Subsidence and Stress Change in the Cerro Prieto Geothermal Field,B. C., Mexico

Previous works have shown that ground deformation and seismicity in the Cerro Prieto geothermal field (CPGF) are due to both tectonics and field exploitation. Here, we use information about current tectonics and data from precision leveling surveys, to model tectonic and anthropogenic subsidence. Our results show that tectonic subsidence constitutes only ∼4% of the measured subsidence. Anthropogenic subsidence was evaluated using a model of rectangular tensional cracks, based on the hydrological model of the field, together with the Coulomb 2.0 program. From the resulting values of the fissure parameters and from extraction and injection data, we calculate that the volume changes caused by closure of the geothermal and cold water reservoirs account for only ∼3% and ∼7%, respectively, of the volume change which should occur due to extraction. Since 18% of the extracted fluids are reinjected, external recharge must compensate for about 72% of the expected volume reduction. An analysis of the changes in Coulomb stress caused by exploitation of the geothermal field suggest that even though the anthropogenic stresses account for only a fraction of tectonic stresses, they are large enough to trigger seismicity.     

Geothermal energy

The potential of a geothermal area is primarily dependent on volume and temperature of the reservoir and adequacy of fluid supply. Inadequate fluid supply may be a more common limiting factor than inadequate heat supply, for heat stored in the upper 10,000 ft of many hot spring systems is 1,000 to 10,000 times their annual natural heat flow. Except in very porous reservoirs, most of this heat is stored in rocks rather than in pore fluids. Geothermal fields can be classified as hot spring systems or as deep insulated reservoirs with little surface expression; gradations also exist. Hot spring systems have high near-surface permeability, at least locally on faults and fractures, permitting fluids to escape at high rates. Owing to vigorous circulation and escaping fluids and heat, near-surface temperatures are high, but temperatures deep in the system are lower than would prevail with inhibited escape. Deep reservoirs with little surface expression require permeable reservoir rocks capped by insulating rocks of low permeability. Larderello, Italy, and Salton Sea, California, have slight leakage, but others may have no leakage. Liquid water, which can be at temperatures far above 100° C because of existing pressures, is generally the dominant fluid. Steam can form by boiling as hot water rises to levels of lower pressure. However, in several explored systems the heat supply is so high and rate of discharge of water so low that steam exists even deep in the system. Dry steam areas are probably rare. About 30 areas in the United States have been explored for geothermal energy, but dry steam has been proved only at « The Geysers ». Extensive utilisation of geothermal energy must therefore depend largely upon steam « flashed » from hot water with decrease in pressure. Problems that confront broad utilisation of geothermal energy include: 1) discovery of reservoirs with adequate supply of energy and natural fluids; 2) deposition of CaCO; or SiO₂; 3) chemical corrosion; 4) objectionable chemicals in some effluents; and 5) inapplicability of existing public laws. The optimum environment for a geothermal reservoir includes:

1. 1.
   Potent source of heat, such as a magma chamber. A depth of at least two miles provides enough pressure to insure water of high temperature; 5 miles may be too deep for effective transfer of heat to circulating water. Such heat sources are most likely to occur in regions of late Cenozoic volcanism.     
   
   

地热正常动态特征的研究

为了研究地热正常动态特征,收集了226个地热前兆观测台站的数据,建立了地热前兆应用数据库。 通过对地热前兆台站观测资料的整理分析,将地热正常动态分为地热的长期正常动态和地热的短期动态两类,归纳出6种类型的地热长期正常动态和4种类型的地热短期正常动态,并结合观测点处的水文地质情况,对不同的地热正常动态类型的成因进行了初步分析。     

Geothermal studies in China

Geothermal studies have been conducted in China continuously since the end of the 1950's with renewed activity since 1970. Three areas of research are defined: (1) fundamental theoretical research on geothermics, including subsurface temperatures, terrestrial heat flow and geothermal modeling; (2) exploration for geothermal resources and exploitation of geothermal energy; and (3) geothermal studies in mines.Regional geothermal studies have been conducted recently in North China and more than 2000 values of subsurface temperature have been obtained. Temperatures at a depth of 300 m generally range from 20 to 25°C with geothermal gradients from 20 to 40°C/km. These values are regarded as an average for the region with anomalies related to geological factors.To date, 22 reliable heat flow data from 17 sites have been obtained in North China and the data have been categorized according to fault block tectonics. The average heat flow value at 16 sites in the north is 1.3 HFU, varying from 0.7 to 1.8 HFU. It is apparent that the North China fault block is characterized by a relatively high heat flow with wide variations in magnitude compared to the mean value for similar tectonic units in other parts of the world. It is suggested that although the North China fault block can be traced back to the Archaean, the tectonic activity has been strengthening since the Mesozoic resulting in so-called “reactivation of platform” with large-scale faulting and magmatism.Geothermal resources in China are extensive; more than 2000 hot springs have been found and there are other manifestations including geysers, hydrothermal explosions, hydrothermal steam, fumaroles, high-temperature fountains, boiling springs, pools of boiling mud, etc. In addition, there are many Meso-Cenozoic sedimentary basins with widespread aquifers containing geothermal water resources in abundance. The extensive exploration and exploitation of these geothermal resources began early in the 1970's. Since then several experimental power stations using thermal water have been set up in Fengshun (Fungshun),     

