| Title | Mapping Hydrothermal Fractures Using Earthquakes and Resistivity |
|---|---|
| Authors | S. Onacha, E. Shalev & P. Malin |
| Year | 2006 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | |
| Abstract | We present results of a joint imaging (JGI) scheme based on closely spaced magnetotelluric (MT) and microearthquake data. Such data from the Krafla geothermal field was used to map structures and faults that are drilling targets for wells. The complex fluid circulation, tectonics, and volcanic activities produce microearthquake waves characteristic of the medium through which they propagate. The fault zones models assume that fluid-filled fractures cause electrical polarization and seismic anisotropy effects below the clay cap. We present evidence of correlations between polarization and anisotropy. Further, using resistivity measurements to determine fracture porosity appears valid for values of more than 5ߟm. The porosity/permeability imaged by the resistivity modeling shows that fracture porosity varies between 10-25%, with higher porosity occurring in between fault cores and host rocks. P-wave velocity models were generated from the resistivity data and used to locate microearthquakes. The microearthquakes appear to occur above the interpreted heat source, on the boundaries between low and high resistivity. Conversions of S to P waves occur close to the heat source, indicating that it is less competent than overlying rock, possibly only partially solid. Fault zones below the clay cap have a low resistivity cores, with increased in resistivity towards fracture zone margins. As a partial validation, a well, contracted prior to our targeting study, was subsequently drilled in an area predicted to be a poor target. The drilling validated the a priori conclusions drawn from the JGI map. While encouraging, we suggest that more complete examples and tests of the JGI methods need to be undertaken and validated for it to become a standard in geothermal exploration. |