Method and device for determining a granular feature of a formation crossing by a boring

摘要

In broad band high frequency acoustic investigation of a subsurface formation around a borehole, like pulses containing a range of acoustic frequencies selected to produce detectable acoustic backscatter from a broad range of grain sizes are transmitted into the subsurface formation and the acoustic backscatter is detected and a spectrum thereof produced and averaged over adjacent subsurface formation regions. A subsurface formation parameter such as the frequency dependency of the attenuation of the backscatter is determined from an analysis of the spectrum to provide an indication of a granularity property of the subsurface formation. In another technique the spectrum of the detected backscatter is modified to remove the effect of the frequency response of the system with which the investigation is made. The modified spectrum may then be used to derive an indication of the granularity property of the subsurface earth formation such as its mean grain size or grain size distribution.