Laboratory and field studies of the application of electromagnetic prospecting for groundwater on Marajó Island, Brazil

Abstract

On Marajó Island, Pará, Brazil, many cattle die due to lack of potable water, since a majority of the shallow groundwater is brackish. Two types of fresh‐water aquifers were delineated using resistivity methods: (1) a deep aquifer at about 100 m depth and (2) shallow buried‐stream channels filled with sand and gravel (ρ = 40 Ω ⋅ m) and embedded in clay and sand beds with brackish water (ρ = 0.8 Ω ⋅ m — 5 Ω ⋅ m). For the demarcation of the shallow buried‐stream channel aquifers, inductive electromagnetic moving source‐receiver methods were tested to replace the cumbersome galvanic resistivity methods. In order to verify the applicability of EM survey techniques for groundwater exploration in hydrogeologic conditions typical of Marajó Island, analog model studies were carried out. The groundwater‐bearing channels buried in the clays or in the sediments with brackish water represent geoelectrically a less conducting body in a more conducting environment. The buried‐channel “Guajará Mirim” on Marajó Island was chosen for test studies. A 500 times reduced model of this channel was constructed using two aluminium sheets of varying thicknesses to simulate host sediments and the sediments in the buried channel. EM profiles were measured in the laboratory across the channel model using horizontal‐coplanar vertical‐coplanar, and vertical‐coaxial coil systems in the broadside and inline modes of operation. Excellent EM model responses were obtained with all six coil configurations. Finally, EM profiles in the field were carried out using horizontal‐coplanar, vertical‐coplanar, and vertical‐coaxial coils in an inline mode of operation. Field and laboratory results are in very good qualitative agreement.