"Acousticore", A new concept in acoustic prospecting for minerals and preconstruction surveying

Abstract

Acousticore is a technique for quantitative measurement of the specific acoustic impedance of marine sediments. A broadband impulsive source and two colinear receivers of fixed spacing are oriented vertically. Acoustic pressure received at the two points is used to compute downgoing and upgoing waveforms which are, in turn, numerically deconvolved to obtain the reflection impulse-response function of sediments below the device. Theoretical relationships are presented between the reflection impulse-response function and sediment acoustic impedance which assume horizontal homogeneity but allow for arbitrary vertical variations including continuous change in impedance. If the range of impedance variation is modest (perhaps 2:1), e.g., in "normal" unconsolidated sediments, the theoretical relationship is simply: <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\int\max{t}\min{0}h(u)du=1/2 ln[z(t)/z_{0}]</tex> where (t) is the reflection impulse-response function and z(t) is impedance as a function of acoustic travel time. Marine sediments encompass a spectrum of impedances, as substantiated by published data regarding acoustic properties of sediments. These data, which are summarized here, show that quantitative measurement of impedance is useful for discrimination between consolidated sediments, gravel, sand, clay, silt, and gassy sediments. In the case of gassy sediments we obtain an undissolved gas percentage profile versus depth (acoustic travel time). Laboratory and field data are shown illustrating the Acousticore technique and its ability to measure acoustic impedance versus depth for multi-layered sediments.