In the last couple of posts, we looked at the challenges of prospecting for geothermal energy. Basically, you need to find water at high temperature and pressure, usually located miles underground. Ideally, you want reservoirs contained in rocks capable of holding lots of water and through which lots more water can flow to replenish whatever you use. Fortunately, these characteristics – temperature, porosity, and permeability – can all be measured without necessarily digging a multi-million dollar hole in the ground. You just need to know a bit about what happens underground.
Let’s start with temperature. After the first few dozen feet you dig into the Earth’s surface, the temperature in most places is mild and fairly uniform – about 10-12 degrees Celsius (50-55 degrees Fahrenheit). Once you go much deeper it starts to get warmer, and where the rocks are fairly uniform in composition (as they are in Iceland), the temperature rises in a fairly predictable fashion. The increasing temperature and depth can be plotted on a chart; the resulting slope is called the thermal gradient.
Curves in the measured slope of the thermal gradient indicate the presence of a dynamic anomaly affecting the temperature underground: if the slope curves downward compared to the predicted gradient, that means there’s something cooling the subterranean earth where the curve is present (as in the graph on the left, below). If there’s an upward curve, it indicates a warm upflow (as in the graph on the right). A warm upflow is a good thing.
In most places, the thermal gradient is about 25 degrees Celsius of increased temperature per kilometer of increased depth (or about 72 degrees Fahrenheit per mile). You can measure the increased temperature by drilling narrow boreholes, called gradient wells, which are comparatively cheap – a few thousand US dollars per well, rather than the millions you’re going to have to spend to drill a production well that accesses the geothermal reservoir. If the temperature rises faster than normal, it means there’s something down there that’s hotter than usual, and that means you’re on the right track.
By digging a series of gradient wells, you can plot out the areas where the temperature rises more quickly. You draw contour lines, just as you would on a topographical map, except instead of showing changes in elevation, you’re connecting areas of equal temperature, creating isotherm lines showing changes in temperature gradient. You want to concentrate on those places where the gradient increases sharply, because it’s there that you’ll probably find the hot rocks you’re looking for.
There are other, more subtle ways of looking for heat. When rocks get very, very hot, they expand slightly, and at high enough temperatures they melt. Most substances are denser when they’re cool than they are when they’re warm (water is an exception to this rule, which is why ice floats). A volume of material that is less dense than an equivalent volume of the same material is less massive, and therefore exerts a slightly small gravitational pull; and so the earth’s gravitational field over a volume of hot rock is fractionally less than it is over cooler rock.
This change in gravity is too minute to be noticed by human beings, but we can build instruments sensitive enough to detect it. A gravity meter is used to measure changes in the earth’s gravitational field from one spot to another; relatively inexpensive models use changes in the strain exerted by a small weight on a metal spring, based on technology developed to allow submarines to plot their location underwater without using sonar or other technology that might reveal their position.
When you find a place where the temperature gradient is steeper than normal, and the earth’s gravity is slightly smaller, chances are good that there’s a mass of unusually hot rock underneath you.
Heat, over time, also changes the chemical composition of minerals. Some of the altered minerals, like zeolites and smectites, have molecular structures that allow ions to flow more freely than would be possible through the original materials. These altered minerals become better conductors of electromagnetism than they were before, and their resistivity can be measured in a number of ways. One method, called transient electromagnetic (TEM) surveying, uses an antenna to emit an electromagnetic signal down into the earth.
Different signal frequencies penetrate different depths and produce data of differing precision: in general, the shorter the frequency, the shallower the depth and the sharper the resolution. By measuring the time it takes the signal to fade away at varying depths, you can build up a picture of the resistivity of the rocks at those depths. This method, coupled with another called magnetotelluric (MT) surveying (which measures minute changes in the flow of ions produced by fluctuations in the earth’s own electromagnetic field), can build up a picture of the resistivity of the rocks many miles beneath the surface. Just as a CT scan or an MRI shows the internal structure of a human body, TEM and MT measurements show the internal structure of the ground beneath our feet.
Geothermal reservoirs often show a characteristic resistivity profile, with a high-resistivity core of molten rock or high-temperature alteration minerals such as chlorites; a low resistivity cap of lower-temperature alteration minerals, like zeolites and smectites, formed by proximity to the heat below; all beneath layers of unaltered minerals of varying degrees of resistivity. Other features, such as the presence of water, can change the resistivity of rock formations as well.
Because there are so many variables, a single method of probing the earth’s interior may not necessarily yield unique solutions: sometimes, for example, there may be more than one possible explanation for the resistivity measurements you achieved. So you combine the data with the results from measurements of temperature, gravity, magnetic fields, and so on.
These geophysical methods of measurement, combined with other kinds of sensing such as recording seismic waves (which can be produced by fracturing of rock during an earthquake, or in much less violent form by the background noise of construction and even auto traffic passing nearby), make it possible to build up a detailed model of the structures underground.
With advanced 3D modeling software, it becomes possible to “see” where heat and water are likely to be located together.
Since the 1960s, experience (and better techniques and equipment) have improved the odds of predicting a successful well site from no better than a fifty-fifty proposition to one where the chances of success exceed 80 per cent. But there are still sometimes expensive failures, and so the work of refining how we look at the earth and what lies beneath continues.
* The meter we used in my surface exploration class was sensitive enough to record differences as small of one-millionth of a per cent change in the earth’s gravity field. We used it to measure the difference between the gravitational pull at top floor of the ISOR building and the basement, and from that calculate the difference in elevation (9.916 meters, if you’re curious).
Cover image: molecular structure of a zeolite. Image credit: Wikimedia Commons.