Jared Peacock

Application of evolutionary methods to 3D geoscience modelling

Geoscience modelling plays a vital role in mapping and tracking the earths resources. Magnetotellurics, which maps the electrical resistivity of the subsurface, is a useful and cost-effective sounding-technique for sensing over a broad scale at depth. However, due to the inherent difficulty in sensing at depth, models produced using MT have a degree of uncertainty. Geoscientists can reduce this uncertainty by producing multiple alternative models, and using multiple modelling techniques and settings, to correlate robust model features with field data responses. Population-based evolutionary search techniques are of interest to MT modelling because they offer an alternative to deterministic techniques, and are able to produce multiple models for analysis. Unfortunately, evolutionary techniques have not been successfully applied to 3D MT modelling. In this work we describe a new, more compact, representation of MT models using volumetric functions. Using this representation we successfully apply evolutionary search techniques to 3D MT modelling for both artificial and real models and show how the development of large scale features during modelling can be correlated with the models fit to field data.

Crustal inheritance and a top-down control on arc magmatism at Mount St Helens

In a subduction zone, the volcanic arc marks the location where magma, generated via flux melting in the mantle wedge, migrates through the crust and erupts. While the location of deep magma broadly defines the arc position, here we argue that crustal structures, identified in geophysical data from the Washington Cascades magmatic arc, are equally important in controlling magma ascent and defining the spatial distribution and compositional variability of erupted material. As imaged by a three-dimensional resistivity model, a broad lower-crustal mush zone containing 3–10% interconnected melt underlies this segment of the arc, interpreted to episodically feed upper-crustal magmatic systems and drive eruptions. Mount St Helens is fed by melt channelled around a mid-Tertiary batholith also imaged in the resistivity model and supported by potential–field data. Regionally, volcanism and seismicity are almost exclusive of the batholith, while at Mount St Helens, along its margin, the ascent of viscous felsic melt is enabled by deep-seated metasedimentary rocks. Both the anomalous forearc location and composition of St Helens magmas are products of this zone of localized extension along the batholith margin. This work is a compelling example of inherited structural control on local stress state and magmatism.Crustal structures are as important as deep mantle melting in controlling magma ascent and the composition and distribution of erupted material, according to 3D resistivity modelling, geophysical data and the distribution of Quaternary volcanism.

Progress towards magnetotelluric time lapse monitoring of enhanced geothermal system fluids

Realization of enhanced geothermal systems (EGS) prescribes the need for novel technology to monitor fluid inclusion at depth. Magnetotellurics (MT) is a relatively cheap method that is sensitive to electrical conductivity contrasts as a function of depth. It is proposed that MT can be employed to monitor areal extent of an EGS reservoir by collecting measurements before, during and after fluids are injected. 3D forward modeling suggests changes in the MT response will be small, on the order of a few degrees in phase. Repeatability of the MT response is important and it is shown that most stations are with in a few percent. Presented are 3D forward models, repeatability confidence from apparent resistivity and phase as well as phase tensor analysis.