Geotechnical assessment of superficial marine sediments using in-situ geophysical probes

Abstract

Abstract A geophysical method is described where electrical resistivity, in terms of formation factor, and compressional wave sound velocity are measured in-situ, on undisturbed sediments, and the measurements then related to porosity using general correlation curves. A survey of a pockmark in the Forties sector of the North Sea detected isolated patches of low geophysical values which showed an increase in inferred porosity of more than 0.10; these patches were assumed to be softer than the surrounding sediments and could represent potential hazards. A resistivity survey in the Irish Sea over a wide variety of sediment types produced a significant correlation between mean size and apparent formation factor (in-situ), which is likely to reflect the natural changes in porosity with mean size. Using values from recent measurements of shear wave velocities in unconsolidated marine sediments, it is suggested that the contribution of the “frame modulus” to the compressional wave velocity is 8–10% whereas that due to the rigidity (as measured by the shear wave velocity) is 2.4%. The propagation of sound and the conduction of electricity in porous media are shown to be controlled by different processes, and as such a combination of these two measurements may uniquely define a third property; cross plots of formation factor and compressional wave travel time showed this to be the case for porosity, and possibly the state of compaction, both properties being assessed directly from the plot.