This study presents a preliminary geophysical assessment of shallow subsurface conditions in the planned permanent housing relocation area of Mendala Village, Brebes Regency, Central Java, Indonesia, following a landslide event that affected settlements, roads, and agricultural land. The relocation area is located in hilly terrain underlain by heterogeneous Pliocene sedimentary materials, where variations in lithology, weathering degree, stiffness, and possible groundwater influence may affect the suitability of the site for permanent housing development. Because detailed geotechnical data such as boreholes, SPT, CPT, MASW, seismic refraction, groundwater monitoring, and laboratory soil testing were not available during the early assessment stage, this study applies a non-invasive single-station microtremor approach as preliminary geophysical screening information.



Microtremor measurements were conducted at 35 points distributed across the proposed relocation area. The recorded ambient vibration data were processed using the Horizontal-to-Vertical Spectral Ratio (HVSR) method in Geopsy to obtain representative H/V curves. The HVSR curves were then inverted using the Dinver module of Geopsy with an ellipticity target and layered subsurface parameterization to estimate shear-wave velocity (Vs), compressional-wave velocity (Vp), density, and the derived Vp/Vs ratio. The inversion results were visualized through two-dimensional profiles and three-dimensional models to evaluate lateral and vertical variations in subsurface stiffness and possible weak or water-influenced zones.



The results show that the near-surface layer is generally dominated by relatively low Vs values, particularly below 750 m/s, extending to depths of approximately 20–30 m. These low-Vs zones are interpreted as soft to medium, weathered, or less competent sedimentary materials. In contrast, higher Vs values exceeding approximately 1000 m/s occur locally at greater depths and are interpreted as more competent or stiffer subsurface materials. The depth and lateral continuity of these higher-Vs zones vary across the study area, indicating heterogeneous shallow subsurface conditions beneath the planned relocation site. The Vp/Vs models show several relatively high anomalies, especially in shallow to intermediate depth intervals. These anomalies may be consistent with weak, deformable, or water-influenced materials, particularly where they coincide with low Vs values and qualitative field evidence of groundwater seepage observed during early excavation.



Based on the integrated interpretation of Vs and Vp/Vs, the northwest to northeast sector of the planned relocation area is considered relatively more favorable from a preliminary geophysical perspective because it is characterized by higher Vs values and lower to moderate Vp/Vs values. Conversely, zones with low Vs values and high Vp/Vs anomalies require greater attention because they may correspond to softer, weaker, or possibly water-influenced sedimentary materials. However, the results should not be interpreted as final geotechnical or engineering design parameters because HVSR inversion is inherently non-unique and Vp/Vs values derived from single-station HVSR inversion contain higher uncertainty than Vs. Therefore, future investigations integrating borehole data, SPT or CPT testing, MASW or other shear-wave velocity measurements, groundwater evaluation, and geotechnical engineering analysis are required before final construction decisions are made.