It has been demonstrated that acoustic echoes, returned from a multi-layered sea floor, are useful in identifying key properties of those sediments. The principal remotely measured classifiers in current use are sediment bulk density, intra-layer compressional wave velocity and attenuation rates.
In typical marine sediments the successive under-lying layers exhibit densities and velocities that usually increase with depth. In Delta areas, as found in the Gulf of Mexico, however, this is not always the case due to the inclusion of biological gas in the soil, which may affect seriously its engineering properties.
This paper reviews the concept of negative reflection coefficients resulting from underlying layers of lower impedance, presence of gas, and their impact on the prediction of physical soil parameters from acoustic probing. The mathematical implications are explored and examples of analyses of acoustic reconnaisance data, taken over Delta soils, are shown.
Acoustic sub-bottom profiling in conjunction with automatic signal processing continues to show promise as a tool for making shallow sub-surface geological and biotechnical assessments. Earlier work by the Raytheon, in typical glaciated sediments, had indicated that certain useful acoustically derived sedimentary parameters may be obtained that relate to physical soil samples. Multi-layer computer-derived indices are now being obtained that may be used to classify certain bulk properties of the underlying layers.
In Delta areas, particularly in the South Pass region of the Gulf of Mexico, the presence of entrapped gas is very common. These soils often appear to be mechanically unstable, and serious hurricane related damage has been reported to bottom supported structures in those area.
This paper reports the results of computer aided analysis of echoes from a normal incidence acoustic system over a site believed to contain substantial amounts of biological gas.
The typical echo from a marine sediment is reflected from those interfaces that pose a change in acoustic impedance. The signal also undergoes scattering by surface layer roughness, by inter-layer reverberation, and it encounters attenuation in its passage through the soil. The returning echo, which is modified by these geologically-related phenomena, may be analyzed to obtain a first order assessment of the causative factors. If acoustic geometries other than normal incidence are available, compressional wave velocity measurements become available, expanding the range and validity of the conclusions.
The simplest acoustic measurable is the reflection coefficient, which, under ideal conditions, is proportional to the percentage of pressure which is reflected back to the receiver, i.e.
(Equation available in full paper)
Where Z represents the acoustic impedance of the medium either side of the interface, increasing in depth as the order of subscripts. The impedance in turn, to a first order approximation is
(Equation available in full paper)
Where ?is the wet bulk density of the soil and c is the compressional wave velocity of sound in that sediment.
