Integration of micro-gravity and very low frequency (VLF-EM) electromagnetic (EM) data analyses to outline hazardous cavities and low rock-strength: Coastal Carbonates, Wadi Shab, Oman

Geotechnical risk in building residential and urban structures in carbonate rocks areas close to the shoreline of southeastern Oman mandates geotechnical site characterization with special attention to near surface geophysical applications relevant to rock strength mapping and detection of subsurface cavities. A coastal area of about 700 m near the mouth of Wadi Shab, southeastern Oman, was selected for building a village to accommodate some residents, from nearby coastal twon (Tiwi), threatened with high probability of collapse subsurface cavities and low strength near-surface zones. For geotechnical evaluation of the selected area, we conducted two joint surveys; one is microgravity and the other is a VLF EM, both with station interval of 5 meters and average line spacing of 10 meters. In support for achieving high accuracy in gravity elevation corrections, elevations were measured to accuracy within ±3 cm. After processing the field data, negative microgravity Bouguer anomalies and resistive VLF in-phase anomalies in spatial agreement outlined a high risk zone, which was interpreted to be a zone with fractures trending parallel to shoreline and subsurface cavities with diameters in the range of 1.0 to 2.0 meters. Sea wave erosional effects and coastal fault zone are interpreted as the prime factors in developing higher geotechnical risk zones in the studied area.