Synthesis Of Modulus Logs From Acoustic Parameters; Some Preliminary Results Available to Purchase

A simple technique has been developed which converts the compressional wave velocity (or travel time) recorded by conventional acoustic logging devices to bulk density values at corresponding depths. A similar conversion yields acoustic velocity from density logs. The conversion technique is based on empirical expressions relating either compressional wave velocity, or density, to elastic moduli (plane wave) measured on a variety of rock types. Hence, the plane wave moduli factor permits the computation of either compressional wave velocity or density when the other variable is known. Collectively, we call these computed curves "modulus" or "elastic parameter" logs. A computer program carries out the conversion. These computed logs have a number of important applications. First of all, synthetic density logs can be computed from more commonly available logs for use in gravity survey evaluations where density layering is important. Secondly, the synthetic logs can be used in the computation of synthetic seismograms which are calculated from reflection coefficient functions. Usually, reflection coefficients are computed only from acoustic logs, assuming constant density, because of the unavailability of density logs. But by the use of synthetic density logs from acoustic logs, or vice versa, more accurate computation of acoustic impedances (velocity times density) and reflection coefficients is possible. Finally, it appears that the relationship between elastic moduli and rock "structure" or lithology which occur make measured and computed density or acoustic curve comparisons a useful technique in lithology identification. The empirical relationships presented here have been tested in a number of wells containing both sandstones and carbonates where conventional density or acoustic logs were available to compare with the synthetic or modulus logs. Reasonable agreement has been obtained between the two sources of density and acoustic data in many relatively "well-behaved" lithologies, provided the holes are not too rough and the acoustic sondes are reasonably well centered during logging. In other well-behaved rocks such as dolomite and shale, and in less well-behaved units such as coal, siderites, and anhydrites, the computed and measured density curves often exhibit a constant shift or separation, even though detailed characteristics are quite similar. These separations constitute the means of lithology identification.