Crustal constraint through complete model space screening for diverse geophysical datasets facilitated by emulation

Deep crustal constraint is often carried out using deterministic inverse methods, sometimes using seismic refraction, gravity and electromagnetic datasets in a complementary or “joint” scheme. With increasingly powerful parallel computer systems it is now possible to apply joint inversion schemes to derive an optimum model from diverse input data. These methods are highly effective where the uncertainty in the system is small. However, given the complex nature of these schemes it is often difficult to discern the uniqueness of the output model given the noise in the data, and the application of necessary regularization and weighting in the inversion process means that the extent of user prejudice pertaining to the final result may be unclear. We can rigorously address the subject of uncertainty using standard statistical tools but these methods also become less feasible if the prior model space is large or the forward simulations are computationally expensive. We present a simple Monte Carlo scheme to screen model space in a fully joint fashion, in which we replace the forward simulation with a fast and uncertainty-calibrated mathematical function, or emulator. This emulator is used as a proxy to run the very large number of models necessary to fully explore the plausible model space. We develop the method using a simple synthetic dataset then demonstrate its use on a joint data set comprising first-arrival seismic refraction, MT and scalar gravity data over a diapiric salt body. This study demonstrates both the value of a forward Monte Carlo approach (as distinct from a search-based or conventional inverse approach) in incorporating all kinds of uncertainty in the modelling process, exploring the entire model space, and shows the potential value of applying emulator technology throughout geophysics. Though the target here is relatively shallow, the methodology can be readily extended to address the whole crust

Joint Inversion of marine MT, Gravity and Seismic Data.

Exploration of sub-basalt targets is difficult because the basalt units reflect and scatter seismic energy, masking the characteristics of the underlying structure. Electromagnetic soundings are less sensitive to the highly resistive basalt units but are strongly influenced by the characteristics of the over and underlying sedimentary structures. So electromagnetic soundings are a valuable compliment to seismic surveys in such areas. We have developed a joint interpretation scheme and inversion algorithm of seismic, MT (magneto-telluric) and gravity data that combines the information content in these data. While each method in itself is able to resolve only a part of the subsurface, we demonstrate that our joint inversion/interpretation technique allows us to identify the base of the basalt and yields information about the underlying sediment. Using synthetic data we demonstrate how the various data types contribute to our inversion/interpretation technique, and show how we recover the sub-basalt structure. We then apply this combined scheme on marine MT, satellite gravity and long-offset seismic data acquired along FLARE-10 profile. This work has been conducted under the EU funded SIMBA project and the FLARE-10 seismic data were made available to us by Amerada Hess