The Successful Use of Microgravity Profiling to Delineate Faults in Buried Bedrock Valleys

Abstract

Site selection for ground‐water production wells was made easier using a microgravity survey technique. The ground‐water regime beneath the Carleton University campus is dominated by fracture flow associated with major faults. These faults are located within buried bedrock valleys where preferential erosion of the bedrock has occurred. A series of faults is inferred from bedrock topography and hydrogeological testing; however, the precise orientation of the faults is to a large extent uncertain. The bedrock surface is masked by fluvial, glacial, and glaciomarine overburden of variable thickness. High precision, small‐scale microgravity profiling was carried out to identify the bedrock valleys. Gravity anomalies of up to 0.05 mgal correspond to valley depths ranging from 5 to 15 meters. The density contrast between overburden and limestone bedrock is estimated as 0.3 g/cm3. Forward modeling using a polygon technique suggests that the observed, corrected gravity profile is a smoothed reflection of the bedrock surface at depth. Based on the locations of the bedrock valleys, several faults were identified and four test wells were drilled as part of the development of Phase 2 of the Carleton Groundwater Energy Project. Three of these wells produced high yields, exceeding 60 I/sec, when converted to 12 inch diameter production wells. The fourth well also encountered a bedrock valley but with a slightly lower yield of water. The favorable drilling results have shown that small‐scale microgravity profiling, in an area that is largely unsuitable for the detection of subsurface features using other geophysical techniques, is an effective method for imaging the bedrock surface. The positions of the faults, which are known to exist on the basis of stratigraphy, geophysical well log correlations, bedrock topography, and calcite‐filled fractures exposed in outcrop or core, have been more accurately defined.

Introduction to geophysical investigations in hard rock

“…The gravity method involves measurement of minor variations in gravitational attraction at a point caused by variations in near surface rock densities and other factors. Some of the classical gravity studies are by West and Sumner (1972), Ibrahim and Hinze (1972), Carmichael and Henry (1977), Stewart (1980), Hansen (1984, Ghatge and Hall (1989) and Allen and Michel (1996). Being a potential field measurement it has inherent ambiguities and may require support from other passive and/or active methods for confirmation.…”