Changes in groundwater levels may also be measured and identified with gravitational methods. This recharge element is imperative in creating productive geothermal systems. Pore density and subsequent overall density are affected by fluid flow and tfore change the gravitational field. When correlated with current weather conditions, this can be measured and modeled to estimate the rate of recharge in geothermal reservoirs. Unfortunately, t are many other factors that must be realized before data from a gravity study can be interpreted. The average gravitational field the earth produces is 920 cm/c^2. Objects of concern produce a significantly smaller gravitational field. Tfore, instrumentation must detect variations as small as 0.00001%. Other considerations including elevation, latitude and weather conditions must be carefully observed and taken into account. Resistivity and magnetotellurics Magnetotellurics (MT) measurements allow detection of resistivity anomalies associated with productive geothermal structures, including faults and the presence of a cap rock, and allow for estimation of geothermal reservoir temperatures at various depths. MT has fully contributed to the ful mapping and development of geothermal resources around the world since the early 1980s, including in the U.S. and countries located on the Pacific Ring of Fire such as Japan, New Zealand, the Philippines, Ecuor, and Peru. Geological materials are generally poor electrical conductors and have a high resistivity. Hydrothermal fluids in the pores and fractures of the earth, however, increase the conductivity of the subsurface material. This change in conductivity is used to map the subsurface geology and estimate the subsurface material composition. Resistivity measurements are me using a series of probes distributed tens to hundreds of meters apart, to detect the electrical response of the Earth to injection of electrical impulses in to reconstruct the distribution of electrical resistance in the rocks. Since flog geothermal waters can be detected as zones of low resistance, it is possible to map geothermal resources using such a technique. However, care must be exercised when interpreting low resistivity zones since they may also be caused by changes in rock type and temperature. The Earth's magnetic field varies in intensity and orientation during the day inducing detectable electrical currents in the Earth's crust. The range of the frequency of those currents allows a multispectral analysis of the variation in the electromagnetic local field. As a result, it is possible a tomographic reconstruction of geology, since the currents are determined by the underlying response of the different rocks to the changing magnetic field. Magnetics Stream in Icelandic geothermal exploration field. |