Tdem In Groundwater Mapping - A Continuous Approach

Culturally developed areas provide a challenge when applying the transient electromagnetic<br>(TDEM) method, for mapping valley aquifers in inhomogeneous glacial geologies.<br>Magnetic coupling between the transmitter coil and cultural conductors such as power lines,<br>metal fences, metal pipes, etc. distort the measurements of the earth responses and generate<br>datasets that in many cases will yield erroneous interpretations. Furthermore, as TDEM<br>soundings are routinely interpreted with one-dimensional (1 -D) inversion schemes (Christensen<br>and Sorensen, 1995), perturbations in data originating from two- and three-dimensional (2-D, 3-<br>D) geological structures have to be revealed in order to ensure reliable 1-D interpretations<br>(Auken, 1995).<br>In Denmark it has become a common practice to perform 16 - 25 soundings/km2. In many cases<br>this number provides basis for estimating the data and interpretation quality. However, much<br>denser data coverage is generally required to discover distorted data and to resolve the multidimensional<br>geology of the valley aquifers. Due to the expense involved in acquiring dense<br>datasets, coverage is unfortunately seldom attained.<br>In order to meet the demands of dense spatial sampling a new method, the Pulled Array<br>Transient Electromagnetic Method (PATEM) has been developed. With the PATEM method a<br>transmitter and a receiver coil system is towed along profile lines while measuring. Obviously<br>this technique generates dense data sets and reduces the cost of the fieldwork drastically.