4-D Microgravity: A Method For Cave Detection And Monitoring

Sinkhole hazard is becoming an apparent problem in the Dead Sea area and a severe obstacle in future land use planning in the region. A feasibility study for the use of microgravity for cave detection (3D) and monitoring the collapse development over time (4D) has been completed in the area. The microgravity was used with the assumption that it could detect subsurface cavities, regardless of their shape or filling material, as long as there is sufficient density contrast. 3-D modeling indicated that subsurface sinkholes could be detected at a depth of up to several times the diameter of the subsurface cavity. Detection was carried out microgravity mapping, using a 5x5-meter grid, with accurate geodetic control in real time. Accuracy is estimated to be in the range of 0.01 to 0.02 mGa1. The microgravity maps indicate the presence of large subsurface cavities underlying the areas where several open sinkholes are observed, suggesting that additional sinkhole development can be expected. Tracing the development of sinkholes, before their roof collapses and they become an apparent surface feature, was attempted by repeated measurements along a road, where such sinkholes were observed in the past. This 4D microgravity monitoring experiment lasted 4 months and revealed apparent changes in the gravity field in the order of 0.03 mGa1, which are thought to be associated with changes in the size of the cavity, probably as a result of roof collapse. The results suggest that microgravity can be an effective technique, not only for detecting and delineating subsurface cavities, but also in monitoring their development in time and prediction of collapse hazards.