Research Collection Review Article Advanced seismic processing/imaging techniques and their potential for geothermal exploration Author(s): Schmelzbach, Cédric; Greenhalgh, Stewart A.; Reiser, Fabienne; Girard, Jean-François; Bretaudeau, François; Capar, Laure; Bitri, Adnand Publication Date: 2016-11 Permanent Link: https://doi.org/10.3929/ethz-b-000188810 Originally published in: Interpretation 4(4), http://doi.org/10.1190/int-2016-0017.1 Rights / License: In Copyright - Non-Commercial Use Permitted This page was generated automatically upon download from the ETH Zurich Research Collection. For more information please consult the Terms of use. ETH Library t Special section: Geothermal energy Advanced seismic processing/imaging techniques and their potential for geothermal exploration Cédric Schmelzbach1, Stewart Greenhalgh1, Fabienne Reiser1, Jean-François Girard2, François Bretaudeau3, Laure Capar3, and Adnand Bitri3 Abstract Seismic reflection imaging is a geophysical method that provides greater resolution at depth than other meth- ods and is, therefore, the method of choice for hydrocarbon-reservoir exploration. However, seismic imaging has only sparingly been used to explore and monitor geothermal reservoirs. Yet, detailed images of reservoirs are an essential prerequisite to assess the feasibility of geothermal projects and to reduce the risk associated with expensive drilling programs. The vast experience of hydrocarbon seismic imaging has much to offer in illuminating the route toward improved seismic exploration of geothermal reservoirs — but adaptations to the geothermal problem are required. Specialized seismic acquisition and processing techniques with significant potential for the geothermal case are the use of 3D arrays and multicomponent sensors, coupled with sophis- ticated processing, including seismic attribute analysis, polarization filtering/migration, converted-wave processing, and the analysis of the diffracted wavefield. Furthermore, full-waveform inversion and S-wave split- ting investigations potentially provide quantitative estimates of elastic parameters, from which it may be pos- sible to infer critical geothermal properties, such as porosity and temperature. Introduction diffusive field geophysical techniques such as gravity Geothermal energy is a promising and increasingly surveying and electromagnetic imaging provide large- popular sustainable energy source for electricity gener- scale subsurface volume information (bulk properties). ation and district heating purposes (Lund, 2009). How- For example, electromagnetic methods have success- ever, considerable further research and development fully been used in geothermal exploration to image is necessary to optimize this renewable energy source the subsurface electrical resistivity distribution, which in terms of the engineering challenges and the geo- is sensitive to fluid content, temperature, and alteration scientific requirements to identify, locate, and produce (Muñoz, 2014). Passive seismic techniques such as am- geothermal reservoirs. In many places, the high temper- bient noise tomography and microseismicity studies atures needed for economical geothermal electricity pro- (Obermann et al., 2015) have been successfully used duction are generally found at depths of at least a few to characterize geothermal reservoirs and to monitor kilometers in the earth’s crust. Suitable subsurface imag- changes. However, it is essentially only active (con- ing tools are required to delineate and characterize geo- trolled source) seismic methods that can provide the thermal reservoirs in sufficient detail at these depths. necessary high-resolution fault and fracture characteri- Seismic reflection imaging has a greater depth of zation at depth necessary for successful well siting. penetration with reasonable resolution compared with Oil companies have explored in great detail many other geophysical methods used to investigate geother- sedimentary basins throughout the world; hence, much mal reservoirs (e.g., gravity, magnetic, electrical resis- can be learned from the oil and gas industry in terms of tivity, and electromagnetic surveys; Barbier, 2002). advanced exploration techniques. For example, frac- Seismic reflection investigations map interfaces associ- ture zone imaging and characterization is of equal ated with elastic-property contrasts (i.e., reflecting/ importance to hydrocarbon exploration as it is to geo- scattering features; Sheriff and Geldart, 1995; Yilmaz, thermal exploration. Key developments in land-seismic 2001) and can provide estimates of physical properties, acquisition and processing for oil and gas exploration such as seismic velocities and anisotropy. Potential and that are relevant to geothermal exploration include Downloaded 08/08/16 to 192.33.105.52. Redistribution subject SEG license or copyright; see Terms of Use at http://library.seg.org/ 1Institute of Geophysics, ETH Zurich, Switzerland. E-mail: [email protected]; [email protected]; fabienne. [email protected]. 