Electrical Resistivity Monitoring of the Single Heater Test in Yucca Mountain, FY98 -1st Quarter Results

Of the several thermal, mechanical and hydrological measurements being used to monitor the rockmass response in the Single Heater Test, electrical resistance tomography (ERT) is being used to monitor the movement of liquid water with a special interest in the movement of condensate out of the system. Images of resistivity change were calculated using data collected before, during and after the heating episode. This report will concentrate on the results obtained after heating ceased; previous reports discuss the results obtained during the heating phase. The changes recovered show a region of increasing resistivity approximately centered around the heater as the rock mass cooled. The size of this region grows with time and the resistivity increases become stronger. The increases in resistivity are caused by both temperature and saturation changes. The Waxman Smits model has been used to calculate rock saturation after accounting for temperature effects. The saturation estimates suggest that during the heating phase, a region of drying forms around the heater. During the cooling phase, the dry region has remained relatively stable. Wetter rock regions which developed below the heater during the heating phase, are slowly becoming smaller in size during the cooling phase. The last set of images indicate that some rewetting of the dry zone may be occurring. The accuracy of the saturation estimates depends on several factors that are only partly understood. Introduction The Single Heater Test (SHT) is one of the in situ thermal tests being conducted in the exploratory studies facility (ESF) in Yucca Mountain to enhance the understanding of the coupled processes. The primary objective of the SHT is to investigate the thermal-mechanical responses of the Topopah Spring tuff in Yucca Mountain. This paper describes electrical resistance tomography (ERT) surveys made during the SHT in order to map the changes in moisture content caused by heating. Of particular interest, is the formation and movement of condensate within the fractured rock mass. In this report, we concentrate on the results observed during the first quarter of FY 98. In this quarter, monitoring of the cooling phase continues. Electrical Resistance Tomography Electrical resistance tomography (ERT) is a geophysical imaging technique which can be used to map subsurface resistivity. Rock mass heating creates temperature and liquid saturation changes which result in electrical resistivity changes that are readily measured. The ERT