Measuring and modeling geothermal resources at Pilgrim Hot Springs

25 June 2014 by Liz O'Connell, posted in Pilgrim Hot Springs

Pilgrim Hot Springs valley / FrontierScientists footage

Pilgrim Hot Springs valley / FrontierScientists footage

There’s a place where the perennially frozen ground of the Alaskan tundra is interrupted by 2 square miles [~ 5 km² ] of thawed soil. There, cottonwoods and thick brush grow among lazily meandering waterways.

The Pilgrim Hot Springs are a pleasant symptom of the geothermal heat which warms the earth deep beneath Alaska’s Seward Peninsula, not so far south from the Arctic Circle. There, deep below the surface, hot water rises through fractures in the bedrock that comprises the valley floor.

The Alaska Center for Energy & Power (ACEP), supported by the U.S. Department of Energy and the Alaska Energy Authority, undertook scientific testing to determine whether the geothermal resource at Pilgrim Hot Springs could provide the city of Nome, Alaska, with a worthwhile power source. ACEP brought together an interdisciplinary team of skilled scientific professionals to explore the geothermal potential of the area.

Enter two team members: Ronald Daanen, geohydrologist in the Department of Natural Resources, Division of Geological & Geophysical Surveys, and Anupma Prakash, professor in geophysics remote sensing at the University of Alaska Fairbanks Geophysical Institute. They utilize their expertise to gain data about the geothermal characteristics of Pilgrim Hot Springs valley which combine to create a valuable computational model.

Remote sensing

High above Earth, Terra, the flagship satellite of NASA's Earth Observing System, threads a complete orbit of Earth every 100 minutes. It carries ASTER, the Advanced Spaceborne Thermal Emission and Reflection Radiometer, which focuses on Earth’s surface to image visible light and measure heat.

ASTER data collected over time helps Anupma Prakash chart out areas with anomalous snow melt and vegetation growth compared with the surrounding countryside – images which indicate an underground heat source. That narrows down the area for her further studies.

Thermal aerial view of Pilgrim Hot Springs / Anupma Prakash

Thermal aerial view of Pilgrim Hot Springs / Anupma Prakash

Prakash utilizes Forward Looking Infrared cameras carried by small aircraft to take high resolution measurements of Earth’s surface. A quadcopter is an Unmanned Aerial Vehicle (UAV) with four helicopter-like propellers. It can be mounted with sensory equipment: optical cameras (which capture up to 20 centimeter resolution pictures of the ground), and thermal temperature sensors (which measure a space as small as 4 centimeters across).

“From the ground temperatures, we go a step further and derive how much heat is coming out from the system underneath.” “And this is the heat coming from under the ground and not just what is heating the ground because of solar radiation. We have to subtract all that, and say that if all that solar radiation was not there, what is the [measure of the] heat coming from the ground? And that is the key information that we are trying to derive from the data set.” “That is an essential piece of information that helps us know what the potential of the system is.” ~Anupma Prakash

Once she’s obtained and manipulated the data, Prakash processes it. She mosaics thousands of overlapping images to create 3-Dimensional topographic maps of the area, recording both visual information and heat data (and how they change over time), carefully checking that each data point lines up correctly.

"You get a very detailed typography from the same image. And we have the technology to do that. Again, with the same data sets, we get the topography, we get the vegetation information. We get the information on every type of land cover... is there snow, is there water, what’s the moisture, is there soil, what’s the temperature? The temperature, of course, is the key in the study.” “And that is the kind of data set that really tells you where the hot water is flowing: where it is hotter, where it is cooler, and what the system is doing.” ~Anupma Prakash

Winter aerial view of Pilgrim Hot Springs / Anupma Prakash

Winter aerial view of Pilgrim Hot Springs / Anupma Prakash

Her technique is an innovative and inexpensive exploration tool that can be used at other prospective geothermal energy sites before a large team of experts or a heavy drilling rig is ever brought in.

A geohydrologist

Ronald Daanen is interested in geology and hydrology– stone and water. They interact at Pilgrim Hot Springs in a unique way. Geothermal water wells up from underground, salty and hot, and mixes with the fresh water flowing down from the surrounding mountains.

“There are only a handful of systems out there – geothermal systems – that have this cross-flowing water, and this is the first one in permafrost.”

“Most geothermal systems are kind of stagnant, you get hot water moving up and very little cold water interaction, typically. [...] In this case it is a lot more complicated because you have cold water that is trying to push the hot water away, at the same time that hot water is strong enough to get to the surface. It makes the system a lot more complex than other systems around the world.” ~Ronald Daanen

Somewhere below, geothermally heated water is rising from cracks in the bedrock. In the 1000 feet [305 meters] or more of sediment that sits atop the bedrock in Pilgrim valley, there’s a mixture of materials. Hot water can be found 1000 feet down, and 100 feet down, but in-between there’s cold water. It’s the complexities of the cross-flowing water coming into play, as well as widespread patches of clay which are less permeable than other soils, forcing hot water to go around instead of through.

Daanen wants to pinpoint a powerful upflow zone where a well would provide great resources (very hot water, strong pressure) for a geothermal energy plant. To do that, the scientists must first define how the water is flowing at Pilgrim Hot Springs, both underground and above. What water reaches the surface? How warm is it, and how fast-moving? What path did it take through the earth below before it reached the surface?

To find out, Daanen uses multiple approaches. Temperature and pressure are charted at hot springs, while tests are performed to measure the same at existing wells – water is pumped out at high quantities to stress the system and to discover how swiftly the water in the wells recharges. At multiple locations, temperatures are measured at 50 and 100 centimeter depths using 1 meter long temperature probes. Other measurements are acquired from depths of 50 meters using a Geoprobe ‘push’ drill system.

