Geophysical Investigation of the Yellowstone Hydrothermal System (original) (raw)
Related papers
2005
In order to investigate the interface conduction in the basaltic rocks of the high temperature fields in Iceland we measured the electrical conductivity for the frequency range 0.1 -100000 Hz versus pore fluid salinity of 12 selected samples of basaltic material from Iceland. These included 2 fresh and completely unaltered samples of recent basaltic lava, 5 samples of basalt and hyaloclastite from the smectite alteration zone and 5 from the chlorite zone. About 2-5% reduction in conductivity is observed per decade in frequency. For the unaltered samples a linear relationship is found between the bulk conductivity and the pore fluid conductivity over almost the whole range of salinities, showing that the pore fluid conduction is always dominant and the interface conduction is negligible. The samples from the smectite zone show almost no dependence on the pore fluid salinity but considerable interface conduction as predicted, the value being from 20-300 µS/cm with the isoelectrical po...
1998
Geothermal zones in volcanic areas are characterized by rough topographic relief and variable shallow resistivities represented by hotsprings, modern and fossil hydrothermally altered zones, and by the presence of different episodes of lavic and explosive volcanism. We examine the performance of in-loop transient electromagnetic (TEM) and vertical electric soundings (VES) in a geothermal area with these characteristics. The inversion of 62 VES and 55 TEM soundings results in similar average stratified models. However, model assessment using the singular value decomposition method show that the TEM soundings possess a higher resolving power. Higher data redundancy, better data quality, less sensitivity to topographic and near-surface inhomogen e- ities, and less sensitivity to equivalence problems associated with thin conductive layers are contributing factors to the supe rior performance of the TEM method.
Electromagnetic mapping of electrical conductivity beneath the Columbia basalts1
Geophysical Prospecting, 1996
Sedimentary rocks beneath the Columbia River Basalt Group are recognized as having potential for oil and gas production, but the overlying layered basalts effectively mask seismic reflections from the underlying sediments. Four electromagnetic (EM) methods have been applied on profiles crossing Boylston Ridge, a typical east-west trending anticline of the Yakima Fold Belt, in an attempt to map the resistivity interface between the basalts and the sediments and to map variations in structure and resistivity within the sediments. The EM surveys detected strong variations in resistivity within the basalts, and in particular the continuous magnetotelluric array profiling (EMAP) revealed resistivity lows beneath the surface anticlines. These low resistivity zones probably coincide with fracturing in the core of the anticlines and they appear to correlate well with similar zones of low seismic velocity observed on a nearby seismic profile. The controlled-source EM surveys (in-loop transient, long-offset transient, and variable-offset frequency-domain) were designed in anticipation of relatively uniform high resistivity basalts, and were found to have been seriously distorted by the intrabasalt conductors discovered in the field. In particular, the resistivity sections derived from 1D inversions were found to be inconsistent and misleading. The EMAP survey provided the most information about the subsurface resistivity distribution, and was certainly the most cost-effective. However, both controlledsource and EMAP surveys call for accurate 2D or 3D inversion to accommodate the geological objectives of this project.
Transient Electromagnetic Resistivity Survey at the Geysir Geothermal Field South Iceland
2003
A general description of the resistivity method is given, and the results of a TEM resistivity survey at the Geysir geothermal field, S-Iceland are presented. The effects of temperature, porosity, salinity, and water-rock interaction on resistivity of rocks are briefly described. Theoretical aspects of DC resistivity measurements are presented and the general principles and application of Transient Electromagnetic (TEM) soundings are discussed. Comparison of DC and TEM resistivity soundings is dealt with briefly. The famous Geysir high-temperature area is in a shallow valley elongated N-S in the upper southern lowlands of Iceland. A total of 9 TEM stations were interpreted with the one-dimensional inversion program, CLTINV. Results reveal the resistivity distribution of the rock formation in the uppermost kilometre. The resistivity in the uppermost 100-400 m is quite high, ranging between 200 and 3000 ohm-m. Below that, the resistivity is low, and at some level it is obviously influ...
The Role of Electrical Interface Conduction in Geothermal Exploration
In order to investigate the interface conduction in the basaltic rocks of the high temperature fields in Iceland we measured the electrical conductivity for the frequency range 0.1 – 100000 Hz versus pore fluid salinity of 12 selected samples of basaltic material from Iceland. These included 2 fresh and completely unaltered samples of recent basaltic lava, 5 samples of basalt and hyaloclastite from the smectite alteration zone and 5 from the chlorite zone. About 2-5% reduction in conductivity is observed per decade in frequency. For the unaltered samples a linear relationship is found between the bulk conductivity and the pore fluid conductivity over almost the whole range of salinities, showing that the pore fluid conduction is always dominant and the interface conduction is negligible. The samples from the smectite zone show almost no dependence on the pore fluid salinity but considerable interface conduction as predicted, the value being from 20-300 µS/cm with the isoelectrical p...
2017
Geothermal exploration involves geology, geochemistry and geophysics. In geophysical exploration, resistivity surveying plays the most important role in delineating the reservoir. The parameters that control the geothermal system show a strong response to electrical resistivity. The resistivity methods that are mostly used in geothermal exploration in Iceland are TEM (Transient ElectroMagnetics) and MT (MagnetoTellurics). The application of these methods is discussed in this report together with an example from the Eyjafjördur low-temperature area in N-Iceland. The resulting resistivity cross-sections and resistivity depth slices show a shallow lying low-resistivity layer and a deep lying low-resistivity anomaly towards the end of the cross-sections. The result of this work has been compared with results from Flóvenz and Karlsdóttir (2000) which interpreted TEM data from the same area. The results are also compared with borehole data and stratigraphy.
Bulls eye! – Simple Resistivity Imaging to Reliably Locate the Geothermal Reservoir
This paper presents interpretations of MT surveys from a number of geothermal fields in steep terrain, where the geophysical interpretation can be constrained by data from deep exploration or production wells. One particular anomaly, namely the shape of the base of the conductive layer overlying the high-temperature geothermal reservoir, strongly correlates with well temperatures and even well productivity. The proposed interpretation method is simple and does not require particularly sophisticated analysis techniques or computing power. It does not even require particularly highquality MT measurements, and has given good results with 10-year old data. As long as the initial survey is welldesigned and executed, the method gives a clear "bulls-eye" target that reliably locates the geothermal reservoir.