The findings offer practical guidance for geophysics researchers, students, and geothermal exploration professionals who use gravity data to investigate underground structures. Better interpretation of these data can support early-stage exploration and help guide further geological investigations before costly drilling begins. However, the gravity patterns identified in this research remain preliminary indicators and require confirmation from independent geological and geophysical evidence.
Why Gravity Anomalies Matter for Geothermal Exploration
Indonesia has substantial geothermal energy potential because of its location along the Pacific Ring of Fire. Developing this resource requires more than identifying volcanic areas. Exploration teams must also understand underground rock properties, geological boundaries, fractures, and possible pathways through which heated fluids circulate.
Gravity surveying is one method used to investigate these conditions. It measures small variations in the Earth's gravitational field caused by differences in the density of underground materials. Dense rocks, porous formations, fractured zones, and rocks altered by hydrothermal activity can produce different gravity responses.
Researchers commonly process gravity measurements into a Complete Bouguer Anomaly (CBA) map. This map accounts for several effects, including latitude, elevation, and terrain, to help reveal gravity variations associated with subsurface density differences. However, a CBA map combines signals from both deep and shallow sources, along with possible measurement and processing noise. Smaller geological features can therefore be difficult to distinguish from broader regional patterns.
Kusuma's research addresses this challenge by applying gravity anomaly enhancement techniques. These methods help highlight changes in the gravity field, making potential geological boundaries easier to examine.
Comparing Two Geophysical Software Platforms
Kusuma processed CBA data from a geothermal prospect covering approximately 1.1 by 1.0 kilometers. The reported gravity anomaly values ranged from about 145 to 155.5 milligals (mGal). The article acknowledges Institut Teknologi Bandung for providing the gravity data and access to software resources.
The workflow involved four main stages. First, the irregularly distributed gravity observations were converted into a regular grid using a method called kriging. This process estimates values between measurement locations and produces a continuous surface for mapping.
Second, the researcher applied two derivative techniques. The First Horizontal Derivative (FHD) measures how rapidly gravity values change across the horizontal plane. Strong changes can indicate boundaries between materials with different densities, including possible faults or contacts between rock types.
The Second Vertical Derivative (SVD) emphasizes shorter-wavelength variations that may help reveal shallow features and sharpen potential structural boundaries. However, these small-scale signals can also be sensitive to noise and processing artifacts.
Third, Kusuma used radial spectrum analysis to classify gravity patterns by wavelength. Broad patterns are generally associated with deeper or larger-scale sources, while shorter-wavelength patterns may reflect shallower features or noise.
Finally, the researcher separated regional anomalies, which represent broad gravity trends, from residual anomalies, which emphasize more localized variations. The processing workflow was compared in Surfer and Oasis montaj Viewer under the configurations available for the research.
Three Key Findings
1. Both programs reproduced the same large-scale gravity pattern.
The CBA map revealed an elongated north–south zone of relatively high gravity values, bordered by lower values on the western and eastern sides. Both Surfer and Oasis montaj reproduced this overall pattern after gridding.
Their main visual differences involved color gradation and shaded relief. Oasis montaj displayed finer color transitions and stronger shading, while Surfer used broader color intervals. These differences demonstrate why researchers should compare the underlying patterns and numerical scales rather than interpreting color differences as evidence of different geological conditions.
2. Spectral analysis identified two important wavelength thresholds.
The radial spectrum analysis produced two cut-off wavelengths that helped distinguish regional, residual, and high-frequency noise components.
- Approximately 525 meters: This threshold separates the regional component from the residual component.
- Approximately 133 meters: This threshold separates the residual component from the higher-frequency noise component.
In the reported classification, features with wavelengths longer than about 525 meters were treated as regional patterns. Features between approximately 133 and 525 meters were classified as residual anomalies, while features shorter than about 133 meters were treated cautiously as potential noise.
These thresholds provide a practical reference for choosing spatial scales during gravity-data interpretation. They do not, by themselves, establish the exact depth or geological identity of an underground structure.
3. Residual anomalies highlighted localized variations in the northern and southern areas.
Spatial filtering in Surfer produced a residual anomaly map with values ranging from approximately −2.2 to +1.4 mGal. Positive residual anomalies appeared mainly in the northern and southern parts of the study area, while smaller negative anomalies occurred along several margins.
These patterns may help researchers identify areas for further investigation. Nevertheless, a positive anomaly does not automatically indicate a particular type of dense rock, and a negative anomaly does not independently prove the presence of a fault, fracture zone, or geothermal reservoir. Additional geological evidence is needed to distinguish among possible explanations.
Software Differences Can Affect Interpretation
One important technical finding concerned the implementation of SVD. In the workflow described by Kusuma, Surfer applied a sign inversion when combining the second horizontal derivatives, while the Oasis montaj Viewer workflow added the derivative grids without that inversion. Consequently, the resulting anomaly polarities appeared reversed.
This difference matters because researchers comparing maps from different platforms could misinterpret positive and negative anomalies if they overlook the mathematical convention used. Explicitly documenting the equations and sign conventions is therefore essential for reproducible geophysical analysis.
The platforms also supported different parts of the workflow. Oasis montaj was used to generate the radial spectrum, while Surfer was used to perform regional–residual separation through spatial filtering. Frequency-domain separation could not be implemented with the software configurations available for the study. This limitation should not be interpreted as a general restriction of every version of either software platform.
Implications for Geothermal Exploration and Geophysics Education
The comparison provides a practical reference for researchers and students selecting tools for gravity-based exploration. Combining the platforms can support complementary tasks, including spectral characterization, derivative mapping, and spatial filtering.
For geothermal exploration, these techniques can help prioritize areas for closer examination. They may also improve the preparation of preliminary subsurface models before more detailed surveys or drilling decisions are considered.
However, the research has important limitations. The software outputs were compared qualitatively rather than through numerical similarity measures. The geological interpretations were not validated against independent geological mapping, resistivity surveys, geochemical evidence, or well data. The moving-average filter used for spatial separation can also be affected by edge effects and the choice of filter window.
Kusuma's findings therefore support a careful, multi-method approach. Future research could compare software outputs quantitatively, apply frequency-domain filters where the necessary tools are available, test additional structural-boundary detection methods, and integrate gravity results with geological, geochemical, and resistivity data.
The central message is that gravity anomaly enhancement can make subsurface patterns easier to interpret, but software outputs must be checked carefully and geological conclusions must be tested against independent evidence.
Author Profile
Wira Hadi Kusuma is affiliated with Universitas Negeri Makassar, Indonesia, and is the corresponding author of the article. His research contribution presented here focuses on gravity anomaly processing, geophysical data visualization, and methods for characterizing subsurface structures relevant to geothermal exploration.
Research Source
Article title: Gravity Anomaly Enhancement for Geothermal Prospect Characterization: A Comparative Application of First Horizontal Derivative, Second Vertical Derivative, and Regional–Residual Separation Using Surfer and Oasis MontajAuthor: Wira Hadi Kusuma
Journal: International Journal of Sustainable Applied Sciences (IJSAS)
Publication year: 2026
Volume and pages: Vol. 4, No. 10, pp. 1433–1448
DOI: https://doi.org/10.59890/ijsas.v4i10.86
Official journal website: http://ijsasjournal.my.id/index.php/ijsas
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