In Search of Thermocons: Brainstorming Subsurface Surveys for Deep Geothermal Energy

By Bob Metcalfe

You can relax. This is not a peer-reviewed paper. This is a brainstorming post. Should be fun.

I view geothermal in its various flavors as the biggest thing I’ve ever seen, and that includes the Internet, now 50 years old. Focusing on the geothermal startup ecosystem, with a focus on oil and gas veterans, is what we are up to at GEO - encouraging startups that will soon help generate gigawatts (later terawatts) of cheap and clean electricity. We aim to generate geothermal energy when the sun is not shining, and when the wind is not blowing – baseload electricity, on or off the grid. The aim is to find economically harvestable geothermal energy anywhere, which in some places in the world means being able, if need be, to drill ~10km deep wells into hot dry rock (HDR) to recover heat at ~350C. So “anywhere” can then mean closer to population centers where energy is needed. Recent progress in drilling technology among oil and gas companies has brought such geothermal goals within reach, and significant global scale is possible within this decade.

Some of our favorite early geothermal startups are teaching us that those depths and temperatures may not be required for economic baseload electricity, at least not at first. Like many good startups, they are talking about “low hanging fruit” - what Silicon Valley calls “product road maps.” They are experts in geology and thermodynamics with good news about the economical harvesting of Earth’s heat from shallower rocks at lower temperatures. They are talking about using mined heat not only to generate baseload electricity, but also to moderate building temperatures, produce hydrogen, and/or mine Lithium. They are also talking about the potential of recompleting abandoned or underperforming oil, gas, and geothermal wells, of which there are millions globally. Some geothermal “anywheres” will prove more economical than others, and the race is on amongst the Founderati to see who wins.

Delivering geothermal electricity profitably, at say (my goal = “cheap”) one cent per kilowatt-hour, might require what oil and gas companies call “exploration of the subsurface.” Where on Earth are there hot rocks willing to yield their heat profitably? Right now, geothermal energy is expensive, mainly because drilling rocks is expensive. Test wells are expensive, and their results not reliable. So, one way of improving our knowledge of the subsurface is to make drilling cheaper. Many geothermal startups are taking that tack - a few of them, like Hypersciences and Strada Global, featured in past articles. This post is going to take a different tack. How can we innovate to explore the subsurface in new and innovative ways?

But first, since we are speaking of tacks, a nautical themed segue. My friend, Dr. Jim Mays, a tide prediction software expert, reminded me that 71% of Earth’s surface is under water at an average depth of 12,000 feet. I reminded Jim that a lot of oil and gas drilling is already miles offshore. Please excuse the use of “feet” and “miles.” At least I didn’t say “2,000 fathoms.” Jim told me about an old technology for assessing the subsurface of navigable waters. His story goes back to sailing ships tacking port and starboard along the foggy coast of Maine. They needed to assess the subsurface to avoid ledges while considering the ebb and flow of Maine’s 10-foot tides. Jim’s story is that in a fog, a young lad would be sent forward by the helmsman with a bushel of potatoes. Standing at the bow, the lad would throw potatoes one at a time forward out into the fog. Then he would listen. If he heard a splash, there was (probably) navigable water ahead. If he heard nothing, or maybe a thump, then his potato had landed on shore. In that case, he would shout to the helmsman to steer away from Maine’s granite jaws. Let’s call this subsurface survey technology “potatosonics.” I would guess our oil, gas, and geothermal startups are well beyond this technology. Of course, nowadays ships use Earth-orbiting satellites in the Global Positioning System (GPS) to navigate at night or in the fog. GPS tells the helmsman where she is. Nautical charts translate from where she is to where the ledges are around her. The charts are the result of repeated surveys over the years that now organize water depth data around GPS locations. Most water depths, thanks to Earth-Moon gravity interactions, vary with time. Hence we have tide charts.

