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Electromagnetic exploration of the Exmouth and Vøring rifted margins

  • Author(s): Myer, David Gerard
  • et al.

In this work, we present improvements to and the use of two marine electromagnetic exploration methods at the Exmouth and Vøring Plateaus. First we discuss the marine controlled-source electromagnetic method (CSEM), specifically the formation and characteristics of custom binary and ternary waveforms as well as improvements to time-series processing which allow derivation of noise estimates. We then apply these improvements to the study of the Scarborough gas reservoir beneath the Exmouth Plateau off the northwest shelf of Australia, over which we collected the world's largest academic CSEM dataset. Using these data we (1) discuss methods for determining uncertainties in CSEM, (2) investigate the resolution of CSEM in an environment where multiple resistive geologic layers lie in close repose, (3) develop the prejudiced minimum norm as a regularization method in 1D CSEM inversion, discussing its strengths and limitations, and (4) develop the 1D/2D and 1D/3D inversion methods in which the forward model is 1D but the regularization is 2D or 3D. All of the new inversion methods derive more realistic results than 1D smooth inversion alone and may form a good starting point for higher dimensional inversion. Finally we target the deep structure of the Exmouth Plateau with marine magnetotellurics and compare the results to the Vøring Plateau off the northwest shelf of Norway. Magnetotelluric data reveal that both plateaus host a body of unexpectedly low resistivity (̲< 0.1 [Ohm]m) at ̃10 km depth. We show that the depth of each body is consistent with known sills and theorize that the low resistivity is due to magnetite cumulates precipitated as layers in mafic -ultramafic sills. Both plateaus exhibit the features of volcanic passive margins in which the final continental rifting event was relatively quick and accompanied by voluminous volcanism. Our data are consistent with this explanation and suggest that a large volume of mafic material was injected into the crust forming a relatively interconnected network of conductive minerals

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