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Electrical and electromagnetic methods for submarine massive sulfide exploration: a case study of the Palinuro Seamount, Tyrrhenian Sea
Safipour, Roxana G.
Safipour, Roxana G.
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2018
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Recent years have seen increasing interest in exploring for mineral resources on the seafloor. I examine the capabilities of two geophysical methods in exploring for and characterizing seafloor massive sulfide (SMS) deposits: the electromagnetic (EM) and the self-potential (SP) methods. Working with a team from the Helmholtz Center for Ocean Research Kiel (GEOMAR) in Germany, I carried out a first test of a new marine EM configuration consisting of a towed-loop inductive source transmitter and remote ocean-bottom electric dipole receivers. The system was tested at the Palinuro Seamount, offshore Italy, where shallowly buried massive sulfides had previously been recovered from drill cores. Data from the first test of this EM configuration were collected using a horizontal loop transmitter, and analysis of the data found higher apparent conductivities when the transmitter was in proximity to the zone of known mineralization, suggesting that the buried massive sulfides were detected by the system. I also carried out 3D forward modeling which suggests that this configuration is sensitive to a shallowly buried conductive target of dimensions consistent with the drilling zone at a remote receiver up to ~100 m away from the transmitter when the target is located in line between the transmitter and receiver. I used both 1D and 3D forward modeling to compare the sensitivity of a horizontal loop transmitter vs a vertical loop transmitter in the marine EM configuration. The horizontal loop was found to be more sensitive than the vertical loop to the thickness of the target, which is advantageous in attempting to characterize the depth extent of mineralization. The vertical loop is more sensitive than the horizontal loop to a resistive target, such as a gas hydrate deposit. The vertical loop is also less affected by changes in the transmitter towing depth caused by the bathymetry of the field area. In addition to EM, I investigated the SP method in exploring for SMS deposits. A test of a marine SP system consisting of two perpendicular electrode pairs towed above the seafloor was carried out at the Palinuro study area. To my knowledge this was the first test of a marine SP system at a buried SMS site. The SP data showed elevated electric field strengths on the order of 1 - 3 mV/m over the zone of known mineralization, demonstrating that a shallowly buried massive sulfide occurrence can be detected by a marine SP system. My results show that both the marine SP and EM methods have applications in the exploration for SMS occurrences. Furthermore, data collected with the EM and SP systems both suggest that the mineralization at Palinuro extends southward of the drilling zone by ~40 m, which demonstrates that these methods are not only useful in detecting SMS deposits, but in characterizing their size and geometry as well.
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