Effects of terrain on borehole gravity data

Abstract

The effect of terrain on gravity measurements in a borehole and on formation density derived from borehole gravity data is studied as a function of depth in the well, terrain elevation, terrain inclination, and radial distance to the terrain feature. The vertical attraction of gravity [Formula: see text] in a borehole resulting from a terrain element is small at the surface and reaches an absolute maximum at a depth of about one and one‐half times the radial distance to the terrain element, then decreases at greater depths. The effect of terrain on calculated formation density is proportional to the vertical derivative of [Formula: see text] and is maximum at the surface, passes through zero where |[Formula: see text]| is greatest, and reaches a second extremum of opposite sign to the first and of much lower magnitude. Accuracy criteria for borehole‐gravity terrain corrections show that elevation accuracy requirements are most stringent for a combination of nearby terrain features and near‐surface gravity stations. Sensitivity to terrain inclination is also greatest for this combination. The measurement of the free‐air gradient of gravity, commonly made’slightly above the ground surface, is extremely sensitive to topographic irregularities within about 300m of the measurement point. The effect of terrain features 21.9 to 166.7 km from the well [Hammer’s (1939) zone M through Hayford‐Bowie’s (1912) zone O] on calculated formation density is nearly constant with depth. At these distances, the terrain correction will be equivalent to a dc shift of about [Formula: see text] of average elevation above or below the correction datum. The effect of topography beyond 166.7 km is not likely to exceed [Formula: see text].

Borehole Gravity Measurements in the Salton Sea Scientific Drilling Project Well State 2–14

“…We assume that the high-density zone is a cylinder centered in the borehole and embedded in a material with the density of the zones above it. Using the formula for the density effect of a cylinder [Hearst et al, 1980], we find that to produce a 90% response, the cylinder radius must be greater than 5 times the thickness, or several hundred meters in this case. Some zones may be much smaller in extent.…”

Borehole gravimetry reviews

“…LaFehr and others (1979) stated that 20 percent of EDCON's BHGM business is for this purpose. However, few case histories in sedimentary rocks of these types of surveys have been published (Bradley, 1976;Clark and Hearst, 1983;Hearst and McKague, 1976;LaFehr and Dean, 1983;Schmoker, 1977;and Schmoker, 1980). There are several underground gravity applications in mining districts that contain anomalous lateral mass distributions; see for example Sumner and Schnepfe (1966).…”

“…Most BHGM studies have been related to petroleum exploration and production (Bradley, 1976;Parley, 1971;and McCulloh and others, 1968), and some surveys have been related to mining studies (Robbins, 1979;. Other topic applications are (a) ground-water studies (Head and Kososki, 1979;Robbins, 1986;and Tucci and others, 1983), (b) radioactive waste-disposal site studies (LaFehr and Dean, 1983;Robbins, 1986;Robbins and others, in press;and Schmoker, 1980), and (c) other research studies relating to structurally complex areas (Hearst and McKague, 1976;Schmoker, 1977). Chapter C contains references to all BHGM studies and case histories presently in the literature (1986).…”

Borehole gravity surveys; theory, mechanics, and nature of measurements

“…Practical implications of their work are that the effect of terrain features from 21.9 to 166.7 km from the well on calculated formation density is nearly constant with depth (like a dc shift) and that corrections for topography beyond 166.7 km are not likely to shift calculated densities by more than 0.01 g/cm. Schmoker (1980) presents a simple method to estimate if the gravitational disturbances due to cultural features, such as basement excavations, gravel pits, mine dumps, etc., will significantly affect densities calculated from the borehole gravity measurements. Such disturbances are rarely significant below well depths of saveral hundred feet and often not significant below depths of several tens of feet (Figure 2-15).…”