A FIVE SPOT WELL CLUSTER FOR HYDRAULIC AND THERMAL TOMOGRAPHY

We present a five spot groundwater well arrangement for investigating hydraulic and thermal conditions in the subsurface. The experimental facility is located on the North-campus of the University of Gottingen/Germany. In 2008, one well was drilled next to a geothermal borehole heat exchanger, both reaching a depth of 80[m] in a clay stone formation. In summer/autumn of 2012 four additional boreholes were drilled down to the same depth to get a spatially splayed test site in a 5-spot arrangement. At one borehole the core was taken, in addition to geophysical borehole measurements performed in all boreholes. One unique feature of the site is the relatively small distance of the wells from 1.5[m] to 6[m], which is important for several kinds of aspired subsurface heat transport experiments. All five wells are constructed equally. The casing is divided into 9 full cased and 9 slotted parts to achieve and investigate connections to different geological strata. The wells are equipped with a comprehensive system of temperature measurement devices. A chain of 27 pt1000 resistance thermometers, an optical fiber cable for spatially highly resolved temperature measurements and an actively heated cable are installed in the filling grout of each hole. This measurement system can optionally be adapted by a chain of mobile thermometers with a relative resolution of 0.3[mK] for operation inside the well casing. The new test site provides various experimental options for the development and test of geothermal exploration and identification methods. The arrangement with numerous well screens and spatially as well as temporally resolved temperature measurements is especially suitable for the application of tomographic methods.