Satellites and Gravity Data Reveal Hidden Faults Beneath Indonesia’s Lokop Geothermal Prospect

Deep beneath the rolling terrain of Aceh Province in northern Sumatra, hot water is circulating through a labyrinth of fractures that scientists have now mapped in unprecedented detail without ever drilling a single well. A research team led by Faisal Abdullah of Universitas Syiah Kuala has combined satellite gravity measurements with Landsat imagery to chart the hidden plumbing of the Lokop geothermal prospect, revealing low-density zones saturated with hydrothermal fluids, a network of faults that channel hot water toward the surface, and clays and iron oxides that betray the passage of scalding fluids through the rock. The study, published in Earth Science Informatics, arrives at a moment when Indonesia is aggressively pursuing geothermal energy as a cornerstone of its renewable energy strategy, and it demonstrates how two of the cheapest exploration tools available can converge on the same underground target from completely different directions.

The appeal of Lokop is easy to understand. The prospect sits on Sumatra, an island stitched together by the Great Sumatran Fault, the massive strike-slip structure that accommodates the oblique collision between the Indo-Australian and Eurasian plates. That tectonic violence fractures the crust and provides the pathways that deep, hot fluids need to rise. Lokop is also readily accessible and surrounded by a substantial population and growing business districts, meaning that any power plant built there would have customers close at hand. The Indonesian government has pledged strong commitment to green and renewable energy initiatives, with geothermal among its priorities, yet the geological framework and geophysical properties of the Lokop prospect remained poorly constrained, and published scientific studies on the area were markedly limited. The new research set out to close that gap with two explicit goals: mapping the hydrothermal alteration zones at the surface and tracing the fault zones that feed them from below.

The first line of attack came from gravity. Gravity surveying is one of the oldest tools in geophysics, but it remains remarkably powerful for geothermal work because the physical property it measures, rock density, changes systematically as fluids and alteration destroy the original fabric of a volcanic terrain. The team conducted field gravity measurements using a CG-5 Autograv gravimeter, an instrument sensitive enough to detect variations in gravitational acceleration of a few hundredths of a milligal, and then processed the data through a suite of mathematical filters designed to sharpen the edges of buried density contrasts. Techniques such as horizontal gradient and enhanced derivative analysis amplify the signatures of steep contacts, which in a geothermal setting usually correspond to faults and fracture systems. The result was a structural map of the subsurface that no surface mapping alone could produce.

The gravity analysis delivered a clear verdict. Across the Lokop field, the Bouguer anomaly, the corrected measure of gravitational attraction that strips away the effects of elevation and surrounding topography, drops to values between -20 and -15 milligal. In geophysical terms, such pronounced gravity lows are the fingerprint of rocks that have been rendered porous and permeable, their density reduced by open fractures and, crucially, by the hydrothermal fluids filling them. Dense, intact volcanic rock pulls slightly harder on a gravimeter than fractured, water-saturated rock does, and the difference, though tiny, is measurable and mappable. The spatial distribution of these low-density zones allowed the researchers to delineate the faults and fracture systems across the Lokop field, effectively drawing the skeleton of the geothermal reservoir in three dimensions and identifying the conduits along which hot fluids ascend.

While gravity probed downward, the second half of the study looked up, or rather out, from more than 700 kilometers away. Landsat satellites have been photographing Earth continuously since the 1970s, and their multispectral sensors record reflected sunlight in narrow wavelength bands that go far beyond what the human eye can distinguish. Hydrothermally altered minerals have diagnostic spectral signatures: iron oxides absorb strongly in the blue and green portions of the spectrum, producing characteristic red-stained outcrops, while clay minerals such as kaolinite and the greenish mica chlorite absorb in the shortwave infrared. By computing band ratios and spectral indices that contrast these absorption features, the team converted ordinary satellite imagery into mineral maps, flagging pixels where the surface rocks had been chemically transformed by circulating hot fluids.

