Prediction of Deep-Sea Sediment Properties: State-of-the-Art
The accuracy of predictions of sediment properties is dependent on relationships between various physical properties and/or their expectable statistical variability. In shallow water, sediment types, and consequently their properties, may vary greatly over short distances. In the deep sea, sediment types are fewer, and the main types occupy vast areas. Within the term “deep-sea sediments” must be included the turbidites which form large abyssal plains, rises, and fans in the deep sea.
Prediction of sediment surface properties in deep-sea sediments falls into two categories: (1) a sediment sample is available, or (2) location, only, is provided. Given a sediment surface sample in which water content, grain size, and grain density can be measured, porosity and density can be computed or measured. Given only a dried sample, grain size data provide predictive relations with the other properties. Having derived one or more of these properties, velocity of compressional waves can be predicted within about 1 to 2% because of its relations with grain size and porosity. Given certain restrictive conditions, reflection coefficients and bottom losses at normal incidence can be computed. All of these properties can be corrected to in situ conditions. With less confidence, values of compressional wave attenuation can be approximated because of relations with grain size and porosity, and values of elastic properties (e.g., shear-wave velocity) can be computed. The values of attenuation and the elastic properties require additional confirmation by future in situ measurements. There is apparently no usable relationship between soil mechanics shear strength and velocity, but there may be a relation between dynamic rigidity and attenuation of compressional waves.
If sediment samples are not available and location, only, is provided, the geologist or geophysicist must use available information to predict, in sequence, the physiographic province, sedimentary environment, and sediment type. After the sediment type is predicted, available tables can be entered to determine averaged laboratory properties which can then be corrected to in situ values.
Although surface properties may be reasonably predicted, there is a lack of data to predict many property gradients with depth in the sediment body. At present, compressional velocity vs. depth in sediments can be reasonably predicted because of sonobuoy measurements, but more measurements are needed. The attenuation of compressional waves with depth is not known, but is speculated to decrease with overburden pressures. A small amount of data is available on consolidation, which, if supplemented, will allow prediction of the variations of density and porosity with depth. The least known properties in deep-sea sediments are the velocity and attenuation of shear waves.
Tables of sediment properties previously published by the writer are supplemented by additional measurements from 12 different sources, and new regression equations and diagrams illustrate some interrelationships.
KeywordsBulk Modulus Marine Sediment Sound Velocity Silty Clay Sediment Type
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