Residual Gravity Map Research Articles

Geophysical investigation of the structural framework of the Newark-Gettysburg Triassic basin, Pennsylvania

A residual gravity map of the Newark-Gettysburg Triassic basin has been constructed using approximately 5,000 gravity stations that are nominally 1 to 5 km apart. In a general way, the map shows gravity lows over Triassic sedimentary rocks and gravity highs over Triassic diabase bodies and pre-Triassic rocks that flank the basin. Steep gravity gradients within the basin, but near the margins, are interpreted to be due to vertical offset on faults within the basin. The presence of these presumed faults along parts of the southern margin indicates that the basin is not a simple half-graben, but is internally faulted, with irregular sediment thicknesses.

CHART TO CHECK ELEVATION FACTOR EFFECTS ON GRAVITY ANOMALIES

The effect of varying surface densities on gravity anomalies is a more common problem in areas of topographical relief than is generally recognized. There is an unjustified tendency to assume that gravity maps are unique and final, even though one basic assumption (density of surface rocks) is inherent in all gravity maps. The use of incorrect elevation factors will produce gravity anomalies over any topographic feature. Both positive and negative gravity anomalies can be produced by either a topographic hill or valley depending on the degree of error in the elevation factor. These “elevation factor anomalies” are especially troublesome on residual gravity maps. The interpretation of gravity data should always include an analysis of the elevation factor effect as well as a study of the surface geology.

GRAVITY CASE HISTORY: DAWN NO. 156 POOL, ONTARIO

The Dawn No. 156 pool in southwestern Ontario was discovered by the Union Gas Company of Canada Limited, in January, 1952, by drilling on a gravity maximum. The drilling supported the original interpretation that this maximum was produced by a Silurian reef. Initial production of 6.5 million cubic feet per day was obtained from the upper fourteen feet of the Guelph in highly porous reef dolomite at a depth from 1,720 to 1,734 feet below the surface. The geology of the area is discussed as a background for the gravity interpretation. Bouguer and residual gravity maps of the original survey are presented.