I have always found it fascinating that GPS has no clue about, but can still tell you about, where you are. Far from Maine and its charts, Geophysics Professor Dr. Maria Zuber, now MIT’s VP of Research, is interested in subsurface surveys of the Moon. But she didn’t plan to drill into the Moon to survey its density and look for large subsurface objects. Nor did she plan to throw potatoes. Instead, she launched two satellites (Ebb and Flow) to orbit the Moon under NASA’s GRAIL – Gravity Recovery And Interior Laboratory. Detecting small differences in the Moon’s gravity between Ebb and Flow, and with some careful modeling and computation, GRAIL was able to deduce the pull of varying densities and to locate subsurface mass concentrations, or “mascons,” in the Moon’s crust. Knowing where the mascons are is very helpful in landing on the Moon, if you are particular about where. Planetary scientists believe that Moon’s mascons were left behind by asteroid impacts many moons ago. It took several moons, but now thanks to GRAIL we have a map of the Moon’s density. And we know a lot more about Moon’s mascons. After the GRAIL subsurface survey, Dr. Zuber crashed her Ebb and Flow satellites down into the Moon so they would not become space junk.

Gravity is also of interest to Air Force Major General (Retired) Dr. Ken Wisian, a Co-Investigator at GEO who recently became the Assistant Geologist of the State of Texas and Assistant Director of the Bureau of Economic Geology (BEG) at UT Austin. General Wisian is a geophysicist and geothermal expert, but his DOD experience has left him both interested and concerned about the fragility of our Global Positioning Systems. GPS radios are jammable, for example. He thinks that someday gravity sensors might replace GPS’s radios. General Wisian’s idea is to GRAIL the Earth. Then, to find out where you are, just scan Earth’s nearby gravity field and match it to Earth’s gravity survey. It will not prove practical to drill Earth for gravity surveys. Maybe instead we can fly satellites like GRAIL’s Ebb and Flow to map the gravity fields of Earth and use them like GPS to find ourselves. Maybe gravity surveys will discover Earth’s mascons. And better yet, maybe gravity surveys will discover heat concentrations, let’s call them “thermocons.”

I wonder if Earth’s mascons and thermocons are the same.

Oliver Strimpel has been many things and is now a part-time geologist. He writes about geology at Geology Bites. Oliver was telling me about how geologists are now using vibrations (sounds) from earthquakes to map Earth’s subsurface. Some call it seismic tomography. They can find what Dr. Zuber might call mascons in Earth’s crust. Maybe seismic tomographers could, with a lot of modelling and computation, detect the attenuation, frequency, and phase of sounds taking different paths, deduce their speeds, image objects, and assess their compositions and learn their…temperatures. Imagine surveying Earth with surface microphones instead of drilling. Imagine buying up heat leases on acreage determined by satellite surveys to offer easy access to the porous hot rocks of thermocons.

UT Austin Professor Dr. Neal Hall has for a long time been working on microphones. His are optical and made from microelectromechnical systems, or MEMS. They offer very wide dynamic range; they can hear whispers across a crowded room. His initial focus was on microphones for cell phones, and he formed a startup, Silicon Audio. Arriving customers had other plans for Professor Hall’s microphones. They were interested in detecting seismic signals, and so he started making seismonitors. It turns out that his technology may apply to a variety of geothermal flavored challenges, including seismicity detection for EGS applications, among others. I’m spending time with Professor Hall to see if he can build sensors with which to survey Earth’s crust for thermocons. If so, then where would we find the software to process Hall’s sensor data to image the subsurface? Turns out there are many seismic tomographers. They use vibrations from Earth’s ongoing earthquakes to image the subsurface. My hunch about finding heat for deep geothermal is that we should start growing a sensor network that will over time gather and image various Earth vibrations. As the network grows and the data gathers, we would over time get an improving survey of the subsurface.

So let’s think back to the birth of the internet, 50 years ago, and look now at the next generation internet, the augmented video mobile gigabit internet, connecting 7.7 billion humans and our 8 billions “things” - imminent and making its debut with 5G. In my view, geothermal is poised for bigger, faster growth. Seismic tomography sensor networks are already expanding, and we can expect this to accelerate. And if things go the way of the internet, pretty soon we will have geothermal’s Holy GRAIL.

Bob Metcalfe is Professor of Innovation and Murchison Fellow of Free Enterprise in the Cockrell School of Engineering at The University of Texas at Austin. He is Principal Investigator of the Geothermal Entrepreneurship Organization (GEO) in the Texas Innovation Center.