MSc thesis presentation by Joakim Mårdh

A TEM study, with complementary ERT, of Punata alluvial fan, Bolivia


The Punata alluvial fan, situated in central Bolivia (Valle Alto) between the Altiplano and the lowlands, is an important aquifer for the local rural population. Due to rapid development and population growth in the area, the demand for fresh water has increased in recent year. In combination with decreasing annual rainfall, in the already semi-arid climate, the groundwater level is steadily sinking and thus depleting shallow wells. The current solution is to drill new, deeper, wells and continue to overexploit the reservoir in an unsustainable manner. In order to map the aquifer geometry this paper presents a TEM (Transient Electromagnetic Method) survey with the aim to find the sediment – bedrock boundary, which is thought be at >300 m depth. The survey acts as a compliment to the previously conducted ERT (Electrical Resistivity Tomography) surveys in the area. Valle Alto is a tectonic basin, in the department of Cochabamba, with predominantly Palaeozoic sedimentary bedrock (Ordovician and Silurian) and minor Mesozoic formations from the late Cretaceous. The lithology varies between shales, siltstones and sandstones, deposited in a marine environment during the Palaeozoic and in a continental rift basin during the Mesozoic. Valle Alto is the result of tectonic activity during the Pliocene, which yielded an enclosed lake in the area. An unquantified amount of lacustrine clay was deposited in the basin before the lake was drained due to renewed tectonic activity. On top of the clay there is intermingling alluvial fans and colluvial deposits of different generations, with the Punata alluvial fan being one of them. In an attempt to achieve adequate depth of penetration, TEM was used in this survey. The method utilizes the fact that an electrical field always yields a proportional magnetic field and vice versa. In short, an electrical pulse is sent through transmitter loop and then abruptly turned off. This induces an electromotive force which propagate into the ground and in turn induces currents proportional to the resistivity. The currents yield a secondary magnetic field which can be measured by receiver coils, and then used to create resistivity models of the subsurface. Unfortunately, the lacustrine clay restricted the depth penetration to 90 – 200 m and the models did not reach the bedrock boundary. The survey did however present a few other interesting features such as a distinct thin layer with very low resistivity, interpreted to be brine, on top of the alluvium – clay boundary. There might also be a fault line beneath the fan, but the results of the survey are inconclusive and further studies of the tectonic regime necessary in order to verify or disregard the hypothesis.


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