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Title: Locating an active fault zone in Coso geothermal field by analyzing seismic guided waves from microearthquake data

Abstract

Active fault systems usually provide high-permeability channels for hydrothermal outflow in geothermal fields. Locating such fault systems is of a vital importance to plan geothermal production and injection drilling, since an active fault zone often acts as a fracture-extensive low-velocity wave guide to seismic waves. We have located an active fault zone in the Coso geothermal field, California, by identifying and analyzing a fault-zone trapped Rayleigh-type guided wave from microearthquake data. The wavelet transform is employed to characterize guided-wave's velocity-frequency dispersion, and numerical methods are used to simulate the guided-wave propagation. The modeling calculation suggests that the fault zone is {approx} 200m wide, and has a P wave velocity of 4.80 km/s and a S wave velocity of 3.00 km/s, which is sandwiched between two half spaces with relatively higher velocities (P wave velocity 5.60 km/s, and S wave velocity 3.20 km/s). zones having vertical or nearly vertical dipping fault planes.

Authors:
Publication Date:
Research Org.:
University of North Carolina, Chapel Hill, NC; Duke University, Durham, nC
Sponsoring Org.:
USDOE
OSTI Identifier:
889361
Report Number(s):
SGP-150-16
TRN: US200619%%802
Resource Type:
Conference
Resource Relation:
Conference: Proceedings, Twentieth Workshop on Geothermal Reservoir Engineering, Stanford University, Stanford, CA, January 24-26, 1995
Country of Publication:
United States
Language:
English
Subject:
02 PETROLEUM; 15 GEOTHERMAL ENERGY; CALIFORNIA; DRILLING; GEOTHERMAL FIELDS; MICROEARTHQUAKES; P WAVES; PRODUCTION; RESERVOIR ENGINEERING; S WAVES; SEISMIC WAVES; SIMULATION; VELOCITY; Geothermal Legacy

Citation Formats

SGP-TR-150-16. Locating an active fault zone in Coso geothermal field by analyzing seismic guided waves from microearthquake data. United States: N. p., 1995. Web.
SGP-TR-150-16. Locating an active fault zone in Coso geothermal field by analyzing seismic guided waves from microearthquake data. United States.
SGP-TR-150-16. Thu . "Locating an active fault zone in Coso geothermal field by analyzing seismic guided waves from microearthquake data". United States. doi:. https://www.osti.gov/servlets/purl/889361.
@article{osti_889361,
title = {Locating an active fault zone in Coso geothermal field by analyzing seismic guided waves from microearthquake data},
author = {SGP-TR-150-16},
abstractNote = {Active fault systems usually provide high-permeability channels for hydrothermal outflow in geothermal fields. Locating such fault systems is of a vital importance to plan geothermal production and injection drilling, since an active fault zone often acts as a fracture-extensive low-velocity wave guide to seismic waves. We have located an active fault zone in the Coso geothermal field, California, by identifying and analyzing a fault-zone trapped Rayleigh-type guided wave from microearthquake data. The wavelet transform is employed to characterize guided-wave's velocity-frequency dispersion, and numerical methods are used to simulate the guided-wave propagation. The modeling calculation suggests that the fault zone is {approx} 200m wide, and has a P wave velocity of 4.80 km/s and a S wave velocity of 3.00 km/s, which is sandwiched between two half spaces with relatively higher velocities (P wave velocity 5.60 km/s, and S wave velocity 3.20 km/s). zones having vertical or nearly vertical dipping fault planes.},
doi = {},
journal = {},
number = ,
volume = ,
place = {United States},
year = {Thu Jan 26 00:00:00 EST 1995},
month = {Thu Jan 26 00:00:00 EST 1995}
}

