Electrical Resistivity Monitoring Of The Thermomechanical Heater Test In Yucca Mountain

A test is being conducted in the densely welded Jopopah Springs tuff<br>within Yucca Mountain, Nevada to study the thermomechanical and<br>hydrological behavior of this horizon when it is headed. A single 4 kW heater,<br>placed in a horizontal borehole, was turned on August, 1996 and will continue<br>to heat the rockmass until April 1997. Of the several thermal, mechanical and<br>hydrological measurements being used to monitor the rockmass response,<br>electrical resistance tomography (ERT) is being used to monitor the movement<br>of liquid water with a special interest in the movement of condensate out of the<br>system. Four boreholes, containing a total of 30 ERT electrodes, were drilled to<br>form the sides of a 30 foot square with the heater at the center and<br>perpendicular to the plane. Images of resistivity change were calculated using<br>data collected before and during the heating episode. The changes recovered<br>show a region of decreasing resistivity approximately centered around the<br>heater. The size this region grows with time and the resistivity decreases<br>become stronger. The changes in resistivity are caused by both temperature<br>and saturation changes. The observed resistivity changes suggest that the rock<br>adjacent to the heater dries as heating progresses. This dry region is<br>surrounded by a region of increased saturation where steam recondenses and<br>imbibes into the rock.