Plum Acquisition Corp. IV and Controlled Thermal Resources Outline a Large-Scale Geothermal and Critical Minerals Platform Plum Acquisition Corp. IV (“Plum IV”) and Controlled Thermal Resources Holdings Inc. (“CTR”) are moving toward a proposed business combination that would create a public company focused on one of the most ambitious geothermal and critical minerals developments in the United States. The combined entity is expected to trade on Nasdaq under the pro forma ticker CTRH, reflecting a platform built around clean baseload power, lithium recovery, and broader critical minerals production. The investor presentation dated August 2026 frames the transaction around CTR’s Hell’s Kitchen project in Imperial County, California, which is presented as a strategically positioned resource opportunity with energy, minerals, infrastructure, and permitting advantages. At the center of the presentation is a clear message: this is not just a power project, and it is not j...
Seeing Through Stone: How the Géoscan Project Is Opening a New Frontier for Geothermal in Greater Paris
The Île-de-France region, home to the sprawling metropolis of Paris and its surrounding suburbs, is already the undisputed champion of deep geothermal energy in Europe. With 54 active installations as of 2025, no other region on the continent comes close. The eastern and northern parts of the region have been tapping the hot waters of the Dogger limestone aquifer for decades, heating tens of thousands of homes with clean, renewable energy.
But what about the west? What about the south?
For years, those areas remained a geothermal blind spot. The subsurface was less understood. The geology was more complex. Project developers hesitated to invest because the risk of drilling a dry well was simply too high to justify. The result was a lopsided geothermal landscape: thriving activity in the east and north, cautious silence in the west and south.
On May 19, 2026, that silence was broken. The French state, via ADEME Île-de-France, the Île-de-France region, and the BRGM (the French Geological Survey), presented the results of the ambitious Géoscan program. After two years of intensive geophysical surveys, data analysis, and 3D modeling, the partners can now say with confidence: the west and south of the region also hold real geothermal promise.
This article dives deep into the Géoscan project—how it worked, what it found, and why it matters not just for Paris but for every city sitting on uncertain geology.
Part 1: The Heat Beneath Our Feet – Why Île-de-France Needs Géoscan
To understand the urgency of Géoscan, one must first grasp the scale of the heating challenge in Île-de-France. Nearly 45% of the region's final energy consumption goes toward producing heat. That is an astonishing share—higher than electricity, higher than transportation. And despite decades of renewable energy progress, much of that heat still comes from burning fossil fuels, primarily natural gas.
Decarbonizing heat is therefore not a side project. It is the central challenge of the region's energy transition.
Deep geothermal energy offers a uniquely elegant solution. The principle is simple: drill down 1,500 to 2,500 meters into certain limestone layers, known as the Dogger and Oxfordian formations. These layers are naturally porous and permeable, meaning they are full of tiny holes and fractures that hold hot water. At those depths, the water temperature typically ranges from 55 to 85 degrees Celsius—perfectly suited for district heating networks. Pump the water up, extract its heat through a heat exchanger, distribute that heat to homes and buildings, and then reinject the cooled water back into the same geological layer to be reheated naturally.
The beauty of the system is its circularity. No fuel is burned. No smoke rises from a chimney. The only moving parts are pumps. Once the wells are drilled and the infrastructure is built, the operating costs are low and stable, insulated from the wild price swings of global gas markets.
But the system only works if you drill in the right place. And knowing the right place requires detailed knowledge of the subsurface. That is where the east and north have always had an advantage. Decades of drilling and production have created a rich dataset. Developers know exactly where the Dogger is thick, where it is permeable, and where it is not.
The west and south, by contrast, remained terra incognita. Few wells had been drilled. The existing geological maps were coarse and outdated. A developer considering a project in Versailles or Rambouillet would be flying blind. That is precisely the problem that Géoscan was designed to solve.
Part 2: The Géoscan Method – A Geological Ultrasound for 2,000 Square Kilometers
The Géoscan program was launched in November 2023 with a clear mandate: study the subsurface of nearly 300 communes across approximately 2,000 square kilometers of western and southern Île-de-France. The target area stretched from the southern Val d'Oise down through the northern Essonne, and from the eastern Yvelines across to the Hauts-de-Seine.
To achieve this, the project team designed a geophysical acquisition campaign of unprecedented scale. The centerpiece was a fleet of "vibrator trucks"—heavy vehicles equipped with vibrating plates that press against the ground and send controlled acoustic waves deep into the earth. This technique, known as seismic reflection, works much like an ultrasound on a human body. The sound waves travel down through the geological layers, bounce off boundaries between different rock types, and return to the surface where sensitive receivers record them.
By analyzing the timing and strength of the returning waves, geophysicists can construct detailed images of the subsurface, revealing the depth, thickness, and structure of the target reservoirs.
The Géoscan campaign involved approximately 280 kilometers of seismic lines, crisscrossing nearly 100 communes across all six departments of the region. The work was done at night, between March and April 2024, to minimize disruption to traffic and daily life. It was a logistical feat requiring coordination with local authorities, road closures, and public communication to reassure residents about the strange rumbling trucks passing through their neighborhoods at midnight.
