Three‑Dimensional Gravity Inversion and Horizontal‑Derivative Analysis for Regional Structural and Density‑Domain Characterization of the Southern Mountain Zone, East Java, Indonesia

Abstract

The Southern Mountain Zone of East Java is a structurally complex volcanic‑arc region characterized by strong lateral lithological heterogeneity and limited subsurface constraints. This study uses regional gravity‑anomaly analysis, horizontal‑derivative methods, and regularized three‑dimensional inversion to constrain regional‑scale subsurface density variations and identify candidate structural boundaries. Satellite‑derived gravity data from the GGMplus dataset were processed to obtain a Complete Bouguer Anomaly ranging from –134.3 to 170.5 mGal and a longer‑wavelength Regional Bouguer Anomaly ranging from –130.8 to 132.8 mGal. The regional component is interpreted primarily as a constraint on broad, deeper‑seated density variations rather than as a direct or unique image of subsurface geological structures. Sequential gravity inversion using Singular Value Decomposition (SVD) and Occam regularization was applied to iteratively update the model‑base geometry, density distribution, and block thickness. The normalized RMS data misfit decreased from approximately 15.5% to 3.02%, indicating progressively improved agreement between the observed and calculated gravity responses. This reduction represents improved data fitting and should not be interpreted as a direct measure of uncertainty or geological accuracy because gravity inversion remains intrinsically non‑unique. The resulting model delineates broad low‑, intermediate‑, and high‑density domains that are consistent with the regional distribution of sedimentary, volcanic, carbonate, and intrusive units. Horizontal‑derivative responses integrated with inversion cross‑sections identify several candidate structural boundaries associated with lateral density contrasts. Apparent normal‑ and reverse‑fault styles, including horst‑graben‑ and half‑graben‑like geometries, are inferred where prominent derivative responses coincide with density‑block variations. However, fault dip, displacement, and kinematics are not uniquely resolved from the gravity data. The results therefore provide regional‑scale constraints on major subsurface density domains and candidate structural boundaries rather than a unique high‑resolution reconstruction of the subsurface.