Patent GB-1360282-A (GB)
Title: Method for locating impedance discontinuities in a wave propagating  medium
IPC: G01V1/28
Filed: 1971-07-06
Abstract: 1360282 Locating impedance discontinuities in a medium TEXAS INSTRUMENTS Inc 6 July 1971 [6 July 1970 (4)] 31621/71 Heading H4D A data image, representing impedance discontinuities in a wave propagating medium, is obtained in an automatic data processing machine from digital input records of refracted, reflected or diffracted waves and has a plurality of co-ordinate points, each such point having a composite amplitude value derived by computing the wave travel time from a source point to a receiver via a potential discontinuity point represented by the co-ordinate point, weighting amplitude values of the respective input records which values occur at the respective computed travel times, and combining the weighted values. The invention is described in detail as applied to the digital processing of seismic traces obtained by using several source points Si and spaced receiver locations Ri in, e.g. a conventional roll-along or common depth point surveying method, although it is stated that the invention is applicable to mapping acoustically in three dimensions any solid, liquid or gaseous medium (e.g. for flaw detection) and that the wave-energy may be electro-magnetic for investigating outer space. The seismic source is an explosive, an hydraulic thumper, a gas exploder or a dropping weight and the traces are preferably phase and amplitude recordings. Data processed using the invention is displayed as a two-dimensional (2-D) cross-section of the medium or as a three-dimensional (3-D) image by stereo pairs, holograms or 3-D drawings. In the Fig. 3 flow chart of one embodiment the processing of an input trace S 1 R 3  will be considered, being similar for all other traces. The image space is represented by a 2-D or 3-D matrix of co-ordinate points in columns below stations such as 92 ... 100 in Fig. 4. The first step comprises spreading (copying) the trace amplitude, at each trace time Ti, across several image columns to give a set of spread traces S 1 R 3 a 0 -S 1 R 3 a n . Each spread trace is then corrected for moveout as illustrated in Fig. 5, whereby the corrected times for the copied amplitude define the locus of all possible impedance discontinuities which would contribute to the energy returned for trace S 1 R 3  at trace time Ti. The resulting spread and moveoutcorrected traces, denoted S w R x b y , derived from S 1 R 3  and the other input traces, provide each image co-ordinate point with several amplitude values, selected ones of which are summed. The selection procedure rejects those values which are invalid and would contribute "noise" to the sum, e.g. in the case of specular reflectors with a known range of dip angles those values representing returned energy from possible reflectors outside this dip range are rejected. The valid values for emphasizing specular reflectors in the image are thus those for energy returning from within a given sector (Fig. 8, not shown). Alternatively, to emphasize diffractors instead of specular reflectors, the values for energy returning from within that sector are rejected. The values retained for summing are weighted, greater or lesser weighting being given to those values representing energy returned from the central angular region of the selected sector, according to whether reflectors or diffractors, respectively, are to be emphasized. Further selection prior to summing is made, on the basis of the degree of correspondence between adjacent input traces, at 26b and 28b in Fig. 3. Thus, to emphasize reflectors whose dip is such as to give a time shift #t in the returned energy on adjacent traces the amplitudes of, e.g. trace S 1 R 3  at time T i , trace S 1 R 3  at time T i -#t and trace S 1 R 4  at time Ti + #t are compared (by addition, multiplication or correlation) to obtain a "gain factor" (representing the degree of similarity) which is then used to weight the S 1 R 3  amplitude, giving weighted traces S 1 R 3 c a - S 1 R 3 c n  (details illustrated in Figs. 9 and 9A, not shown). To emphasize diffractors, similar "gin factors" are derived by comparing at 28b amplitudes for the same depth point on adjacent traces, and these factors are then used to weight the trace amplitudes to give weighted traces S 1 R 3 d 0 -S 1 R 3 d n  &c. The amplitude values selected and weighted as above are finally summed to give respective composite amplitude values for the image co-ordinate points. In a second embodiment (Fig. 11, not shown) input traces (which may be the results of stacking common depth point traces) are spread and corrected for moveout as described above and the corrected traces are collated according to their associated stations in the image space. A print-out of the amplitude values of the collated trace group for a station enables the dip of interfaces below that station to be analyzed from alignments of those values (Fig. 13, not shown). In addition an automatic "sector selection" of valid amplitudes to be included in a subsequent summation, or an automatic evaluation of the correlation "gain factors" mentioned above, can be made by determining the degree of amplitude correspondence of selected groups of amplitude values in the collated traces (by determining signal power content in the groups), the trace amplitudes subsequently being weighted by interpolated factors derived from these gain factors, and the weighted traces finally being summed for display. Details of this automatic selection and evaluation are given (Figs. 18-25, none shown). Two-dimensional arrays of seismic source points and receivers, not necessarily arranged in straight lines, may be used for a 3-D imaging of the subsurface (Figs. 26-28, not shown).