Conference:
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  • A large number of microearthquake seismograms have been recorded by a downhole, three-component seismic network deployed around the Coso, California geothermal reservoir. Shear-wave splitting induced by the alignment of cracks in the reservoir has been widely observed in the recordings. Over 100 events with body wave magnitude greater than 1.0 from microearthquakes recorded since March of 1992. have been processed. The results show that most events with paths within the critical angle that defines the shear-wave window, display clear shear-wave splitting, and the fast shear waves have predominant polarization directions for most stations. The rose diagrams of fast shear-wave particlemore » motion suggest that there are three predominant fracture strikes (or directions of maximum horizontal stresses) in the Coso geothermal field: N 40°-60°E, N 0°-25°E, and N 25° - 35° W, which are consistent with photographically or magnetically mapped alignments on the surface. From the delay time of split shear waves, we estimate that the crack density in the most active geothermal reservoir area (above 3.00km depth) ranges between 0.030 and 0.055. values commonly found in other hydrocarbon or geothermal reservoirs.« less
  • The Coso geothermal area, California, has produced hot water and steam for electricity generation for more than 20 years, during which time there has been a substantial amount of microearthquake activity in the area. Seismicity is monitored by a high-quality permanent network of 16 three-component digital borehole seismometers operated by the US Navy and supplemented by a ~ 14-station portable array of surface three-component digital instruments. The portable stations improve focal sphere coverage, providing seismic-wave polarity and amplitude data sets sufficient for determining full moment-tensor microearthquake mechanisms by the linear-programming inversion method. We have developed a GUI-based interface to thismore » inversion software that greatly increases its ease of use and makes feasible analyzing larger numbers of earthquakes than previously was practical. We show examples from an injection experiment conducted in well 34-9RD2, on the East Flank of the Coso geothermal area. This tight well was re-drilled February – March 2005 with the intention of hydrofracturing it, but instead, pervasive porosity and fractures were encountered at about 2660 m depth. Total drilling mud losses occurred, obviating the need to stimulate the well. These mud losses induced a 50-minute swarm of 44 microearthquakes, with magnitudes in the range -0.3 to 2.6. Most of the largest microearthquakes occurred in the first 2 minutes. Accurate relative relocations and moment tensors for the best-recorded subset reveal fine details of the fracture that was stimulated. This comprised a fault striking at N 20° E and dipping at 75° to the WNW, which propagated to the NNE and upward. Co-injection focal mechanisms reveal combined crack-opening and shear motion. Stress release and mode of failure differed between the pre-, co- and post-swarm periods. Some post-swarm events involved cavity collapse, suggesting that some of the cavity opening caused by the fluid injection was quickly reversed. Stress & mode of failure had not returned to pre-swarm conditions within 1 month following the injection, posing the question of how long stress perturbations persist following a stimulation experiment. This question may be answered by processing data spanning a longer post-injection period, work that is currently underway and will be reported in this presentation.« less
  • Recent field reconnaissance, petrography, nanno and foraminifera age determinations, and seismic stratigraphy of the Sepik and Piore subbasins of northern New Guinea reveal the existence of an extensive, tectonically unstable, Miocene-Pliocene carbonate shelf system. These findings represent the first recorded evidence of northern Papuan limestones coeval in age to those of the hydrocarbon productive Salawati Basin of Irian Jaya. Moreover, these observations also demonstrate the significance of episodic activities of the northern New Guinea fault zone upon the changes in carbonate sedimentation and diagenesis. During the Neogene, algal biosparites to foraminiferal biomicrites defined the clean portion of a mixed clastic-carbonatemore » shelf system of the northern New Guinea basin, which began at the central New Guinea cordillera and deepened northward. This shelf was interrupted by coral-coralline algal boundstone fringing- to patch-reef buildups with associated skeletal grainstones. Clean carbonates were spatially and temporally restricted to basement blocks, which episodically underwent uplift while terrigenous dilutes carbonates were more common in adjacently subsiding basement block bathymetric lows. These tectonic expressions were caused by the spatially transient nature of constraining bends of the evolving north New Guinea faults. As shown by seismic stratigraphy, by the late Miocene to the early Pliocene the uplift of the Bewani-Torricelli Mountains sagittally divided the shelf of the northern New Guinea basin into the Ramu-Sepik and the Piore basins. Continued regional sinistral transpression between the Pacific and the New Guinea leading edge of the Indo-Australian plates led to the reverse tilting of the Piore basin, the shallowing of the former distal shelf with concomitant extensive biolithite development (e.g., on subsiding volcanic islands) eventual uplifting of the Oenake Range, and en echelon faulting of the Bewani-Torricelli Mountains.« less
  • The fault arrays within fault zones are commonly highly organized, suggesting that simple processes are controlling the overall evolution of the system. Most analyses of slip data depend upon a dynamic interpretation, typically formulated as an inverse modeling scheme. A more realistic forward model of the finite kinematics should be provided by consideration of secondary stresses due to proximity of a rupture front. Approximating the secondary stress field using linear elastic fracture mechanics, the merits of the homogeneous dynamic theory can be evaluated by comparison with results from simulations of superimposed slip events. It was found that sets of randomlymore » oriented fractures may record numerous overlapping slip events and large variations in lineation orientation accompany spatial variations in rupture termination, complexities not represented in a dynamic scenario. A robust feature of slip sets formed in an odd axis stress state are ubiquitous great circle distributions of M-poles the pole to which lies collinear to the odd axis. The computer model was field tested using fault geometries measured in the Black Mountain Frontal Fault Zone. Results of this modeling shed light on several features commonly observed in slickenside data sets, including; bi-clustering and great circle distributions of lineations, curvilinear slickensided surfaces, great circle M-pole distributions and anomalous lineation clusters with respect to movement on nearby major faults. These observations can be explained by combinations of; incomplete resetting, incrementally shallowing fault planes, spatial variations in fault terminations, incremental and conjugate slip about an inclined principle stress. These results strongly suggest that thorough documentation of local fault geometries is essential for proper interpretation of slip data obtained from large fault zones and that a regional dynamic approach may be overly simplistic in such studies.« less
  • Fault-zone guided waves have been identified in microearthquake seismograms recorded at the Coso Geothermal Field, California. The observed guided waves have particle motions and propagation group velocities similar to Rayleigh wave modes. A numerical method has been employed to simulate the guided-wave propagation through the fault zone. By comparing observed and synthetic waveforms the fault-zone width and its P- and S-wave velocity structure have been estimated. It is suggested here that the identification and modeling of such guided waves is an effective tool to locate fracture-induced, low-velocity fault-zone structures in geothermal fields.