Interpreted fracture anomalies: joint imaging of geophysical signals from fluid-filled fracture zones in geothermal fields

Stephen Onacha, E Shalev, P Malin, P Leary, L Bookman

Research output: Chapter in Book/Report/Conference proceedingConference PaperResearch


We present the theory and application of a joint imaging and interpretation approach to seismic shear-wave splitting and electrical resistivity polarization due to aligned, fluid-filled fracture zones. Previously, we have developed and tested methods for joint geophysical imaging (JGI) and interpretation of seismic velocity and electrical conductivity. The methods included coupling of physical properties as well as geological structures.

Our examples come from microearthquake (MEQ) recordings and magnetotelluric (MT) soundings in geothermal fields in Kenya and Iceland. We find that two main polarization directions align with both younger fractures and tectonically activated older fracture systems. The complex interconnection between these fracture zones causes heterogeneity and anisotropy, which determines fluid movement. As such, our field data and their joint interpretation have been used to successfully target geothermal exploration and production wells.

Our experience suggests that the best targets for drilling geothermal production wells are found along fracture zones that are aligned with tectonically activated fractures. Intersections of different trending fractures also form important fluid up-flow channels. The alignment of fractures can be used to successfully drill high production wells. Due to heterogeneity and anisotropy, it is difficult to determine and predict fluid flow pathways and therefore routinely drilling across interpreted surface fractures does not always guarantee success. In our Kenyan example, the most successful wells in a new field were targeted along NW trending fractures which form a zone of enhanced permeability. A similar result was found in the Icelandic example. From these cases we have concluded that the remedy for finding successful drilling targets is to focus on understanding the heterogeneity and anisotropy at depth, an instance of which is the joint-imaging methods described here.
Original languageEnglish
Title of host publicationProceedings of the World Geothermal Congress
Number of pages8
Publication statusPublished - 2010
Externally publishedYes
EventWorld Geothermal Congress 2010 - Bali, Indonesia
Duration: 25 Apr 201029 Apr 2010


ConferenceWorld Geothermal Congress 2010

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