GB1175700A - Digital Reflection Searching and Section Plotting - Google Patents

Abstract

1,175,700. Seismic prospecting. MOBIL OIL CORP. 23 Dec., 1966, No. 57740/66. Heading H4D. A migrated seismic depth section is derived by converting data samples representing the travel time of seismic energy reflected from a region of a subsurface interface to first and second physical representations of the reflection occurrence time and attitude of the interface, respectively, deriving from these representations values indicative of the actual depth and horizontal location of the region, and plotting the interface conformally with the actual depth, horizontal location and attitude. In an embodiment for processing a multi-trace time section recorded using a geophone spread and a plurality of shot-points and corrected for static and normal movement errors, the traces are first adjusted in amplitude so that they have equal RMS energy (thereby obviating amplitude variations due to instrument sensitivity differences or distance from the shotpoint). A selected (reference) trace is then compared with a threshold amplitude and the occurrence times Ti of reflections exceeding this threshold are stored as being indicative of primary reflections. Next, at each stored occurrence time the amplitudes of traces adjacent to the reference trace are sampled and summed along each of several possible "dip" lines and the attitude of the line giving the greatest sum is stored as indicative of the dip Di of the associated interface. For each value of Ti the mean amplitude of the samples giving rise to the said greatest sum is compared with a second threshold and if greater than the threshold (suggesting a primary reflection) is stored as indicative of the interface reflection coefficient. From the stored Ti and Di values and from seismic velocity data for the subsurface formations the actual vertical depth and horizontal locations of the reflecting regions relative to the shot points are computed. The reflecting interfaces are then plotted as a depth section by an automatic plotter supplied with these computed values. The presence of multiple reflections can be determined by obtaining all possible sums of the stored Ti values, all possible sums of the stored Di values and comparing these in pairs with pairs of the stored Ti and Di values. Correspondence between a pair of sums and a stored Ti, Di pair indicates a multiple reflection, which can be rejected and plotted in a separate depth section. The static error corrections for the traces may be obtained by considering the occurrence times of a given reflection in a group of adjacent traces as e time series wherein the errors are represented by high frequency components, filtering out these components to obtain a smoothed series, and applying the differences between the original series and the smoothed series as time corrections to the traces. To determine whether reflections of energy from two adjacent shot points are from the same subsurface interface, Ti, Di values of a trace are compared with predicted values computed from the Ti, Di values of an adjacent trace. If the comparison indicates correspondence the reflection is plotted as a continuous line on the depth section. Reflections which are not predictable may be plotted as a separate depth section and a composite of the two sections may be made. In a modification of the method of selecting reflections in the traces, summations are made along various possible "dip" lines at each of a number of sampling intervals along the traces, rather than only where there is an indicated reflection on the reference trace, thereby ensuring reflections will be detected even if they are of low amplitude on the reference trace. In a further modification, where the original seismic time section has been marked with continuous lines indicating reflecting interfaces, a photo-electric curve follower or a light pen and cathode-ray tube display of the section may be used to obtain incremental co-ordinate values of reflection time Ti and horizontal displacement along these lines. From these values and from seismic velocity data the dip Di of the interfaces is computed. The Ti and Di values are then processed as before to give the depth section.