Seafloor electromagnetic induction studies in the Bay of Bengal

Seafloor magnetometer array experiments were conducted in the Bay of Bengal to delineate the subsurface conductivity structure in the close vicinity of the 85° E Ridge and Ninety East Ridge (NER), and also to study the upper mantle conductivity structure of the Bay of Bengal. The seafloor experiments were conducted in three phases. Array 1991 consisted of five seafloor stations across the 85° E Ridge along 14° N latitude with a land reference station at Selam (SLM). Array 1992 also consisted of five seafloor stations across 85° E Ridge along 12° N latitude. Here we used the data from Annamalainagar Magnetic Obervatory (ANN) as land reference data. Array 1995 consisted of four seafloor stations across the NER along 9° N latitude with land reference station at Tirunelveli (TIR). OBM-S4 magnetometers were used for seafloor measurements. The geomagnetic Depth Sounding (GDS) method was used to investigate the subsurface lateral conductivity contrasts. The vertical gradient sounding (VGS) method was used to deliniate the depth-resistivity structure of the oceanic crust and upper mantle. 1-D inversion of the VGS responses were conducted and obtained a 3-layer depth-resistivity model. The top layer has a resistivity of 150–500 Ωm and a thickness of about 15–50 km. The second layer is highly resistive (2000–9000 Ωm) followed by a very low resistive (0.1–50 Ωm) layer at a depth of about 250–450 km. The 3-component magnetic field variations and the observed induction arrows indicated that the electromagnetic induction process in the Bay of Bengal is complex. We made an attempt to solve this problem numerically and followed two approaches, namely (1) thin-sheet modelling and (2) 3-D forward modelling. These model calculations jointly show that the observed induction arrows could be explained in terms of shallow subsurface features such as deep-sea fans of Bay of Bengal, the resistive 85° E Ridge and the sea water column above the seafloor stations. VGS and 3-D forward model responses agree fairly well and provided depth-resistivity profile as a resistive oceanic crust and upper mantle underlained by a very low resistive zone at a depth of about 250–400 km. This depth-range to the low resistive zone coincide with the seismic low velocity zone of the northeastern Indian Ocean derived from the seismic tomography. Thus we propose an electrical conductivity structure for the oceanic crust and upper mantle of the Bay of Bengal.