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Constable, S, Kowalczyk P, Bloomer S.  2018.  Measuring marine self-potential using an autonomous underwater vehicle. Geophysical Journal International. 215:49-60.   10.1093/gji/ggy263   AbstractWebsite

The marine self-potential (SP) method is used to explore for hydrothermal venting and associated seafloor mineralization. Measurements are commonly made in deep water using instruments towed close to the seafloor, which requires dedicated ship time, is limited to slow speeds, and is subject to navigation errors. Instead, we mounted a three-axis electric field receiver on an autonomous underwater vehicle (AUV), and tested the method with data collected in the Iheya area of the Okinawa Trough, off Japan. Parts of this prospect have documented hydrothermal venting and seafloor massive sulfide (SMS) deposits. An International Submarine Engineering Limited explorer-class AUV was fitted with a controlled-source electromagnetic (CSEM) amplifier and logging system, modified to collect DC SP data using silver chloride electrodes on approximately 1 m dipoles. A 1 km x 1 km area was surveyed with a flight pattern of six lines, collected three times to assess repeatability and noise levels. The entire data set was collected in a single day on station with a 10 hr AUV deployment. Flying height was 70 m, navigation errors were less than 3 m, collection speed was 1.1 m s(-1) and electric field noise levels were less than 5 mu V m(-1). Localized anomalies of 0.3 mV m(-1) were observed, from which potentials were estimated using regularized inversion, yielding negative SP anomalies of 15-25 mV. Modelling electric field data as dipoles shows that the negative poles causing the anomalies are localized near the seafloor with a diffuse return current deeper than 1000 m below seafloor. Apparent conductivities as high as 30 S m(-1) were derived from CSEM data collected during the same deployment, which strongly suggests that SMS mineralization is associated with one of the SP anomalies, although the localization near the seafloor and the lack of a dipolar signal suggest that the causative mechanism for the SP anomalies is due to hydrothermal venting. In either case, we have demonstrated that AUV-mounted instrument systems are an efficient, effective and low noise means of collecting marine SP data.

MacGregor, L, Sinha M, Constable S.  2001.  Electrical resistivity structure of the Valu Fa Ridge, Lau Basin, from marine controlled-source electromagnetic sounding. Geophysical Journal International. 146:217-236.   10.1046/j.1365-246X.2001.00440.x   AbstractWebsite

In December 1995 we carried out a comprehensive controlled-source electromagnetic survey of the Valu Fa Ridge at 22 degrees 25'S in the Lau Basin. The Valu Fa Ridge is a back-arc spreading centre of intermediate spreading rate and is a site of extensive hydrothermal activity. Seismic studies have imaged a melt lens at an average depth of 3.2 km below the seafloor, surrounded by a zone of lowered seismic velocity, interpreted as a region of partial melt in the crust. The electromagnetic experiment was part of a multidisciplinary study which included wide-angle and reflection seismics, bathymetry and potential field measurements. Electromagnetic signals at frequencies between 0.25 and 40 Hz were transmitted from a horizontal electric dipole towed close to the seafloor and were recorded by an array of 11 sea-bottom receivers fit ranges of up to 20 km from the source. Over 80 hr of data, consisting of the magnitude of the horizontal electric field at the seafloor, were collected. These data have extremely low scatter compared to similar data from previous surveys. The data were interpreted using a combination of 1- and 2-D forward modelling and inversion. The vertical resistivity gradient in the upper crust at the Vain Fa Ridge is abnormally low, with resistivities of less than 10 Ohm m observed throughout layer 2 of the crust to a depth of 3 km. This is significantly more conductive at depth than the axis of the slow-spreading Reykjanes Ridge at 57 degrees 45'N, and the fast-spreading East Pacific Rise at 13 degreesN, where similar data sets have been collected in the past. Although the structure of layer 2 is well constrained by the electromagnetic data, its extremely low resistivity causes rapid attennuation of electromagnetic signals diffusing through it, and hence the data are not sensitive to the structure in layer 3, in particular the structure of the melt lens or surrounding low-velocity zone. The seismic velocity structure of the Valu Fa Ridge, determined from the coincident wide-angle seismic study, is similar to that observed at other mid-ocean ridges, with a steep seismic velocity gradient through layer 2 (although overall velocities are slightly lower). The seismic velocity anomaly calculated relative to an average off-axis structure is also small. This suggests that the very low resistivities observed at the axis are not caused by an upper crust of abnormally high porosity. However, hot and/or saline fluids permeating the crust can explain the low resistivities without affecting the seismic velocity. Since the conductive region extends unbroken from 3 km depth to the seafloor, it is probable that these fluids circulate to (or close to) the magma chamber itself.