In this study a TEM investigation where preformed on the Bolivian altiplano north of the city Oruro. There is a deficit of drinking water in Oruro and the aquifer Challapampa is their main source of water. With this investigation the object was to map the bottom limitation of the Challapampa aquifer and, if possible, restraining structures within the aquifer, aquitards. The surrounding bedrock consists mainly of sandstones and shale’s that has been penetrated by intrusions in several locations. Quaternary sediments on top of the bedrock hold most of the ground-water in the area. The general structures in the bedrock surrounding the Quaternary sediments are in a northwest to southeast direction and similar structures are expected to be found underneath the sediments.
The method used in this investigation was TEM, which uses transient electromagnetic (EM) waves to penetrate the ground. TEM is a geophysical technique that creates a magnetic field by passing an electrical current through an ungrounded cable. When the current is cut off, as quickly as possible, the magnetic field induces a current in its surrounding ground that results in a secondary magnetic field. The secondary magnetic field induces a current in the receiver antenna and electrical potentials are measured and registered in the instrument. The variation of the secondary magnetic field depends on the ground properties.
The TEM method is well suited for investigating groundwater properties in sand- and gravel-dominated deposits. A depth of investigation (DOI) to 850 m b.s. (meters below surface) was achieved in several points, which is over expectations. Two low resistivity valleys was found and interpreted as water saturated sedimentary valleys, extend in a northwest to southeast direction over the studied area. The depth to the bedrock is interpreted to be around 75 m b.s., both in the western and eastern part of the area. The basal boundary for the interpreted sedimentary valleys crossing profile 3 is around 130 m b.s. for the eastern valley and 120 m b.s. for the western valley.