FY2002 Final Report for EMSP Project No.70108 Effects of Fluid Distribution on Measured Geophysical Properties for Partially Saturated, Shallow Subsurface Conditions

Our goal is to improve geophysical imaging of the vadose zone. We are achieving this goal by providing new methods to improve interpretation of field data. The purpose of this EMSP project is to develop relationships between laboratory measured geophysical properties and porosity, saturation, and fluid distribution, for partially saturated soils. Algorithms for relationships between soil composition, saturation, and geophysical measurements will provide new methods to interpret geophysical field data collected in the vadose zone at sites such as Hanford, WA. This report summarizes work after 32 months of a 3-year project. We modified a laboratory ultrasonics apparatus developed in a previous EMSP project (No.55411) so that we can make velocity measurements for partially-saturated samples rather than fully-saturated or dry samples. Modifications included adding tensiometers and changing the fluid system so that pore fluid pressure can be controlled and capillary pressure can be determined. We made a series of measurements to determine properties of partially saturated Ottawa sand and Santa Cruz aggregate samples as well as sand-clay samples and some preliminary measurements on natural soils. Current measurements include investigations of effects of pore fluid chemistry on grain cementation and velocities for calcite-cemented sand samples. We analyzed these measurements as well as velocity and electrical properties measurements made as part of the earlier EMSP project and developed relationships between measured geophysical properties and parameters of interest, including lithology, fluid content and distribution, and soil microstructure. Our laboratory velocity measurements have confirmed recent field observations of extremely low seismic velocities of a few hundred m/s in shallow soils, and we have shown that these values are consistent with effective medium theories. We have shown that the laboratory velocities for partially saturated sands, collected at ultrasonic frequencies, behave as predicted by Gassmann's static result, and thus laboratory ultrasonic velocities can be considered analogous to velocities at seismic frequencies. We have shown that shallow soils may have very steep velocity gradients, a matter of some importance for seismic field data that are interpreted using ray-tracing codes that may assume straight, rather than curved, ray paths. We have also found that packing effects can be important, and this will be significant for seismic surveys conducted using hammer sources since the properties of the soil below the plate could change after repeated hammer blows. The presence of clay reduces the nonlinearity, and this may be apparent in seismic field data. We found that Vp/Vs ratios are a useful indicator of clay content, particularly when considering changes with depth. We were able to apply self-consistent and differential effective medium theories to understand velocities in sandy soils with relatively low concentrations of clays or organic materials, but for concentrations above about 20% we had to use theories that included grain contact effects. A new method was developed for relating compressional and shear wave velocities to fluid content and distribution, and tests on laboratory data were successful. It would be useful to test whether this new technique can be used to investigate fluid distribution in the vadose zone using seismic data. Our results suggest that the approach for this EMSP research project has been appropriate, that microstructure is an important factor for measured geophysical properties, and that seismic field experiments should be designed to collect both compressional and shear wave velocity data and to collect wave amplitude as well as velocity information when possible. The lead p.i. has written a new EMSP proposal in collaboration with Ernie Majer of LBNL for a new project that would involve collecting seismic data at appropriate sites including Hanford and applying these new interpretation techniques to the data to further develop the methods.

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