Once the acoustic data was collected, the BRGM took over. The agency's scientists spent months processing the raw seismic records, correcting for noise and distortions, and integrating the new data with existing well logs and geological studies. The result was a pair of digital masterpieces: a three-dimensional geological model of the entire study area, and a "reservoir model" specifically focused on the two primary geothermal targets, the Oxfordian and Dogger limestone formations.
Part 3: The Results – Three Reservoirs, One Clear Answer
The Géoscan study focused on three geological reservoirs, listed by increasing depth. Each has its own characteristics and potential applications.
The Oxfordian lies between 700 and 1,600 meters below the surface. It is the shallowest of the three targets, which generally means lower drilling costs. However, shallower also often means cooler. The water temperatures in the Oxfordian are generally lower than in deeper reservoirs, making it more suitable for direct heating applications rather than electricity generation. The Géoscan results showed that the Oxfordian is present and potentially exploitable across significant portions of the study area, though its characteristics vary.
The Dogger sits between 1,000 and 2,000 meters deep. This is the workhorse of Parisian geothermal energy. The Dogger has been exploited for decades in the eastern and northern parts of the region, and for good reason. It is thick, porous, and holds water at temperatures ideal for district heating. The Géoscan results confirmed that the Dogger extends into the western and southern study area, though with some variations. In some zones, the layer is thinner than in the east. In others, the permeability—the ability of water to flow through the rock—is lower. But critically, the study identified specific zones where the Dogger retains favorable characteristics, creating clear targets for future development.
The Trias lies deepest, between 1,200 and 2,500 meters. This older formation has been less exploited historically because it is deeper and therefore more expensive to reach. However, deeper often means hotter. The Trias could offer water temperatures exceeding 85 degrees Celsius in some locations, opening up more efficient heating and even potential for electricity generation via binary cycle technology. The Géoscan results provided the first consistent picture of the Trias across the study area, identifying zones where it is thick, permeable, and thermally promising.
The overarching conclusion from the three reservoirs is clear and encouraging: yes, the west and south of Île-de-France have geothermal potential. No, it is not identical to the east and north. But with the detailed maps now available, developers can target the specific zones where the geology is most favorable, avoiding the areas where it is not.
Part 4: Unlocking the West – What the Map Changes
Before Géoscan, a mayor or a housing authority in the western suburbs or the southern countryside had no reliable way to assess whether geothermal was a viable option for their community. They could look at the success in the east and wonder: why not us? But they could not answer that question with data. The risk of moving forward was simply too high.
Now they have an answer. The Géoscan results, which are being made publicly available through a dedicated website, provide a color-coded favorability map. Areas shaded in dark green are highly favorable. Yellow areas are moderate. Red areas are low. For the first time, a commune in the Yvelines can look at its specific location and see, at a glance, whether the Dogger beneath its streets is likely to produce hot water.
This changes everything. A favorable map does not guarantee a successful well. Drilling always carries residual risk. But it reduces the uncertainty dramatically. Developers can now focus their exploration budgets on the most promising zones. Municipalities can include geothermal in their climate action plans with confidence. Private investors can model project economics based on realistic assumptions rather than wishful thinking.
The prefect of the Île-de-France region, Marie Gautier-Melleray, put it succinctly: the results will allow the region to "remain an example in Europe." The ambition is not just to maintain the existing geothermal fleet but to expand it aggressively into new territories.
Part 5: The Policy Context – Heat, Sovereignty, and Decarbonization
The timing of the Géoscan results is no accident. France has set ambitious targets for reducing greenhouse gas emissions and fossil fuel consumption. The war in Ukraine and the subsequent energy crisis exposed the vulnerability of relying on imported natural gas for heating. Every therm of gas burned in a Parisian boiler is a therm that could be cut off by a foreign power or inflated by a global market.
Geothermal offers a path to energy sovereignty. The heat beneath Île-de-France belongs to France. No pipeline can be sabotaged. No tanker can be rerouted. No dictator can raise the price. Once the wells are drilled, the heat flows, and the price is stable for decades.
The regional government, led by Vice-President Yann Wehrling, has made the expansion of geothermal a strategic priority. The goal is to "massify" the use of renewable heat across the entire region—not just the traditional geothermal heartland in the east and north, but everywhere the subsurface allows. The Géoscan results provide the roadmap.
The ADEME regional director, Amélie Renaud, emphasized that the results "open the way to a concrete acceleration of projects." This is not vague political rhetoric. With the favorability maps in hand, project developers can move to the next phase: feasibility studies, drilling permits, financing arrangements, and ultimately, construction. The two years of Géoscan research have compressed what might have taken a decade of piecemeal exploration into a single, coordinated campaign.
Part 6: The Technology – Why Vibroseis Trucks Worked at Night
For residents of the affected communes, the most memorable part of Géoscan was likely the strange procession of heavy trucks rumbling through their streets in the middle of the night. The choice to work at night was deliberate and thoughtful.
Vibroseis trucks generate significant ground vibration. While the energy levels are carefully controlled and well below thresholds that would damage buildings, the noise and vibration are noticeable to anyone nearby. Working at night, when traffic is minimal and fewer people are awake to be disturbed, reduced the impact on daily life. It also improved the quality of the seismic data, as the acoustic noise from cars, trains, and industrial activity is much lower after midnight.
