ALBERT

Description: We study a new marine electromagnetic configuration which consists of a ship‐towed inductive source transmitter and a series of remote electric dipole receivers placed on the seafloor. The approach was tested at the Palinuro Seamount in the southern Tyrrhenian Sea, at a site where massive sulfide mineralization has been previously identified by shallow drilling. A 3D model of the Palinuro study area was created using bathymetry data, and forward modeling of the electric field diffusion was carried out using a finite volume method. These numerical results suggest that the remote receivers can theoretically detect a block of shallowly‐buried conductive material at up to ∼100 m away when the transmitter is located directly above the target. We also compared the sensitivity of the method using either a horizontal loop transmitter or a vertical loop transmitter and found that when either transmitter is located directly above the mineralized zone, the vertical loop transmitter has sensitivity to the target at a farther distance than the horizontal loop transmitter in the broadside direction by a few 10s of meters. Furthermore, the vertical loop transmitter is more effective at distinguishing the seafloor conductivity structure when the vertical separation between transmitter and receiver is large due to the bathymetry. As a horizontal transmitter is logistically easier to deploy, we conducted a first test of the method with a horizontal transmitter. Apparent conductivities are calculated from the electric field transients recorded at the remote receivers. The analysis indicates higher apparent seafloor conductivities when the transmitter is located near the mineralized zone. Forward modeling suggests that the best match to the apparent conductivity data is obtained when the mineralized zone is extended southward by 40 m beyond the zone of previous drilling. Our results demonstrate that the method adds value to the exploration and characterization of seafloor massive sulfide deposits.

Description: Gas hydrates are a potential energy resource, a possible factor in climate change and an exploration geohazard. The University of Toronto has deployed a permanent seafloor time-domain controlled source electromagnetic (CSEM) system offshore Vancouver Island, within the framework of the NEPTUNE Canada underwater cabled observatory. Hydrates are known to be present in the area and due to their electrically resistive nature can be monitored by 5 permanent electric field receivers. However, two cased boreholes may be drilled near the CSEM site in the near future. To understand any potential distortions of the electric fields due to the metal, we model the marine electromagnetic response of a conductive steel borehole casing. First, we consider the commonly used canonical model consisting of a 100 m, 100 m thick resistive hydrocarbon layer embedded at a depth of 1000 m in a 1 m conductive host medium, with the addition of a typical steel production casing extending from the seafloor to the resistive zone. Results show that in both the frequency and time domains the distortion produced by the casing occurs at smaller transmitter-receiver offsets than the offsets required to detect the resistive layer. Second, we consider the experimentally determined model of the offshore Vancouver Island hydrate zone, consisting of a 5.5 m, 36 m thick hydrate layer overlying a 0.7 m sedimentary half-space, with the addition of two borehole casings extending 300 m into the seafloor. In this case, results show that the distortion produced by casings located within a 100 m safety zone of the CSEM system will be measured at 4 of the 5 receivers. We conclude that the boreholes must be positioned at least 200 m away from the CSEM array so as to minimize the effects of the casings.

Description: Active mud volcanoes, where changing salinities of pore fluids, large temperature gradients and occurrences of free gas are frequently observed, potentially exhibit significant variability in their internal resistivity structure. In marine environments, these resistivity variations may be investigated using controlled source electromagnetic (CSEM) measurements. Within a RWE Dea funded investigation at the North Alex Mud Volcano (NAMV), we have developed a new high resolution CSEM system. The system consists of several autonomous electric dipole receivers and a lightweight electric dipole transmitter, which was mounted on a small remotely operated underwater vehicle (ROV). In an experiment carried out in November 2008, ten receivers were deployed over the surface of NAMV at a total of 16 receiver locations. During three successful ROV dives, the transmitter was deployed at a total of 80 locations. Measured transients are interpreted using 1D inversions, where good data fits can be achieved by models containing 2-3 layers. Generally, models show low resistivities close to the ocean floor, indicative for penetrating salt water and/or high temperatures. Toward greater depths, increasing resistivities presumably are due to a combination of compaction of sediments (i.e. reduced pore space), an increased presence of fresh water and possible occurrences of free gas. The increase in resistivity may exceed a factor of 10 or more and layer interfaces are indicated down to depths of up to 100m. A combination of 1D models reveals lateral resistivity contrasts, which are well in agreement with structures evident in 3D seismics.