Environmental impact analysis of potential geothermal resource areas : Circular C-106

State of Hawaii. Department of Land and Natural Resources, Division of Water and Land Development
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Division of Water and Land Development, Department of Land and Natural Resources, State of Hawaii
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The compilation of public geophysical data for the Kilauea east rift zone has been completed at a scale of 1:100,000. The known geothermal resource is well expressed as a maximum in the self-potential data and as one of several resistivity lows in the electrical resistivity data base. Regional aeromagnetic data indicate a complex geologic structure for the east rift zone that is not apparent in the gravity or regional resistivity data. I conclude that a detailed, low level aeromagnetic survey offers the most promise for defining the subsurface structural features (and possibly the gross temperature distribution) of the east rift zone in a cost effective manner. The subsurface temperature data base is limited to a small portion (Puna area) of the east rift zone, and the self-potential data base is also incomplete.The HGP-A geothermal reservoir appears to be localized by the intersection of the center of the rift zone and a north-northwest trending structure, probably a transform fault. The structure is indicated by gravity, aeromagnetic, microearthquake and self-potential data. This reservoir is not well delineated by the resistivity data due to numerous problems in obtaining detailed electrical resistivity data in this volcanic environment. Although the HGP-A area appears to be the most promising setting for a geothermal reservoir, based on the existing data, a resistivity (8) and self-potential anomaly (A) seven kilometers southwest of HGP-A also seems promising. Other geothermal reservoirs may be present but not clearly indicated because of the irregular distribution of electrical resistivity, self-potential and well temperature data. A detailed, low level aeromagnetic survey is recommended as the highest priority for further exploration. Such a survey offers the possibility of determining the position and geometry of structural features and relative magnetizations along the rift. Suggested survey parameters are 1) line spacing no greater than 400 m in the Puna area but as much as 800 m for areas to the west; 2) a smoothly draped flight path between 200 and 300 meters above mean terrain level; 3) flight direction north-south or northwest perpendicular to the rift; 4) several tie lines along the trend of the rift to closely define irregularities within the rift itself. Acquisition of additional (proprietary) geophysical data may well be cost-effective if the duplication with existing data is not great, and the data are of good quality. It is apparent that all roads and trails of even marginal accessibility have been considered for ground electrical work (resistivity andself-potential) and acquiring new survey data without reasonable access will be both costly and slow. Pending a review of available proprietary data and detailed aeromagnetic data, preferably at a scale of 1:24,000, the cost-effectiveness of new surveys should again be evaluated. Controlled source audiomagnetotelluric (CSAMT) may be a cost-effective technique for mapping the low resistivity second layer in detail, but the expected responses should be numerically modeled prior to undertaking new surveys. Seismic refraction surveys have provided a useful regional velocity model and the best estimates to the top of the dike complex. In a conceptual sense one would like to map the detailed velocity structure including and surrounding the geothermal areas. In practice this would be difficult to carry out, would require large receiver arrays and should be considered high risk in terms of results versus costs. Shot-to-receiver distances of 5 to 15 km would be required for velocity mapping for depths of one to two km. This would probably require charges greater than 10 pounds of Aquagel, based on Suyenaga's experience, and would dictate offshore or remote area shot locations. There may be some merit to considering refraction fan arrays in hopes of determining small velocity lags near the geothermal sites which could be interpreted in terms of fracturing or alteration. Any additional refraction surveys should be given a lower priority than the proposed detailed magnetic survey and the acquisition of proprietary resistivity data. There is much interest in determining the total power production potential for the east rift zone to facilitate economic planning. Present estimates of the lateral extent of the reservoir, based on microearthquake data must be considered premature and subject to considerable error. It is apparent that maps of microearthquake epicenter locations and density (i.e. Plate VII) are heavily biased by the location and extent of the seismographnet and the limited period of observation for the data reviewed here. A geometric definition of the reservoir through magnetic and resistivity data, integrated with multiple well production test results is required to determine the power generating potential of the known geothermal reservoir.
geothermal resource subzones, Hawaii, DLNR circulars, Geothermal resources--Environmental aspects--Hawaii--Kilauea (Hawaii Island), Geothermal resources--Environmental aspects--Hawaii--Puna, Environmental impact analysis--Hawaii--Kilauea (Hawaii Island), Environmental impact analysis--Hawaii--Puna
142 pages
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