ALBERT

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: 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.

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: 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 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.