Patent GB-1099483-A (GB)
Title: Optimum horizontal stacking
IPC: G01V1/20
Filed: 1965-03-30
Abstract: 1,099,483. Seismic prospecting. TEXAS INSTRUMENTS Inc. March 30, 1965 [April 2, 1964], No. 13468/65. Heading H4D. [Also in Division G4] In a system for enhancing primary reflections relative to multiple reflections in a composite signal produced by combining a plurality of signals representative of seismic waves detected after travel to and from the same subsurface reflecting point over a corresponding plurality of paths all of different lengths, each said signal is corrected for normal moveout and filtered, prior to combination, by modifying the time distribution of the multiple reflections therein relative to the primary reflections in dependence upon the geometry of the associated path and upon the velocity profile along said path. In a specific embodiment three-fold coverage of each of a plurality of common subsurface reflection points is provided by detonating shots at the two ends successively of a line of geophone locations (1 to 24) (Fig. 1), transferring one third of the geophone locations from one end of the line to the other and repeating this procedure. For each reflection point, detected seismic signals are recorded as three tracks X 1 , X 2,  X 3  on a magnetic medium 29. The tracks are played back by moving medium 29 past reproducing heads 40-44, 70-74, 80-84 whose outputs are weighted by associated resistors and summed in amplifiers 53, 61, 63 respectively to provide time-domain filtering of each track. The filtered signals are combined by summing unit 65. The timedomain filtering separates " signal " (i.e. primary reflections) from " noise " (i.e. multiple reflections) on the basis of the difference in their apparent horizontal velocities, which difference is manifested by their different relative delays on the three tracks X 1 , X 2 , X 3 . Optimum filter parameters can thus be determined from the " residual moveout " of multiple reflections, i.e. moveout errors remaining after a seismic trace has been corrected for static errors and normal moveout of primary reflections, the " residual moveout " (Figs. 5 and 6, not shown) being calculated as a function of record time, the geometry of a seismic wave travel path and the velocity profile along the path (the latter being obtained from a velocity log or further reflection data). In the arrangement of Fig. 8 for calculating the required impulse-response of the filters, matrix elements Sij and Mij of a matrix equation relating the " signal " and " noise " components of the three tracks, the desired filtered signal and the response of the filters in the frequency domain are derived from the " residual moveout " data and represented by voltages in units (100), (105-107), (101), (110-112), (102), (115-117). These voltages are combined by multipliers (120-122) and adders (123) to solve the matrix equation for each filter. The resulting frequency/amplitude plots at (125) are applied to units (127) to provide Fourier transforms, i.e. the filter impulse-responses, at (128). From these impulse-responses the values of the weighting resistors for the time-domain filtering (Fig. 3) of tracks X 1 , X 2 , X 3  are determined. A ganged switch system 89 in the Fig. 3 arrangement permits different weighting resistors to be used in different increments of record time, thus providing time-variant inverse filtering of the traces. In a modification, a digital computer supplied with the seismic reflection data and design data for the optimum filters may replace the analogue processing system. A detailed mathematical treatment of the filtering is given.