2BRGM, Orléans, France and Institut de Physique du Globe de Strasbourg, Strasbourg, France. E-mail: [email protected]. 3BRGM, Orléans, France. E-mail: [email protected]; [email protected]; [email protected]. Manuscript received by the Editor 29 January 2016; revised manuscript received 18 March 2016; published online 4 August 2016. This paper appears in Interpretation, Vol. 4, No. 4 (November 2016); p. SR1–SR18, 9 FIGS. http://dx.doi.org/10.1190/INT-2016-0017.1. © 2016 Society of Exploration Geophysicists and American Association of Petroleum Geologists. All rights reserved. Interpretation / November 2016 SR1 3D and repeated 3D (4D; time-lapse) technology associated with expensive drilling programs. Moreover, (Lumley, 2001; Brown, 2004), multicomponent seismic seismic imaging enables more intelligent and selective measurements (Hardage et al., 2011), advanced and ef- drilling. Targets in geothermal exploration are per- ficient prestack migration schemes (prestack time and meable zones of sufficiently high temperature and fluid depth migration, reverse time migration; Yilmaz, 2001), movement; such zones are mostly controlled by faults seismic attribute and amplitude variation with offset and fractures. Hence, the focus in seismic geothermal analyses (Luo and Evans, 2004; Chopra and Marfurt, exploration is mapping deep sedimentary and basement 2007), and anisotropy studies (Crampin, 1985). structures, such as faults and fracture zones. Enhanced geothermal systems (EGS) have their Various sophisticated seismic imaging techniques maximum potential in crystalline basement rock have been developed for the oil and gas industry as well (Tester et al., 2007) in which temperatures are suffi- as for the mining industry, not only for exploration but ciently elevated but the permeability is low and in need also for monitoring of reservoirs during production. It of artificial stimulation. So far, extensive seismic explo- needs to be investigated how these techniques can be ration studies over hard-rock environments (for a com- adapted and applied to geothermal sites to improve the prehensive review, see Eaton 2003) are mainly related planning and development of geothermal reservoirs. In to mineral exploration (Milkereit et al., 1994; Drum- this paper, we (1) summarize the challenges and current mond et al., 2003; Malehmir et al., 2012), mapping frac- status of geothermal seismic exploration, (2) discuss ture zones primarily for finding suitable underground how geothermal seismic exploration can benefit from repositories for radioactive waste (Green and Mair, experience with seismic methods in hydrocarbon and 1983; Juhlin and Palm, 2003; Schmelzbach et al., 2007), metalliferous-ore exploration, and (3) review a selec- and assorted geologic studies, especially crustal inves- tion of advanced seismic processing techniques that tigations (Clowes et al., 1984; Juhlin et al., 1998; Cook could potentially be of value for the seismic imaging et al., 1999; Schmelzbach et al., 2008a, 2008b). and evaluation of geothermal reservoirs. There are several key challenges one needs to face in hard-rock seismic exploration (Salisbury et al., 2003; Status of geothermal seismic exploration Greenhalgh and Manukyan, 2013): (1) reflection ampli- Permeable fracture zones control the fluid flow in tudes are generally weak, leading to low signal-to-noise sedimentary and hard-rock geothermal reservoirs and, ratio (S/N) that makes it difficult to image features therefore, need to be considered in the planning of within crystalline rocks; (2) reflectors are often small, wells. Some examples of seismic reflection investiga- steeply dipping, and laterally discontinuous due to tions in geothermal areas for EGS and hydrothermal more complex morphology, lithology, and deformation; systems are described in the following two subsections. (3) the high velocities in crystalline basement result in a loss of resolution due to the relatively longer wave- Hydrothermal systems lengths; (4) obtaining reliable velocity information in A geothermal system that contains a naturally occur- deep basement is problematic without long aperture re- ring permeable layer or layers and a large amount of cordings; and (5) anisotropy can be introduced through fluid or vapor that circulates in the subsurface is called fractures and layering, leading to complex wave propa- a hydrothermal system (Barbier, 2002). Whether the gation. Milkereit and Eaton (1998) discuss in detail the system is economically