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Abstract:
Marine electromagnetic measurements are used to derive information about the conductivity structure beneath the seafloor. For the correct interpretation of electromagnetic data it is important to know the measurement geometry, including the orientations of receivers. From an experimental standpoint, this can be challenging because stations are often deployed free falling, thus, ending up in arbitrary orientations on the seafloor. The orientations are frequently derived from electronic compass measurements or magnetometers which record all components of the magnetic field. However, these measurements may be distorted by magnetic parts on stations (e.g. batteries), biased by local inhomogeneities in the local field or difficult to perform if no reliable reference data from a nearby observatory is available. A possible remedy for such problems may come from space physics. Given a grid of stationary magnetometer stations, surrounding the area of interest but at relatively large distances, the method of spherical elementary current systems (SECS) can be used to reconstruct equivalent ionospheric currents and their resulting time variations of the magnetic field at any point within the grid. We have successfully applied the method to marine data sets. The SECS method qualitatively reproduces the magnetic variation as observed by the seafloor stations. Here we investigate the results from the above mentioned EM data sets, and discuss the applicability, accuracy and constraints of the SECS method for EM data calibrations. Furthermore, we illuminate the possibility for using SECS as an interpolation tool for other applications at remote offshore locations, such as measurement while drilling (MWD) operations.
