GRAVIMETRIC DETERMINATION OF ANOMALIES LATERAL TO BOREHOLES

• US 3,747,403 A
• Filed: 07/27/1970
• Issued: 07/24/1973
• Est. Priority Date: 07/27/1970
• Status: Expired due to Term

• Current Assignee: Sulhi H. Yungul
• Original Assignee: Sulhi H. Yungul
• Inventors: Yungul, Sulhi H.
• Primary Examiner(s): Myracle, Jerry W.
• Application Number:
  US05/059,783
• Time in Patent Office: 1,093 Days
• Field of Search: 73/151,152,382
• US Class Current: 73/152.02
• CPC Class Codes: G01V 7/00 Measuring gravitational fields or waves; Gravimetric prospecting or detecting

Abstract

A portable gravimeter is traversed over a selected interval in an exploratory borehole. Formation density data is obtained for the same selected interval. This gravity and density information is then combined with the gravity value at the surface of the borehole when no anomaly is present, the '"'"''"'"''"'"''"'"'free air'"'"''"'"''"'"''"'"' correction and the Bouguer correction to obtain a measured perturbation effect. The measured perturbation effect represents the net perturbation in the gravity values measured within the borehole due to the presence of a gravimetric anomaly lying lateral to the borehole. The measured perturbation effect is subjected to high pass filtering to render it solely a function of the lateral distance from the borehole to the adjacent flank of the gravimetric anomaly. A trend is subtracted from the filtered measured perturbation effect. Anomaly models are then employed to simulate the gravimetric anomaly which produces the measured perturbation effect. A calculated perturbation effect is generated for the anomaly model. This calculated perturbation effect is high pass filtered and is subjected to trend subtraction. Finally, the filtered measured perturbation effect is compared with the filtered calculated perturbation effect and differences are noted. The parameters of the anomaly model are varied to reduce these differences and further comparisons are made. The process of varying parameters and making comparisons is repeated with the aid of iterative computer programs employing regression techniques such as the method of least squares. When the differences between the filtered measured perturbation effect and the filtered calculated perturbation effect are smaller than some preassigned arbitrary number the anomaly model represents the actual anomaly. The distances between the anomaly model and the simulated borehole are the actual distances between the gravimetric anomaly and the exploratory borehole.

Claims

1. The method of gravimetrically prospecting for geologically anomalous bodies lateral to an exploratory borehole and for determining the lateral distance thereto, comprising the steps of:

a. measuring the density of the formations surrounding said borehole by running a gamma-gamma density logging device within said wellbore over the interval under study;

b. traversing a borehole gravimeter within said wellbore over said interval under study to obtain measurements of the acceleration due to gravity as a function of depth;

c. determining the net gravitational perturbation effect of a geologically anomalous body in accordance with the following relationship;

2. The method of gravimetrically prospecting for geologically anomalous bodies lateral to an exploratory borehole and for determining their lateral distances from said borehole over a given depth interval, comprising the steps of:

a. measuring the densities of the geologic formations surrounding said borehole at a plurality of successive points along said borehole over said given depth interval;

b. computing a space-derivative of the gravitational potential that would exist over said given interval of said borehole if said mEasured densities represented geologic formations that extended laterally away from said borehole to infinity, with no interruptions by lateral anomalous bodies, the first order space-derivative of said gravitational potential being represented by the following equation, with higher order space-derivatives being derivatives of said equation;

3. The method of claim 2, wherein said gravity-responsive instrument is a borehole gravimeter which measures the acceleration due to gravity.

4. The method of claim 2, wherein said gravity-responsive instrument measures a space-derivative of gravitational acceleration thereof.

5. The method of claim 2, wherein the densities of said geologic formation are measured in core samples removed from said formations.

6. The method of claim 2, wherein the densities of said formation are measured by a density logging device traversed through said borehole.

7. The method of gravimetrically prospecting for geologically anomalous bodies lateral to an exploratory borehole and for determining their lateral distances from said borehole over a given depth interval comprising the steps of:

a. measuring the densities of the geologic formations surrounding said borehole at a plurality of successive points over said given depth interval;

b. traversing a borehole gravimeter within said borehole over said given depth interval to measure the acceleration due to gravity as a function of depth;

c. determining the net gravitational perturbation effect of a geologically anomalous body in accordance with the following relationship;

8. The method of claim 7 wherein said gravitational acceleration derivative of said net perturbation effect is directly measured as a function of depth over said given depth interval by means of a gravity gradiometer.