GB1359719A - Mapping subsurface structures in the earth - Google Patents

Abstract

1359719 Seismic exploration UNITED GEOPHYSICAL CORP 27 July 1971 [27 July 1970] 35319/71 Heading H4D A digital image of a subsurface region of the earth is built up in memory cells of a computer by deriving digital representations of the amplitude of seismic wavelets from seismic record traces; allocating the cells to respective elemental parts of the image; and storing replicas of said amplitude representations in cells corresponding to isochronic image points (i.e. those image points defining the locus of various possible subsurface points which, for the same seismic source-receiver geometry as was used in obtaining the wavelet record, would return a seismic wave to the receiver with the two-way travel time associated with that wavelet) whereby the replicas accumulate algebraically in the cells and establish the image. Field procedure.-The records to be processed may be obtained by a roll-along or split spread (common reflection point) continuous profiling method, with linear or areal source-receiver arrays and automatic gain control of receiver amplifiers, and may be in analogue or multiplexed digital form. Twenty or more records preferably are made per 1000 ft. along a survey line. A marine survey method is described. Signal processing.-Stacking and normal movement corrections prior to the above storage procedure are optional. The records are reproduced as, or converted to, digital signals for input to the computer. For each source-receiver geometry and seismic travel time the isochronic image points are computed and the replicas are transferred to the memory cells corresponding to those points. Algebraic accumulation of data in the cells enhances the signal-to-noise ratio. For imaging a twodimensional vertical profile of the earth as shown in Fig. 13, the memory cells are notionally allocated to a two-dimensional array of image points C(IJ) and the isochronic points for a source-receiver combination S, R form sets which lie on respective curves DL1, DL2 &c. respectively associated with the various travel times. For example assuming a constant seismic velocity, these curves will be ellipses with S and R as foci. Replicas of successive portions of a wavelet may be stored in a subset of cells, e.g. as on a vertical line VL. In practice the image point array may be three dimensional. Visual images in two or three dimensions are displayed by reproducing the cumulative data stored in the cells. The display may be a plot, e.g. photographic, of a selected sectional plane through the image space, or a three-dimensional display in which subsurface structures are represented by semi-transparent objects.