The remote sensing results dovetailed neatly with the geophysics. The altered minerals identified across the prospect were primarily iron oxides, kaolinite, and chlorite, a classic assemblage for the outer, cooler margins of a geothermal system where descending meteoric water reacts with volcanic rock at moderate temperatures. Significantly, these alteration minerals were concentrated mainly in the hot spring areas, exactly where one would expect fluid upflow zones to be. The coincidence of surface alteration, thermal springs, and subsurface gravity lows paints a coherent picture of a working geothermal system: faults acting as highways for fluid ascent, porous fractured reservoir rock at depth, and a halo of altered minerals marking where the fluids have leaked toward the surface over geologic time.

The team added a temporal dimension by analyzing time series of land surface temperature derived from the thermal infrared bands of Landsat. Rather than relying on a single snapshot, which can be distorted by weather, season, or solar heating, the multi-year record isolates the persistent thermal component of the Lokop hot spring. The springs registered temperatures ranging from 36 to 38.5 degrees Celsius, with the lowest value recorded in 2023 and the highest in 2020. That the maximum predates the minimum is more than a curiosity: the researchers interpret the warmer 2020 peak as evidence of an increased heat supply from the geothermal reservoir, a reminder that these systems are dynamic and that their surface expressions can vary on timescales of just a few years. For exploration geologists, such thermal variability is a clue about how vigorously the reservoir is being recharged and heated from below.

What makes the study methodologically interesting is its economy. Traditional geothermal exploration escalates quickly in cost, from geological mapping through gravity and magnetic surveys to magnetotellurics, seismic reflection, and finally slim-hole drilling, which can run into millions of dollars per well. By integrating freely available Landsat data with a modest ground gravity campaign, the Lokop team extracted a structural framework and an alteration map for a fraction of the usual expense, and they did so in a region where prior published data were scarce. The approach follows a well-established logic in geothermal exploration worldwide, from the East African Rift to Turkey and the Philippines, where the combination of potential field geophysics and optical remote sensing routinely serves as the screening tool that decides where expensive instruments and drills should be pointed next.

The regional tectonic context strengthens the case that Lokop is worth that attention. Sumatra’s geothermal resources are fundamentally a byproduct of the Great Sumatran Fault system, whose segmentation controls where volcanic centers and hot springs emerge along the 1,900-kilometer structure. Previous studies by some of the same Universitas Syiah Kuala researchers have used gravity data to trace the fault’s continuity, including its offshore extensions, and to map shallow fracture systems around volcanic prospects such as Jaboi and Seulawah. The Lokop work extends that playbook to a previously understudied field, showing that the same density contrasts and fault-controlled permeability that characterize Sumatra’s better-known geothermal systems are present here as well.

For Indonesia, the implications are practical. The country sits along one of the most volcanically active arcs on Earth and holds some of the world’s largest estimated geothermal reserves, yet only a fraction of that capacity has been developed. Prospects like Lokop, which are accessible, close to demand, and now supported by a first-pass scientific characterization, are natural candidates for the next stage of exploration, which would likely involve magnetotelluric soundings to image the conductive clay cap at depth, geochemical sampling of the springs, and eventually temperature gradient wells. The authors present their results as essential insights for enhanced characterization of the Lokop geothermal system, and in doing so they offer a template for how satellite data and a gravimeter in a backpack can turn a blank spot on the exploration map into a well-defined drilling target, all before a single rig arrives on site.

Subject of Research: Integrated gravity and Landsat remote sensing analysis for mapping hydrothermal alteration and fault zones in the Lokop geothermal prospect, Sumatra, Indonesia

Article Title: Integrated gravity and landsat analyses for hydrothermal alteration and fault zone mapping in the lokop geothermal prospect, Indonesia

Article References: Abdullah, F., Yanis, M., Adhari, M. R., Darisma, D., Nugraha, G. S., Zainal, M., & Ismail, N. (2026). Integrated gravity and landsat analyses for hydrothermal alteration and fault zone mapping in the lokop geothermal prospect, Indonesia. Earth Science Informatics, 19(10), Article 178. https://doi.org/10.1007/s12145-026-02229-0

Image Credits: AI Generated

DOI: 10.1007/s12145-026-02229-0

Keywords: geothermal energy, Lokop, Sumatra, gravity survey, Landsat, hydrothermal alteration, fault mapping, Bouguer anomaly, remote sensing, kaolinite, Great Sumatran Fault, Indonesia