Recent Vertical Deformation in Mexicali Valley and its Relationship with Tectonics, Seismicity, and the Exploitation of the Cerro Prieto Geothermal Field, Mexico

—The interpretation of the results of regional and local leveling which began in 1977 in the Mexicali Valley and the local short profile precision leveling which started in 1994 are discussed. The relation of vertical deformations around the Cerro Prieto Geothermal Field (CPGF) and along the Imperial fault, with local tectonics and seismicity in the Mexicali Valley, is reviewed. Also the relation between vertical deformation and fluid operation in the CPGF is analyzed. The subsidence observed in the field seems to be induced by fluid extraction. The way in which fluid production influences surface changes along the Imperial fault is not clear. The possibility that seismicity is triggering subsidence in the area and vertical movement on the Imperial fault is discussed.     

白家疃井地热异常与地震监测

通过近４年的地热前兆监测工作，对地热资料分析方法的实用性、地热井孔的映震效能进行了检验。结果表明，地热井孔地震信息丰富，同时，也进行了试验性地震预报实践。     

Geothermal resource assessment of the United States

Geothermal resource assessment is the broadly based appraisal of the quantities of heat that might be extracted from the earth and used economically at some reasonable future time. In the United States, the Geological Survey is responsible for preparing geothermal assessments based on the best available data and interpretations. Updates are required every few years owing to increasing knowledge, enlarging data base, improving technology, and changing economics. Because geothermal understanding is incomplete and rapidly evolving, the USGS complements its assessments with a broad program of geothermal research that includes (1) study of geothermal processes on crustal and local scales, (2) regional evaluations, (3) intensive study of type systems before and during exploitation (4) improvement of exploration techniques, and (5) investigation of geoenvironmental constraints.     

Geothermal fields of Hengill Volcano,Iceland

This paper provides a review of geological, geophysical, and geochemical studies for three geothermal zones of Hengill Volcano, Iceland: Nesjavellir, Hellisheidi, and Hveragerdi. We discuss the relationships between global tectonics and high-temperature geothermal systems in Iceland. The bulk of this review is devoted to studies of the physical, geochemical, and mineralogic parameters for the three areas. A separate discussion concerns surface phenomena, as well as the origin of thermal water. This review covers studies of the main aquifer complexes: Miocene/Lower Pliocene plateau basalts, Upper Pliocene/Pleistocene lavas and volcanoclastics involving tillite horizons, an aquifer complex of Holocene lava sheets as thick as 1 km, and an aquifer complex of Upper Pleistocene/Holocene alluvial eolian deposits and formations of bottom moraines. We consider a conceptual model of geothermal reservoirs characteristic for the Hengill geothermal fields.     

Heat source for Tongonan Geothermal Field

Abstract The primary mineral and whole-rock chemistry of 46 core samples from the host rocks of the Tongonan Geothermal Field (the Philippines) have been used to infer the likely composition of the heat source for the system. The host rocks consist of andesite lavas (with intercalated fossiliferous early to mid–Miocene shales and limestone), and a plutonic rock basement ranging in composition from gabbro to granite. The whole rock TiO₂, Fe₂O₃ (total iron), MgO, P₂O₅ and V data for volcanic and plutonic rocks are colinear on conventional Harker diagrams. This, along with similar hornblende chemistry, age and close spatial relationship suggests that the basement and cover rocks are cogenetic and evolved by low-pressure crystal fractionation. Crystal fractionation models indicate that separation of 60% plagioclase and 30% hornblende from original magma controlled the chemistry of the host rocks. The original Miocene magma chambers beneath the Tongonan field crystallized inwards from the walls at approximately 750°C and 1 kb pressure (3–4 km depth) thus forming a series of plutons or a batholith at drilled depths. A supercritical hydrothermal fluid trapped in the crystallizing, hornblende-granite-pegmatite core of a crystallized Miocene diorite batholith was gradually being released to shallower levels through antithetic cross fractures during creep and uplift along the main branches of the Philippine Fault from the Pliocene. This ascending fluid is now thought to be responsible for the present thermal activity of the field.     