The trucks themselves are marvels of engineering. Each truck carries a vibrating plate, typically about a meter and a half across, that presses down onto the asphalt with several tons of force. A hydraulic system then causes the plate to vibrate at precisely controlled frequencies, typically sweeping from low to high over a period of several seconds. The vibrations propagate downward through the road surface, through the shallow soil and gravel, and deep into the bedrock below.
Along the seismic lines, strings of geophones—sensitive listening devices—were laid out on the ground, often spaced every few dozen meters. These geophones recorded the returning echoes from the deep geological boundaries. A single shot from a vibroseis truck might generate several seconds of recorded data, containing information about rock layers more than two kilometers down.
The entire operation required careful coordination. Trucks had to move slowly along predetermined routes, stopping every few hundred meters to perform a vibration sequence. Geophones had to be laid out, collected, and relaid as the line progressed. Traffic had to be managed. Residents had to be informed. The fact that the campaign was completed successfully is a testament to the professionalism of the field teams and the cooperation of local authorities.
Part 7: The BRGM's Role – From Raw Data to 3D Model
The BRGM is France's national geological survey, a public institution with more than six decades of experience in mapping and understanding the country's subsurface. For the Géoscan project, the BRGM played the central role of data processor and interpreter.
Raw seismic data is not a map. It is a chaotic collection of wiggly lines representing sound waves bouncing off rocks. Converting that raw data into a usable geological model requires sophisticated mathematics, powerful computers, and deep geological expertise.
The BRGM team first applied a series of processing steps to clean up the seismic data. Noise was removed. Spurious signals were filtered out. The timing of reflections was corrected for variations in near-surface geology. The result was a set of "stacked" seismic sections—essentially vertical slices through the earth, showing the arrangement of geological layers along each seismic line.
Next came the interpretation. The geologists identified specific reflections that correspond to the top and bottom of the Oxfordian, Dogger, and Trias layers. They tracked these reflections continuously across the survey area, building a three-dimensional framework. Where well data existed, they used it to calibrate the seismic interpretation, ensuring that the depths and thicknesses were accurate.
Finally, the team integrated additional data sources: existing well logs, temperature measurements, and production data from existing geothermal plants in the east. The result was not just a geometric model of the layers but a "reservoir model" that estimates permeability, porosity, and water temperature across the study area.
This model is the crown jewel of Géoscan. It lives on a computer server, but its implications are entirely physical. Every developer considering a project in the study area will now consult this model before choosing a drill site.
Part 8: What Comes Next – From Maps to Megawatts
The presentation of results on May 19, 2026, marks the beginning of the next phase, not the end. The maps and models are now public. The next step is for project developers, municipalities, and energy companies to use them.
Several immediate actions are expected. First, municipalities in favorable zones will likely commission feasibility studies. These studies will assess not only the subsurface but also the surface: where are the district heating networks? Where are the buildings that need heat? Who will own and operate the geothermal plant?
Second, the national government and the region will likely adjust their subsidy and risk mitigation programs to reflect the new knowledge. If the maps show that certain zones are highly favorable but have no existing projects, there may be a case for targeted incentives to jumpstart development.
Third, and most speculatively, the success of Géoscan may inspire similar programs in other French regions. If the model of a coordinated, publicly funded seismic campaign can unlock geothermal potential in one place, why not in others? The BRGM's expertise and the vibroseis fleet are national assets that could be deployed elsewhere.
The BRGM's director for energy and decarbonization, Frédéric Glanois, described Géoscan as "a model of its kind." That is not mere modesty. The combination of state, region, and scientific agency, working together on a two-year timeline with a clear deliverable, is replicable. Other regions and other countries facing similar geological uncertainty could learn from the French approach.
Conclusion: Seeing Clearly, Drilling Wisely
The story of geothermal energy in Île-de-France has always been one of geological fortune. The Dogger aquifer beneath the eastern and northern suburbs is a gift—a vast, hot, permeable reservoir sitting at a drillable depth. That gift has heated homes and created jobs for decades.
But fortune favors the prepared mind. For the west and south, geological fortune was ambiguous. The Dogger exists, but its quality varies. The Oxfordian and Trias are present, but their characteristics were poorly known. Without good information, developers stayed away. Potential went unrealized.
The Géoscan program changed that equation. By investing public money in a comprehensive seismic campaign, the French state, the Île-de-France region, and the BRGM have transformed uncertainty into knowledge. The west and south are no longer blind spots. They are mapped territories, with favorability zones clearly marked.
Now the baton passes to the market. Developers will study the maps. Municipalities will debate the options. Investors will calculate the returns. Some projects will move forward. Others will not. But whatever happens next will be based on data, not guesswork.
That is the quiet revolution of Géoscan. No drilling has yet occurred as a result of the project. No new heat has flowed. But the foundation has been laid—a geological map of unprecedented detail, built at unprecedented scale, delivered on an unprecedented timeline. The heat is down there. Now everyone can see exactly where.
Source: BRGM
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