A rock-physics investigation of unconsolidated saline permafrost: P-wave properties from laboratory ultrasonic measurements

Abstract

Saline permafrost is sensitive to thermal disturbances and is prone to subsidence, which renders it a major source of geohazard in Arctic coastal environments. Seismic methods could be used to map and monitor saline permafrost at scales of geotechnical interests because of the ice-content dependencies of seismic properties. We have developed a comprehensive study of the ultrasonic P-wave properties (i.e., velocity and attenuation) of synthetic saline permafrost samples for a range of salinities and temperatures, and measurements conducted on a fine-grained permafrost core obtained from Barrow, Alaska. The resulting data consist of P-wave properties presented as functions of temperature and salinity. Notable observations include the following: P-wave velocities showed marked reductions in the presence of dissolved salts and complex variations resulting from the water-to-ice phase transitions; strong P-wave attenuation was present in the temperature intervals in which the samples were partially frozen. When presented as functions of ice saturation, the data sets lead us to two key findings: (1) neither a purely cementing nor a purely pore-filling model of the pore-scale distributions of ice could adequately fit the observed velocity data and (2) although the velocities increase monotonically with increasing ice saturations, P-wave attenuation reaches a maximum at intermediate ice saturations—contrary to the ordinary expectation of decreasing attenuation with increasing velocities. The observed ice-content dependencies of P-wave properties, along with the implications on the probable pore-scale distributions of ice, provide a valuable basis for rock-physics modeling, which in turn could facilitate seismic characterizations of saline permafrost.