Preparation of Residual Gravity Maps for the Southern Cascade Mountains, Washington Using Fourier Analysis

Abstract

This report represents a continuation of gravity work in the Cascade Mountains of Washington supported by the Division of Geology and Earth Resources since 1974. The purpose of this research has been collection of baseline gravity data for use in geothermal resource evaluation. Results of the Division's gravity studies to date are given in Danes and Phillips (1983a, 1983b). One of the problems encountered when analyzing gravity data is distinguishing between those parts of the data that represent geologic structures of interest, and those that do not. In many cases, the features of interest are relatively small, near-surface features, such as those sought in mineral, petroleum, or geothermal exploration. Gravity anomalies caused by such structures may be distorted or masked by anomalies caused by larger, deeper geologic structures. Gravity anomalies caused by relatively shallow, small geologic structures are termed residual anomalies. Those due to broad, deep-seated features can be described as regional anomalies. The purpose of this report is to describe a Fourier analysis method for separating residual and regional gravity anomalies from a complete Bouguer gravity anomaly field. The technique has been applied to gravity data from the Southern Cascade Mountains, Washington. Residual gravity anomaly maps at a scale of 1:250,000 are presented for various regional wavelength filters, and a power spectrum of the frequency components in the South Cascade gravity data is displayed. No attempt is made to interpret the results of this study in terms of geologic structures.