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: Fluids entering the subduction zone play a key role in the subduction process. They cause changes in the dynamics and thermal structure of the subduction zone1, and trigger earthquakes when released from the subducting plate during metamorphism. Fluids are delivered to the subduction zone by the oceanic crust and also enter as the oceanic plate bends downwards at the plate boundary. However, the amount of fluids entering subduction zones is not matched by that leaving through volcanic emissions4 or transfer to the deep mantle, implying possible storage of fluids in the crust. Here we use magnetotelluric data to map the entire hydration and dehydration cycle of the Costa Rican subduction zone to 120 km depth. Along the incoming plate bend, we detect a conductivity anomaly that we interpret as sea water penetrating down extensional faults and cracks into the upper mantle. Along the subducting plate interface we document the dehydration of sediments, the crust and mantle. We identify an accumulation of fluids at ~20–30 km depth at a distance of 30 km seaward from the volcanic arc. Comparison with other subduction zones5–14 indicates that such fluid accumulation is a global phenomenon. Although we are unable to test whether these fluid reservoirs grow with time, we suggest that they can account for some of the missing outflow of fluid at subduction zones.