Geothermal exploration using surface mercury geochemistry

Shallow, soil-mercury surveys can be used effectively in exploration for geothermal resources. Soil-mercury data from six areas in Nevada, California and New Mexico are analyzed using contour maps, histogram and probability graphs. Plotting on probability graphs allows background and anomalous populations to be resolved even when considerable overlap between populations is present. As is shown in several examples, separate soil-mercury populations can be plausibly interpreted. Mercury data can significantly enhance the structural understanding of a prospect and can yield important information about the geothermal resource.     

Geothermal prospecting by ground radon measurements

Radon-222 was measured using Kodak LR-115 film in the soils of 2500 locations near the Ngawha hot springs region, New Zealand, which is being exploited for geothermal power; the object was to determine its usefulness for predicting good drill sites. Unlike other surveys, which have shown large areas with consistent high radon values, anomalies here were scattered, and corresponded mainly with fault lineaments. The results suggested a major previously unnoticed fault. The sampling distance was 50 m.There was a strong seasonal effect on ground radon levels, with summer levels about ten times higher than winter levels.Swamps usually had measured radon levels of near zero because of the slow diffusion of radon in stagnant water, and even thermal areas (mainly in the swamps) usually had low measured values. However, where a fault crossed swamp it was sometimes detected, and with high signal/noise ratio, so swamps should be surveyed.Arguments from the radon levels found on different geologies show that at Ngawha radon has a maximum half value diffusion thickness of 7 m for lacustrine sediment and 25 m for basalt unless a permeable area is present (e.g., a fault).There was a weak correlation of radon levels with the temperatures found on drilling deep bores.Comparisons with the ROAC system and Alphacard system for measuring radon showed no statistical inter-correlations, but some qualitative correspondence of radon contours.The main strength of the method in regions with impermeable soils (such as at Ngawha), seems to be in detecting or confirming the presence of faults, and possibly (through them) indicating geological structure as deep as 300 m.     

大姚地热动态特征分析

通过大姚高精度水温观测井水地质、井孔条件的分析，震例观测资料、实验观测数据的处理，以及井水水质分析结果，表明大姚水温周期性变化的突出位置是井深75m左右，根本原因是该地层石膏脉稳定释热，含水层热水与井内冷水混合过程中，在通道内形成钙盐类沉淀物（如CaSO4CaCO3），堵塞对流通道，含水层内外压差突破被阻塞通道，热水反复侵入井内，便形成水温周期性变化。地震的孕育、发生和调整过程，由于应力场作用，产生附加地热场，水温基值发生变化，周期畸变或消失，这种附加地热动态是可以恢复的。所以认为大姚地热动态是井区特殊地层地热信息的反映。     

基于新编《系列世界地图》的全球板块分布图

采用新编《系列世界地图》绘制出一套四种全球板块分布图,具体分为,“东半球版”、“西半球版”、“北半球版”和“南半球版”.其中,“东半球版”和“西半球版”为“经线全球板块分布图”,适用于反映东、西半球的板块构造分布;“南半球版”和“北半球版”为“纬线全球板块分布图”,适用于反映南、北半球的板块构造分布.基于《系列世界地图》的全球板块分布图以东、西、南、北四种视角,从经度、纬度两种方向,将全球板块构造以多元化的形式地表达出来,为深入研究板块相互作用和运动机制,提供一种新的图形工具.     

生成GoogleMap地震震中分布图的一种方法

利用程序读取EQT格式的地震目录,生成一个GoogleMap格式的地震震中分布图,该图不需要联网即可在局域网应用,适合在地震会商、地震应急指挥中使用.     

