The Wavelet-based multi-scale analysis is a powerful tool for multiple decomposition of potential field data. Discrete wavelet transform generates low-order wavelet details that keep invariant scales with the total order number; while the change in the total order number can only increase wavelet details of high orders and the degree of smoothness of the final approximation. Thus, one can combine wavelet details based on regional geology to decompose gravity anomalies, producing anomalies correlated to certain geological sources. Applying the traditional frequency spectrum analysis techniques, one can estimate the everage depth of sources for every wavelet detail, and obtain the statistic correlation between the decomposed wavelet details and the source depth. A typical example for triple decomposition of local anomalies is given by using gravity data obtained from the Chinese continental scientific drilling site. An example for triple decomposition of gravity data from an Australian continental margin area is also presented.

Discrete Wavelet Transform for Multiple Decomposition of Gravity Anomalies

Based on the power spectra of gravity anomalies in Tarim Basin, the anomalies can be decomposed to the following three components: a sub-anomaly formed mainly by the basin crystallized basement, a sub-anomaly formed mainly by deep sedimentary layers, and that by shallow sedimentary layers A special wavelet transform analysis scheme and a density inversion method are designed and applied to the decomposition and inversion of gravity sub-anomalies, which are correlated with regional geology and drilling data The results indicate that the deep and the shallow sub-anomalies show some relations with ancient fluid active zones The negative density disturbances inversed from the shallow sub-anomaly are mainly caused by Mesozoic fluid active zones, whereas the negative density disturbances form the deep sub-anomalies are mainly correlated with Paleozoic fluid active zones As the ancient fluid movement was good for formation of oil/gas fields, the multi-scale inversion methods for locating the ancient fluid movement zones seem to be helpful and valuable to oil/gas exploration

Multi-scale inversion of density structure from gravity anomalies in Tarim Basin

Interpretation of magnetic anomalies of dikes using Fourier transforms

The problem of inversion of potential field data is a challenging one because of the difficulty in obtaining a unique solution. This paper identifies various types of nonuniqueness and argues that it is neither possible nor necessary to remove all categories of nonuniqueness. Some types of nonuniqueness are due to human limitations and choice and these would always persist.

Inversion of potential field data

The elegance and ease with which the Fourier transformation method can be employed in the interpretation of potential field geophysical data (Dean, 1958; Gudmundsson, 1966) has resulted in the formulation of a number of frequency‐domain approaches for interpreting the magnetic effects of specific source geometries, e.g., Bhattacharyya (1966), Sengupta (1974), Sengupta and Das (1975), Bhattacharyya and Leu (1975, 1977), etc. In these techniques, the parameters of interest are estimated from the characteristics of the amplitude and phase spectra, a process that is often beset with several problems which adversely affect the estimation of parameters. These problems arise partly from the ill‐defined nature of characteristic points in the spectra, either due to spectral noise or due to inadequate digitization spacing, and partly from the fact that the errors in the estimation of certain parameters which are used for subsequent estimation of the other parameters usually cause added spectral distortions. Rao et ...

Frequency domain least-squares inversion of thick dike magnetic anomalies using Marquardt algorithm

The Fourier transform of the gravity field due to a finite dipping dike is derived and its real and imaginary components are separated. Analysis of these two functions in a certain high-frequency range yields simple relations that can be used to estimate the unknown parameters of the dike. The theoretical considerations are tested on synthetic data after performing the discrete Fourier transform (DFT), and the validity of the method of interpretation is established from a comparison of the actual and estimated parameters.

R. Nagendra

Papers

Interpretation of gravity anomalies due to finite inclined dikes using Fourier transformation