Magnetotellurics is a technique in which certain wavelengths of energy are measured. The electric and magnetic data can be analyzed to calculate information about underground structures – the nature of the soil, clay or stone below.

Pilgrim Hot Springs simulation data points (above ground) / Anupma Prakash

Pilgrim Hot Springs simulation data points (above ground) / Anupma Prakash

Computer model

Once the data is obtained from sources on the ground and in the sky, Daanen assembles it into a computer model as though the data were pieces of a puzzle. The computer model can be used to test the feasibility of developing Pilgrim Hot Springs into an active geothermal energy resource.

“In particular, I’m involved with the part where we take that data and actually put it in a computer model. So we create this virtual space [...] create a model that actually is similar to what you find in Nature.” “We run the physics: the pressure connections, the flow connections and the temperature associated with each.”

“The advantage of having a model then, is that [...] you can start thinking: ‘Well, what happens if you start pumping?’” ““You can always speculate, ‘Maybe this is happening, let’s look what results you get if you assume these conditions.’” “So you can put in a well at a particular location in the virtual space that you created and see what the effects might be on the system. You can actually predict what a well in a particular location might produce in terms of the amount of water and the amount of heat that you get out of the ground.” ~Ronald Daanen

Fine-tuning the model is a complicated process. Daanen compares lines of data from the real-life physical wells already drilled at Pilgrim Hot Springs to lines of data simulated by the model, and adjusts the model parameters until the data lines match. Data which aligns properly is a good sign that the model is correctly simulating the study area. A well-tuned model will help calculate the best spot to drill additional test wells, and perhaps even a main well to serve a future power plant. That’s great news, because drilling a well is expensive. Using the model helps explorers decide where to commit real resources.

Pilgrim Hot Springs simulation data points (below ground) / Ronald Daanen

Pilgrim Hot Springs simulation data points (below ground) / Ronald Daanen

Colors and the human brain

Images from the computer simulation are alight with rainbow colors. The colors make the many equations and strings of data easier for the human brain to comprehend.

“The data itself is just data, it is numbers. But when we produce an image out of it, we put a color code on it because people understand color better than just numbers.” “To the human eye, it’s easier to interpret when we see the color values.” ~Anupma Prakash

Daanen recognizes the importance of human intellect in utilizing the computer model to search for meaning.

“The only thing a computer can do is calculate, so it takes a number, multiplies it, and you get another number. What we need to do is make meaning. So you attach a unit or a value to a number; there are programs out there that can do that and they create out of the 3-Dimensional field of numbers a 3-Dimensional field of colors of meaning.”

“If you do that in slices in time you can create a movie and the movie plays a particular view. And it is really complicated because you are dealing with what I call 4-Dimensional data, not just 3-, because you have your spatial dimensions and Time. You need a movie where you can walk in 3-D space or fly through it.” “Once you have the colors you can sit down and look at it, interpret it through the brain. That’s the only thing we [alone] can do; the computer can not do that.” ~Ronald Daanen

Laura Nielsen
Frontier Scientists: presenting scientific discovery in the Arctic and beyond

Geothermal energy in remote Alaska (Laura Nielsen) Pilgrim Hot Springs, Energy

References:

  • FrontierScientists interviews with project scientists
  • {*} ‘Development of a reservoir stimulation model at Pilgrim Hot Springs, Alaska using Tough2’ Chittambakkam, A., Daanen, R.P., Prakash, A., Haselwimmer, C., and Holdmann, G., Thirty-Eighth Workshop on Geothermal Reservoir Engineering, Stanford, California. SGP-TR-198, 13p. Full manuscript. (Feb 11-13 2013)
  • {*} ‘Geologic model of the geothermal anomaly at Pilgrim Hot Springs, Seward Peninsula, Alaska’ Miller, J.K., Prakash, A., Daanen, R., Haselwimmer, C., Whalen, M., Benoit, D., Cumming, W., Clark, A.C., Mager, M. and Holdmann, G., Thirty-Eighth Workshop on Geothermal Reservoir Engineering, Stanford, California. SGP-TR-198, p. 1326-1334. Full manuscript. (Feb 11-13 2013)
  • {*} 'Geothermal Exploration in Pilgrim, Alaska Using Airborne Thermal Infrared Remote Sensing' Haselwimmer, C., Prakash, A., and Holdmann, G., Geothermal Resource Council, 35th Annual Meeting, San Diego, California. Full manuscript. (Oct 23-26 2011)
  • {*} 'Investigating geothermally-heated ground at Pilgrim Hot Springs, Alaska using remote sensing observations of anomalous snow-melt and in-situ shallow temperature measurements' Haselwimmer, C.E., Prakash, A., Parr, C., Stowell, L., Miller, J.K., Daanen, R.P., and Holdmann, G., AGU Fall Meeting, San Francisco, California. Abstract no.V13A-2808. (Dec 3-7 2012)
  • {*} ‘Investigating low-temperature hydrothermal alteration in drill cuttings from Pilgrim Hot Springs, Alaska using a suite of low-cost analytical techniques’ Miller, J.K., Haselwimmer, C.E., and Prakash A., Geothermal Resources Council Annual Meeting, Las Vegas, Nevada. Full manuscript. (Sep 29 - Oct 2, 2013)
  • {*} 'Use of COMSOL multiphysics to develop a shallow preliminary conceptual model for geothermal exploration at Pilgrim Hot Springs, Alaska' Daanen, R.P., Chittambakkam, A., Haselwimmer, C., Prakash, A., Mager , M., and Holdmann, G., Geothermal Resource Council, 36th Annual Meeting, Reno, Nevada. Full manuscript. (Sep 30 - Oct 3, 2012)

Leave a Reply


8 − two =