Pseudo velocity logs can be obtained by seismogram inversion, using true amplitude processing and detailed investigation of move‐out velocities. The precision of the results depends on the quality of the seismic data and on the possibility of deconvolving without increasing the noise. An investigation is made of the deformation of pseudo logs due to seismic signal variations and to imperfections of deconvolution. Both marine and land examples are shown, in some cases with adjustment on well logs. When the dips are large, time sections must be migrated and pseudo velocity logs must be computed from migrated sections. Comparison of sonic logs with pseudo velocity logs obtained in the same area is usually good enough to obtain information on lithological parameter variations by adjustment of pseudo velocity logs on sonic logs. Even when no well is available, pseudo velocity logs can give some indications on the nature of sediments between seismic horizons.

Acousticimpedance, the product of seismic velocity and density, is a basic physical property of rocks.Seismic traces are converted into pseudoreflection-coefficient time series by appropriate initial processing. then into acoustic impedance by the inversion of the time series. Such pseudologs are rough11 equivalent to logs recorded in wells drilled at every seismic trace location. They yield important information concerning the nature of the rock and variations in lithology To obtain the best quality pseudologs, careful initial processing is necessary: true-amplitude recovery. appropriate deconvolution. common-depth-point (CDP) stack. wave-shaping, wave-equation migration, and amplitude scaling. The low frequencies from moveout velocity information are inserted. Both the short-period information computed from reflection amplitudes and the long-period trend computed from reflection moveout are displayed on acoustic impedance logs. Possible causes of pseudolog distortions are inaccuracies of amplitude recovery and scaling, imperfection of deconvolution and migration. and difficulties of calibrating the pscudolog to an acoustic Iof derived from well logs. Such calibration increases the precision; facies variations observed in well logs cau bc extrapoled to large distances from the wells. leadin, (7 to a more accurate estimation of hydrocarbon resel.ves.

The new process of automatic determination of seismic velocities by well to well measurements (Bois et al., 1971, Geophysical Prospecting 19, 42‐73) gives the possibility to increase the knowledge of oil reservoirs, by detecting large inhomogeneities between wells. It can also give useful informations on the mechanical properties of rocks in mining exploitation and civil engineering, by gallery to gallery measurements. An application of the method is given to the search for the proper location of an underground hydroelectric power plant, where the problem was to investigate the rock properties in a horizontal plane between two exploration galleries.