Abstract

Borehole gravity surveys of oil and gas reservoirs are usually run at depths of thousands of feet, and such surveys are not likely to be significantly affected by the ordinary works of man. However, borehole gravity surveys for engineering applications and the evaluation of mineral deposits and aquifers may have maximum depths of only a few hundred feet and, therefore, are more likely to be affected by the gravity fields of man-made surface features such as basement excavations, quarries, gravel pits, sewage plants, road fills and cuts, storage piles, mine dumps, tank farms, etc.Examples of the terrain effect of a few common man-made surface features upon densities computed from near-surface borehole gravity data are shown in Figure 1. Such features can degrade the accuracy of a survey if not taken into account, since the precision of densities computed from borehole gravity measurements can approach 0.01 g/cm 3 (Schmoker, 1978).FIG. 1. Examples of the terrain effect of selected cultural features upon densities computed from subsurface gravity measurements. Data are from borehole gravity surveys in south Florida. The rectangular drilling pad measured 200 X 380 X 7 ft (61 X 116 X 2.1 m), with an estimated density of 1.9 g/cm 3 . The irregularly shaped water-filled excavation had an average depth of 29 ft (8.8 m), a surface area of 193 X 10 3 ft 2 (18 X 10 3 m 2 ), an estimated density contrast of -0.9 g/cm 3 , and was centered 325 ft (99 m) from the well. Two above-ground water tanks contained 113 X 10 3 ft 3 (3.2 X 10 3 m 3 ) of water centered 90 ft (27 m) from the well and 63 X 10 3 ft 3 (1.8 X 10 3 m 3 ) of water centered 140 ft (43 m) from the well. It is difficult to estimate the terrain effect of cultural features as a function of depth and hence determine the need for terrain corrections. It is also time consuming to compute precise terrain corrections for a cultural feature because accurate dimensions of the feature are often difficult to obtain and because its shape may not be well suited to the zones and compartments of conventional terrain-correction schemes. (A case in point is the 'water tanks' example of Figure 1 where the author's calculations were not only laborious, but proved to be unnecessary because all gravity stations were below the depth of significant effect.) A simple chart (Figure 2) has been developed for estimating the significance of the terrain effect of cultural features upon densities computed from borehole gravity data. By signaling the need for data corrections, it can help the user maximize survey accuracy while avoiding unnecessary computations.Figure 2 provides a qualitative estimate of the depth at which the terrain effect of a man-made feature is no longer significant. It is based on a spherical mass anomaly, which can be defined by a minimum of geometric parameters. The chart thus represents a trade-off of precision in favor of simplicity and is best suited to approximately equidimensional cultural features. Figure 2 shows depths below the center of a sphere of a given mass and horizontal distance from the wellbore at which the absolute value of the terrain effect of the sphere upon borehole-gravity density is 0.005 g/cm 3 . A terrain effect of this magnitude or greater assumes importance with respect to other experimental errors. Figure 3 is an enlargement of the upper left region of Figure 2, showing the detail needed for evaluating terrain effects of small cultural features.To use Figure 2 or 3, one estimates the horizontal distance to a cultural feature's center of mass and its mass excess or deficiency (the product of volume and density contrast). The maximum depth at which the absolute value of the terrain effect is 0.005 g/cm 3 is then located on the vertical axis of the chart. ThisFIG. 2. Depths below the center of a sphere of a given mass excess or deficiency, located a horizontal distance X from the borehole, at which the absolute value of the terrain effect of the sphere upon densities computed from borehole gravity measurements is 0.005 g/cm 3 . The example shows a feature with a mass contrast of 10 7 ft 3 .g/cm 3 centered 300 ft (91 m) from the wellhead. Its terrain effect upon measurements deeper than 495 ft (151 m) would be less than 0.005 g/cm 3 . depth represents an approximate cut-off below which the terrain effect of the cultural feature upon borehole-gravity densities can be neglected.Figures 2 and 3 are similar in concept to the accuracy criteria for terrain elevations developed by Hearst et al (1980), but here the criteria are presented in a format better suited to the geometry of cultural features. The figures show that the influence of terrain tends to increase as depth in the well and horizontal distance to the anomalous mass decrease. However, along each curve of constant horizontal distance (except x = 0), there is a discontinuity at which the density calculated from borehole gravity measurements is insensitive to terrain. This discontinuity occurs at that depth where the gravitational attraction of the terrain element reaches an absolute maximum and the vertical derivative of the gravitational attraction, to which calculated borehole-gravity density is proportional, is zero.Approximate depths of significant effect for tabular man-made features can also be obtained from Figures 2 and 3. The relationship between the true depth of significant effect for a tabular body and the approximate depth given by Figure 2 or 3 is not a simple function, but results from the spherical model (Figure 2 or 3) are not likely to be significantly misleading as long as the ratio of height to width of the tabular body is greater than about 0.4. Elongate cultural features pose further difficulties because their orientation is an additional variable to be considered, but information of limited value can be obtained from Figure 2 or 3 by subdividing the elongate body into more equidimensional units.FIG. 3. An enlargement of the upper left region of Figure 2 showing the detail required for evaluating the terrain effects of small cultural features.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not currently have access to this article.