Data processing revisited in electrical and electromagnetic survey data using digital filters.

Abstract

Data processing is one of the essential techniques to obtain accurate responses of the earth. The Fast Fourier Transform (FFT) has been widely used in the processing of magnetotelluric and other electromagnetic data, and gives us electromagnetic response functions when the signal-to-noise ratio is high or if the length of time series is long enough. However, these conditions may fail from time to time depending on the environmental conditions of exploration. We now focus on the data processing techniques using not the FFT but digital filters that have been “proven” effective to process seismic data. Such data processing techniques have been rarely applied to electromagnetic data. In this study, we apply a digital filter called “pole on pedestal” that amplifies or extracts signals at a specific frequency with less distortion of the Fourier spectrum than those obtained by the FFT. We first elucidate how effectively the application of digital filters improves the quality of electrical imaging using “pole on pedestal.” Next, we then apply this filter to marine electrical and electromagnetic survey data to demonstrate the effectiveness of the method. Real marine data-sets acquired by a deep-towed DC resistivity survey system and ocean-bottom instruments are used. It is obviously shown that the pole-on-pedestal filter improves the quality of the response functions and the estimated sub-seafloor resistivity structure.