First Break - Volume 4, Issue 9, 1986

Examination of the autocorrelation functions of seismic traces is widely used to obtain information to be used in the design of deconvolution operators. The length of gap to be used in predictive deconvolution, for example, may be determined from the time-lags at which zerocrossings occur in the autocorrelation functions (Peacock & Treitel, 1969). In order to determine by how much autocorrelation functions vary between traces, the autocorrelation may be appended to the seismic trace so that the trace and its autocorrelation function may be examined together. With the advent of the Fast Fourier Transform (FFT), frequency domain deconvolution procedures become practicable. In this domain, the log of the amplitude spectrum is used to design the amplitude and phase spectrum of the deconvolution operator. Hence it is useful to be able to examine the log spectra of traces in order to specify the frequency domain deconvolution parameters correctly. The BMR in-house seismic processing system (Brassil et al., 1986) made extensive use of frequency domain processing, and for this reason a facility for appending the log of the amplitude spectrum to seismic traces was developed. An appropriate name for this type of display is an F-X plot, as it shows the frequency content of the traces as a function of position. Such displays were first used in the optical processing of seismic data (Jackson, 1965) and some use was made of the displays for interpretation (Fitton & Dobrin, 1967), but this is believed to be the first application to digital data. During the use of this facility for deconvolution parameter estimation, it became clear that F-X plots had a much wider use for data quality control, and this paper aims to draw attention to some of these applications.

The magnetotelluric (MT) method has been greatly improved during the last ten years (Vozoff, 1972; Clarke et al., 1983), so it is now possible to get reliable and accurate results in any kind of survey, even in the presence of cultural noise. A few contractors now offer MT services for low-cost basin reconnaissance, structural surveys, and for geothermal and mineral prospecting. In spite of the range of different design options, some common characteristics can be sifted out from an analysis of available commercial MT systems. The most important of these common characteristics are reviewed in this article, and it is suggested that these should be regarded as 'standard' for any up-to-date MT system. The main differences between the various available acquisition and processing systems are described next, with a brief discussion of the advantages of each technical option. Finally, an automatic technique of timeseries 'pruning' the MT signal is described, which constitutes a pre-processing stage in the time domain.