Uncertain Estimation of Clay Cap Boundaries using Magnetotelluric and MeB Data: Application to Wairākei Geothermal Field

While the magnetotelluric (MT) method has been used in geothermal exploration for many years, the integrated evaluation of the effects of reservoir temperature, clay alteration, salinity and permeability on electrical resistivity have remained highly qualitative. In order to contribute to quantifying these effects, we develop an inversion methodology that integrates methylene blue (MeB) logs from wells with the electrical resistivity distribution from MT surveys. We use this method to infer, under uncertainty, clay cap boundaries in a geothermal field. Our methodology is based on a fast stochastic inversion of MT signals using Markov Chain Monte Carlo (MCMC) to fit a one-dimensional three-layer resistivity model beneath each MT station, laterally constrained by MeB data. The advantage of our approach is the explicit investigation and visualization of inversion uncertainty, yielding inferred depth intervals for clay cap boundaries. The methodology is tested on a profile of MT stations and MeB logs in wells located in the western area of the Wairākei geothermal field in the Taupō Volcanic Zone, New Zealand. Over the long-term, our goal is to improve our knowledge of uncertainty related to MT signals and its graphical representation to enhance our understanding of the relationship between resistivity and the clay cap

Heat Transfer Through the Wairakei-Tauhara Geothermal System Quantified by Multi-Channel Data Modeling

To obtain the fullest picture of geothermal systems, it is necessary to integrate different types of data, for example, surface electromagnetic surveys, lithology, geochemistry, and temperature logs. Here, by joint modeling a multichannel data set we quantify the spatial distribution of heat transfer through the hydrothermally altered, impermeable smectite layer that has developed atop the Wairakei-Tauhara system, New Zealand. Our approach involves first constraining magnetotelluric inversion models with methylene blue analysis (an indicator of conductive clay) and mapping these onto temperature and lithology data from geothermal wells. Then, one-dimensional models are fitted to the temperature data to estimate heat flux variations across the field. As a result, we have been able to map the primary seal that insulates the geothermal reservoir and estimate the heat flow of the system. The approach could be applied in geothermal provinces around the world with implications for sustainable resource management and our understanding of these magmatic systems

Bayesian Inversion for Layered Spherical Symmetric Earth Conductivity Model from Global Magnetic Data

Abstract In the Bayesian perspective, inference on model parameters from observed data is performed by calculating the likelihood of the data given prior model parameters, i.e. to estimate the posterior probability of model parameters. With the advent of computational resources, there are increasing interests in resolving full non-linear inverse problems using global approach. Although the current trends are geared towards algorithms to efficiently explore the model space, we employed the classical “pure” Monte Carlo method to resolve the inverse problem in the global scale induction study. Observatory and satellite magnetic data are used to provide insight on the deep mantle conductivity. In this case, layered (1D) spherical symmetric conductivity model can be considered as adequate to represent the Earth’s conductivity variation with depth. Model parameters (resistivities and thicknesses) with uniform probabilities over predefined intervals are drawn as samples of the model space. Reliable posterior estimates are derived from a large number of samples which are still manageable with the current PC technology. Relatively small uncertainties of the posterior estimates suggest that the Monte Carlo method is adequately sampled the model space with a small number of model parameters. Our results are consistent with a monotonic increase of conductivity with depth with a marked inflexion at about 700-900 km, while discontinuities at 410 km and 660 km known from seismic and petrology data seem unresovable directly from EM data.</jats:p

Etude du champ magnétique anormal d'origine intralithosphérique

The separation of the anomalous magnetic field with respect to the depth of lateral contrasts of conductivity is discussed in this paper. It is shown that, for most geophysical cases, this separation is very simply performed for periods greater than 1800 s: the anomalous field of an intralithospheric source is the difference between the observed anomalous field and the one related to the lateral contrasts of conductivity in the superficial part of the crust (mostly the coast effect).Electromagnetic data observed in California are analysed using this method. An anomalous field of intralithospheric source is shown to exist. The study of this field provides us with indications about lateral variations of the physical and chemical conditions in the lithosphere; the obtained results are in good agreement with data drawn from other Earth sciences.Finally, we show that such results can be obtained in other tectonically active areas: Japan and Peruvian Andes. </jats:p