Geothermal resources in the imperial valley of california

The Imperial Valley is a major rift valley characterized by unusually high heat flow and large quantities of water in storage in the thick fill of alluvium provided by the sediments of the delta of the Colorado River. The inventory of hot water appears to be sufliciently large that if used for water desalination it might add several million acreleet of new water to the resources of the lower Colorado River basin. This distilled water would serve to lower river salinity and provide extra water to help meet the U. S. — Mexico treaty commitments. A major fraction of water desalination costs lie in the cost of energy and are related to desalination technology which is directly related to water chemistry. The discovery of low salinity geothermal waters in the Imperial Valley opened th possibility for a major breakthrough in lowered water desalination costs. We have tried to develop a broad understanding of the origins of the waters of the Imperial Valley and how natural recharge occurs. The chemical composition of the waters of the central portion of Imperial Valley basin waters, while not that of present surface flow of the rivers, nevertheless does have a close affinity to Colorado River water. No sea water seems to be present in the valley although marine sediments appear to occur on basement on West Mesa and on basement to the east in Arizona south of Yuma. Low salinity waters dominate the basin hydrology and waters as saline or more saline than sea water appear to be restricted to the immediate area of the Salton Sea. The isotope work of T. Coplen makes it possible to determine the relative contribution of precipitation runoff from California watersheds and from Colorado River water. Both sources are significant. The Colorado River water in aquifers from 100–400 m appears to have been entrapped from a relatively homogeneous basin which was subject to substantial evaporation. Its original source was snow melt water from the Colorado River. Five types of waters, none of them sea water, were recognized by their salt geochemistry. Bromide/chloride data are particularly effective in resolving different types of water masses. The bromide/chloride data agree with the isotope data and identify rainfall and precipitation runoff from the high mountains to the west. Modern Colorado River water is easily recognized by its salts and two types of ancient Colorado River waters from previous lake stage are proposed on the basis of the bromide/chloride data. One old lake occurring during the pluvial stage associated with the last Ice Age is proposed to account for much of the water in artesian aquifers. Another younger lake stage, possibly with Lake Cahuilla affinities is also suggested. Mountain runoff waters can be distinguished in the subsurface by their relatively lower salinity and high bicarbonate concentration, and their heavy isotopic composition. Revised fluid reserve calculations based on additional porosity data continue to show that the low salinity water resources of the Imperial Valley may exceed two billion acre-feet. The oceanic plate tectonic model is modified in the Imperial Valley by the evidence of a series of complex blocks with the generation of both tensional and compressional features in the valley. Major strike slip faults dominate the tectonic fabric but conjugate features increase complexity by a large degree and a major amount of work will be needed before any geologically sound structural models can be generated. Xenoliths within the obsidians at the volcanoes at the south end of the Salton Sea provide samples of the basement under the Imperial Valley. These xenoliths include partially remelted granitic rocks, fragments of basalt, greenschist, and baked shale and sandstone. This is taken as evidence that the basement in the valley consists in part of partially remelted granite. This would render basement plastic and readily deformed. The source of the heat is suggested to be derived from basalt that comes into the basement and deeper sediments from below. This upward movement of basalt along a spreading zone is the continental equivalent to a sea floor spreading area. In the continental case the insulating blanket of wet sediment retains the heat and appears to produce a major geothermal resource. The geothermal resources of the Imperial Valley are the aggregate of the thermal energy of the large inventory of subsurface water heated by the complex mix of intrusive phenomena. The net result is to generate a polygenetic geothermal resource of very large dimensions.     

Geothermal study of reunion island: Audiomagnetotelluric survey

In 1979 and 1980, 535 magnetotelluric soundings using a frequency range of 1700 Hz-8 Hz were performed on the island of Réunion for geothermal exploration.Few direct hot-water discharges were observed. Consequently, geophysical methods, particularly the audiomagnetotelluric method, were used extensively. Favourable geological conditions for this method were encountered and the results, which were controlled using classical electrical methods on test areas, suggest an unusual distribution of resistivities for lava flows situated above suspected geothermal areas. These layers progressively decrease in resistivity down to a very conductive layer. In areas where these conductive layers were nearest the surface, detailed studies were carried out showing a close correlation between decreasing resistivity and increasing hydrothermal alteration. In addition, gradient wells reveal high geothermal gradients in such areas. The conductive layers revealed by audiomagnetotelluric soundings seem to correlate with thermal effects creating progressive hydrothermal alteration from an inferred hot-water reservoir up toward the surface.     

Exploring for Geothermal Resources with Electromagnetic Methods

Electrical conductivity of the subsurface is known to be a crucial parameter for the characterization of geothermal settings. Geothermal systems, composed by a system of faults and/or fractures filled with conducting geothermal fluids and altered rocks, are ideal targets for electromagnetic (EM) methods, which have become the industry standard for exploration of geothermal systems. This review paper presents an update of the state-of-the-art geothermal exploration using EM methods. Several examples of high-enthalpy geothermal systems as well as non-volcanic systems are presented showing the successful application of EM for geothermal exploration but at the same time highlighting the importance of the development of conceptual models in order to avoid falling into interpretation pitfalls. The integration of independent data is key in order to obtain a better understanding of the geothermal system as a whole, which is the ultimate goal of exploration.