Gravity Anomaly Spectrum Analysis Method and Its Application

Objectives Spectral analysis is to analyze and study various spectra of potential anomalies with wave-number as the independent variable, to solve the problems of anomaly conversion, filtering and forward and inverse analysis in practical work. The spectral characteristics of the potential, which include the spatial distribution, lithology and tectonic characteristics of the subsurface stratum, help the interpretation and analysis of geophysical data and provide information for further understanding of the study area. The amplitude spectrum of the potential anomaly spectrum is directly related to the burial depth, width of the subsurface geological body; the phase spectrum reflects the information of the horizontal position and dip angle of the subsurface geological body. However, in previous studies, most of the researchers' work had been focused on making use of the characteristics of fast calculation of the bitfield data in the frequency domain, doing a lot of forward work as well as applying the relationship between the spectrum and the parameters of the geologic body for inversion, but the analysis of the spectral characteristics of different geologic bodies and the specific effects of the geologic body parameters on the spectrum have been less studied. On the other hand, there are few studies on the joint inversion using different spectra in the frequency domain inversion.

Methods In order to better carry out the study of gravity anomaly in the frequency domain, using the characteristics of simple, solvable and easy to program and fast computation of the expression for the positive inversion of gravity anomaly in the frequency domain, the spectral curves of various models are calculated, and the characteristics of the spectral curves, and detailed effects of model parameter changes on the spectral curves are analyzed too. It also includes how to interpret the amplitude spectral inversion and phase spectral inversion, and proposes a joint inversion method in the frequency domain to accurately invert the geometric parameters of the models.

Results The gravitational anomaly spectrum of a horizontal cylinder, which is of the periodic fluctuation type with monotonically decreasing amplitude spectrum and its phase spectrum is a straight line with a negative slope; the gravity anomaly spectrum of the tilted thin plate is periodically fluctuating, with monotonically decreasing amplitude spectrum and fluctuating phase spectrum; the gravity anomaly spectrum of the tilted thick plate is periodically fluctuating with fluctuating amplitude spectrum and fluctuating and progressive phase spectrum. By analyzing the influence of different parameter changes of the thick plate on the model spectrum, the trend of the subsurface geological body is judged according to the spectrum changes. The amplitude spectrum can determine the burial depth of the model, and the change of its first zero value point position is determined in the horizontal direction of the model, and the phase spectrum is directly related to the model parameters.

Conclusions The joint inversion in frequency domain is an effective method to combine the advantages of amplitude spectrum inversion and phase spectrum inversion to accurately calculate the geometric parameters of the model. The method was applied to the inversion of the target stratigraphy in the Wuqing Sag, and the precise depth and width of the target substratum in the two measured sections were obtained, providing a basis for further seismic studies in the area.