Permafrost
An icewedge in a permafrost exposure.
A substantial fraction of the Earth's surface including its continental shelves is underlain by permafrost ground that has been frozen for two or more years. Some estimates put this fraction in excess of 80% for Alaska! Permafrost is not merely of scientific interest, but because it is the only geologic hazard that is sensitive to the mean annual surface temperature (MAST), it is of vital interest to the geotechnical industry, which must assess the structural integrity of foundation soils. This assessment is primarily done by a relatively-costly, site-specific, drilling and sampling program. However, some methods used in the geophysical exploration industry have proved effective in characterizing soils in a reconnaissance mode for further, more site-specific investigation by drilling. ASTF has funded a project to improve a method used previously, namely the shallow-sounding terrain conductivity method, which is based on electromagnetic induction, as an indicator of the thermal state of the soil (frozen soils are less conductive electrically than unfrozen soils). The modifications were to include the following:
1. Increase the sensitivity of sensors by considering in-phase output, coil-coil separation and/or frequency.
2. Improve boom design.
3. Convert all analog to digital data.
4. Develop a data acquisition system.
5. Develop software for testing, calibration, data storage/retrieval and data interpretation.
Development of a prototype system incorporating the modifications above has been proceeding with circuitry/chips of unique design. At the present time, the overall system is not functioning in its optimal state, because there are severe noise/interference problems. The data acquisition system is working well and may be a commercially-viable, stand-alone system. Calibration and test sites have been established. An alternate design to the boom/coil-coil system is being considered.
ASTF Grant: Scientific personnel; G. G. Walker, K. Kawasaki and J. G. Holt.