Abstract

Gravity geophysics has useful application in mapping buried bedrock topography and in groundwater exploration in some specific geologic situations. One of these is the occurrence of groundwater in the sands and gravels of buried stream valleys on the bedrock surface, where there is an overburden such as glacial drift. We have evaluated the gravity method as an aid in glaciated terrain, to (a) locate local buried valleys for groundwater extraction, and (b) do reconnaissance surveying of the bedrock surface for a groundwater resource inventory. This has been done by modeling analysis and field research studies. For a gravity meter with sensitivity of .01 mgal, and with the standard corrections in field surveys, buried-valley anomalies greater than about 0.14 mgal can be resolved. This requires that elevations be accurate to + or -2 inches. For a drift/bedrock density contrast of 0.3 g/cm 3 , valley depths greater than 45 ft can be identified. Data-analysis techniques can be used to enhance the precision and applicability of the method, namely, (a) the use of theoretical diurnal earth-tidal curves for gravity correction, in lieu of the less accurate and more time-consuming empirical curves; (b) various least-squares surface fittings to improve interpretation and compensate for errors in surface elevations; and (c) conversion of gravity anomaly data to bedrock elevations. Field studies here illustrate the use of gravity in locating a localized buried stream valley and in mapping bedrock under glacial drift in a reconnaissance survey of over 50 sq mi. In the latter, although elevation control was not available to precision, a useful residual anomaly map resulted from subtracting a 3rd degree (regional) surface from a 15th degree (smoothed-elevation-error) surface. The resulting gravity residual was accurate to + or -0.15 mgal; the calculated relief on the bedrock surface is up to 250 ft, below about 200 to 300 ft of glacial drift.

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