Helium isotopes may identify potential sources for geothermal energy

Washington, Dec 2: A new research by geologists has suggested that areas with high resource potential for geothermal energy can be identified using the ratio of helium isotopes.

Geothermal energy is an untapped alternative means of power for humans that lies deep within the Earth's surface. It is quite significant nowadays in the wake of depletion of fossil fuel sources and rise in global warming.

It has been estimated that within the continental United States, there is a sizable resource of accessible geothermal energy – about 3,000 times the current annual U.S. consumption.

Two important reasons this storehouse of energy has not been tapped is that locating the specific energy hot spots is difficult and expensive.

"Since many geothermal resources are hidden, that is, they do not show any clear indications of their presence at the surface, locating them by just using observations made at the surface is difficult," said Matthijs van Soest, associate research professional at the Noble Gas Geochemistry and Geochronology Laboratory within the School of Earth and Space Exploration at Arizona State University.

"Often when people thought there might be a geothermal resource below the surface the only way to determine if their assumption was correct was drilling and drilling is extremely expensive," Soest added.

Now, the new research by Van Soest and B. Mack Kennedy from the Lawrence Berkeley National Laboratory, US, has pointed out helium isotopes as a cheaper and more efficient way to find out areas that could be the source for geothermal energy, without the process of drilling.

"We wanted to show that certain surface indicators, specifically the ratio of helium isotopes, can be used to identify areas with high resource potential for geothermal energy," said Van Soest.

Earth's crust contains a variety of noble gases, one of those being helium. Natural helium occurs as two isotopes, helium-4 (4He) and helium-3 (3He.)

Typically, helium-4 is more abundant in Earth's crust, whereas helium-3 is more abundant in the mantle below. Thus, the helium-3/helium-4 ratio of the gas found in groundwater can provide an indication of the extent to which the water has interacted with volcanic rocks derived from the mantle.

Waters that have equilibrated only with crystal rocks typically have low helium-3/helium-4 ratios, but Kennedy and Van Soest found that some waters from hot springs near the Dixie Valley geothermal power plant in Nevada contained anomalously high ratios.

"When we found the elevated ratios, we knew that the only way these waters could be enriched with helium-3 was if they had interacted with fluids from the Earth's mantle," explains van Soest. "The area directly surrounding the power plant has about two to three times the values found elsewhere in the region," he added.

The analysis of samples taken from more than 60 features (mostly from hot springs and shallow wells) in the northern Basin and Range showed that other areas with characteristics similar to those of Dixie Valley (higher 3He/4He ratios) could be very favorable for geothermal development.

"Areas where we can sample fluids near the surface provide a way of getting a relatively cheap and easy indication of what's happening deep down," said Van Soest. "Applying what we know about the helium ratios makes the exploration for geothermal resources cheaper and faster," he added. (ANI)

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