Research on aquifers in the lateritic deposits
of the city of Jangany (Madagascar)

Purpose of the study and methodologies applied.

The objective of this study is to identify potential sites near the city of Jangany in Madagascar where wells for drinking water can be constructed. The geophysical investigations applied in this case study aim to locate a possible shallow or deeper aquifer compared to those currently exploited, and to determine its position underground in order to construct wells appropriately, yielding satisfactory results to meet the city’s ever-growing water demand. Geophysics employs indirect investigation methods, as they do not directly observe the physical and mechanical characteristics of what lies beneath the ground, as in the case of drilling, but rather indirectly through the propagation and acquisition of various types of energy fields. Following a geophysical survey, the subsurface structure is hypothesized based on its response to the passage of such energy. For this work, the Time Domain ElectroMagnetic (TDEM) method was considered more appropriate, as it is sensitive to changes in electrical resistivity underground, and thus capable of identifying a potential aquifer. Subsurface electrical resistivity is closely related, among other parameters, to its water content. Some advantageous aspects of this method include its ability to take measurements over large areas in short timeframes and to utilize lightweight instrumentation, thereby facilitating logistical challenges. As for disadvantages, this method is highly susceptible to interference in the presence of metal structures or neighboring power grids in the data acquisition area, and it requires pre-existing data, such as studies or previous drilling in the same area of interest, to interpret the results optimally. The TDEM method allows for subsurface investigation using electromagnetic impulses (primary electromagnetic field) generated by the induction of electric current within one or more transmitting coils placed above the area of interest; these coils are usually positioned to form a square whose size varies depending on the target to be investigated, generally increasing the size of the coil increases the depth of investigation at the expense of lower surface resolution. In our case, most acquisitions were made using coils with a side length of 50 meters. The primary electromagnetic field propagating from the surface produces, upon encountering the formations being investigated, a secondary field, which contains the information of interest, namely the variations in resistivity in the subsurface. The secondary magnetic field is measured by the receiving coil, which in the case of TEM-Fast (the instrument used for this survey) corresponds to the transmitting coil, following the interruption of the current in the circuit. The data thus obtained need to be processed, which occurs during the processing phase, which for this work involves transforming the signal acquired by the instrumentation into real electrical resistivity and depth using software.