³.

Over 25 000 boreholes reaching bedrock exist in the area (Fig. 2(a)). The GSC used the borehole data to construct a grid of sediment thickness, which was subtracted from a grid of surface topography to produce a map of bedrock topography (Brennand et al., 1998). The borehole data indicate sediment thickness ranges from 0 to 250 m, averaging 57 m (Russell et al., 1998). Surface topography ranges from 32 m above sea level (m asl) to 520 m asl (average 236 m asl) on the Niagara Escarpment. Bedrock topography, as described by the borehole data, ranges from 16 m below sea level (m bsl) to 495 m asl (average 177 m asl). Little borehole control on the depth to bedrock exists in the area of the moraine itself (kriged area in Fig. 2) where sediment thickness reaches greater values. As a result, there is poor horizontal resolution of bedrock topography. The gravity data acquired over the moraine would in theory improve the definition of bedrock channels in those areas where few boreholes exist.

Fig. 1(a) shows the gravity survey area. The distribution of the gravity stations is shown in Fig. 1(b). Four gravity surveys were completed between 1994 and 1996 covering the moraine and the Laurentian Channel Belisle, 1995, Gill, 1996, Gill, 1997, Jobin, 1997. The gravity readings were initially taken at 500-m intervals in the southwestern part of the moraine. Subsequently, the sampling interval was increased to 1 km. Gravity readings were taken with Scintrex CG-3 Autograv and Lacoste and Romberg G790 gravimeters. Station altitude was measured with an altimeter and psychrometer. In the final survey covering the Laurentian Channel west of Lake Simcoe (Fig. 1(b)), a GPS Sokkia system was used for coordinate (including altitude) measurements. Together with the national regional database, a total of 5681 observations were made available by the GSC for interpretation. The maximum errors on the measurements of horizontal coordinates, altitude, and gravity average 40.12 m, 2.04 m, and 0.066 mGal, respectively. The average error in altitude is about 1 m.