Atlantic Richfield Company

David A Chapin

Charles C Mosher

A method of obtaining gravity gradient well logs over selected baselines, using a well log tool having first and second gravity meters separated by a fixed distance, referred to as the baseline, is disclosed. Gravity gradient measurements are obtained in the wellbore by stepping the tool along the wellbore, where readings at adjacent depths are obtained by setting one of the gravity meters at the same depth at which the other gravity meter is or was located in another reading; alternatively, the well log tool may include a single gravity meter, with readings at adjacent depths made by moving the tool along the wellbore by the baseline. The user or computer system selects a virtual baseline, corresponding to a multiple of the gravity gradient measurements. The gravity gradient is then obtained by summing the individual gravity gradient measurements and dividing by the number of summed measurements. The virtual baseline provides the ability to increase the radius of investigation from that which is provided by the tool itself, and permits selection of varying baselines in deriving the gradient log.

Gravity gradiometry in a wellbore using a virtual long-baseline

A method of analyzing geological survey data of the potential field measurement type is disclosed. According to the disclosed method, discrete measurement values of the potential field, either the gravitational or magnetic field, are retrieved from memory, corresponding to either a one-dimensional or two-dimensional survey region of the earth. The discrete values are first preconditioned by the application of a noise-reduction digital filter, preferably of the Weiner type. The disclosed method then performs decomposition of the filtered discrete values using the Daubechies scale function and wavelet function of length 2. According to the preferred method, the second level low-frequency component of the first level high-frequency component is generated, and expanded by a factor of four to correspond to the range of the input discrete values. Interpolation is performed to fill in values between adjacent coefficients of this component. As a result of the disclosed method, resolution of geological structures is much improved, enabling detection and discrimination of complex and interfering structures at increased depths.

Method and apparatus for identifying geological structures using wavelet

A method for producing isostatically corrected gravity displays using available gravity data sets. The values of crustal density, density contrast between lower crust and mantle, and crustal thickness at sea level required for the Airy-Heiskanen equation are determined by analysis of the data set. Crustal density is determined by fractal analysis. Density contrast is determined by cross plotting Bouguer anomaly values versus elevation. Crustal thickness is determined by plotting power spectra of free-air gravity grids. The values are then used in the Airy-Heiskanen equation to provide the value of depth to crust-mantle boundary at each data point location in order to make accurate isostatic corrections and to produce accurate isostatically corrected displays.

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Plano, TX, United States of America

David A Chapin