METHOD OF GEOPHYSICAL PROSPECTING BY MEASURING THE ATTENUATION OF SEISMIC WAVES IN THE EARTH

摘要

1341422 Seismic exploration WESTERN GEOPHYSICAL CO OF AMERICA 21 May 1971 [18 June 1970 (3)] 16417/71 Heading H4D The lithographic character of a rock stratum is determined by a seismic reflection method in which the absorption of seismic energy in the stratum is measured. In the Fig. 1, embodiment the reflection signal amplitudes of a trace recorded (e.g. by a digital binary-gain method) at a surface station L along a traverse are first corrected to compensate for the geometric (generally spherical) spreading of the seismic wavefronts. Reflection coefficients R for the strata interfaces 1,2, ... i ... n are calculated from seismic velocity and density data (the velocities may be determined by a normal-movement analysis) and applied, together with the corrected signal amplitudes S and travel times t, to a computer. The attenuation coefficient i for the stratum lying between the i the and (i + 1) the interfaces is then computed from the relationship Values of thus found may be displayed similarly to amplitude values in a conventional record section and compared with values of obtained from other stations LSP1/SP to identify horizontal changes in lithologic character within a stratum. In a second embodiment (Figs 3 and 4, not shown) the reflection signals after correction for geometric spreading are converted to the frequency domain by a Fourier, Laplace or other integral transform operation to give transform functions |F i (#)| which are used in place of the amplitude values S in the above relationship. As the transformation involves an averaging effect, the reflection coefficients R can be assumed identical and hence omitted from the above computation of . The attenuation coefficient as a function of travel time, ( y (t), is obtained in a third embodiment (Figs. 5 and 6, not shown) from the amplitude-time trace. (corrected for geometric spreading) recorded at a surface station, where A o is the initial signal amplitude and R y (t) is a reflectivity function for the strata below the station. By analysing a similar trace recorded at an adjacent surface station from the same or a similar seismic source actuation (hence the same or similar A o ) the reflectivity function R x (t) is found and, since it is assumed equal to R y (t), is used in the above relationship between Y(t) and y (t) y (t) for computing.