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: Where several different kinds of geophysical datasets have been acquired from a particular region, each of these can contain valuable information about the Earth, which may not be present in the other datasets. Jointly determining a common model, therefore, often gives a more thorough and more constrained description of the Earth structure than considering each dataset individually. For example, a seismic velocity inversion is only weakly constrained by first arrival seismic refraction data, but considering it alongside Magneto-Telluric (MT) and gravity data can greatly assist in the constraint (Jegen-Kulcsar et al., 2009). Strategies for joint inversion are therefore an active area of research. To date, most schemes for accomplishing this have been deterministic in nature. Using a deterministic technique often means that it is conceptually difficult to include prior beliefs about the system under determination, uncertainties both in measurement and the relationship between the different physical quantities (velocity, resistivity, density), and the discrepancy between the model and the real Earth. Statistical strategies such as MCMC (Markov Chain Monte Carlo) model searches exist for assessing this kind of problem, but the number of potentially computationally expensive forward model runs required to effectively sample the whole model space and thus achieve a meaningful result is normally prohibitively high (〉 105), even for simple 1D models, so such schemes are not generally implemented. However, a technique known as emulation is used in various scientific fields eg. cosmology (Vernon and Goldstein, 2009), whereby computationally expensive forward modelling code (a simulator) is approximated by an uncertainty-calibrated computationally cheap function. Here we apply emulation to the problem of stochastic joint model determination. We show that emulation can be used to quickly exclude large areas of implausible model space, allowing fast updating of beliefs about an Earth structure. It thus provides a means by which the input model space for a deterministic inversion or MCMC scheme can be greatly reduced. We also show how an emulator can, by itself, effectively constrain a region of the Earth. We demonstrate the concept using a 1D model.

Description: Joint inversion strategies for geophysical data have become increasingly popular since they allow to combine complementary information from different data sets in an efficient way. Here, we present a non-linear joint inversion scheme, in which data from different methods are inverted separately and are joined through constrains accounting for parameter relationships. To avoid that the convergence behavior of the inversions is not profoundly disturbed by this coupling, the strengths of the constraints are re-adjusted at each iteration. In contrast to a joint inversion with a fixed parameter relationship, where data is inverted to one common model, this scheme requires no relative weighting of the data sets from different methods. Moreover, we observe that the adaption of the coupling strengths makes the convergence of the inversions much more robust. When we test our scheme with and without adaption on a synthetic 2-D model with seismic tomography, gravity and MT data, the final results with adaption were significantly closer to the true model. Finally, we observe that the adaptive scheme can to some extent handle models with structures for which the assumed parameter relationships are invalid.

Description: We present the first results of an electromagnetic survey for gas hydrates offshore Taiwan using a novel marine controlled-source electromagnetic system. Seismic evidence suggests the presence of gas hydrates and free gas in both accretionary (Four-Way-Closure) and erosional (Formose Ridge) settings to the southwest of the island, but complementary geophysical techniques are required to further quantify the distribution and concentration of the deposits. Electromagnetic experiments were conducted along profiles in both regions and show an increase in apparent resistivity at depth, which may be associated with the presence of methane hydrates. However, both profiles are characterized by severe bathymetric relief, in some cases having slope angles greater than 30 degrees, such that the apparent resistivity section may be biased by tilts of the instruments. We therefore derive a first-order bathymetric correction which can be applied to apparent resistivities and tested the correction procedure on data collected at Four-Way-Closure. Results show that increased apparent resistivities persist and reach up to 7Ωm, which suggests the presence of significant concentrations of hydrate or free gas at this location.

Description: Two marine geophysical methods are tested over a shallowly buried and hydrothermally inactive massive sulfide occurrence at the Palinuro Seamount in the Tyrrhenian Sea. A novel EM configuration consisting of a ship-towed loop transmitter and remote dipole receivers was deployed over the seamount, and electric field transients were successfully recorded by the remote receivers. An SP system consisting of two perpendicular pairs of electrodes towed close to the seafloor was also deployed. Anomalously high electric field strengths were recorded over the zone of known massive sulfide mineralization, demonstrating that the SP method is effective at detecting inactive, buried massive sulfide sites.

Description: We developed a joint inversion algorithm capable to invert gravity, seismic tomography and MT data to one common earth model. The development was motivated by the sub‐basalt imaging problem in the frontier areas of the north‐west European margin. Tests on synthetic data sets show that while no single geophysical method was able to recover the given sub‐basalt structure by itself, the entire structure could be recovered when a joint inversion algorithm was used.