HR: 14:25h
AN: GP23A-04    [Abstracts]
TI: Sinkhole Hazard Assessment Using the Gravity and Magnetic Signatures of the Shallow Subsurface Mass Deficiency
AU: * Rybakov, M
EM: rybakov@gii.co.il
AF: The Geophysical Institute of Israel, 6 Baal-Shem Tov st., Lod, 71100 Israel
AU: Shirman, B
EM: bshirman@soi.gov.il
AF: The Survey of Israel, 1 Lincoln st., Tel Aviv, 66800 Israel
AB: Problems associated with sinkhole development are causing serious concerns in land-use planning in the Dead Sea area. We studied the resolving power of gravity and magnetic surveys for the assessment of sinkhole hazard. Microgravity surveying has been used worldwide to detect karst caves in the dense carbonate rocks. However, the Dead Sea sinkholes are developing very rapidly in the young, thick, low-density sediments that fill the Dead Sea graben. The gravity effect of the entire graben reaches -130mGal with a horizontal gravity gradient up to 10mGal/km. To the best of our knowledge a micromagnetic surveying has never been used to delineate subsurface cavities. Assuming that the collapse is caused by large voids in the shallow (a few tens of meters) depth, we defined our survey designs by 3-D gravity and magnetic forward modeling. A density contrast of such a target ranges from 0.9 to 2.0 gr/c3 and numerous magnetic susceptibility measurements suggested an average magnetic contrast of 3*10-4 SI. The modeling showed that high resolution detailed gravity and magnetic surveys can reliably delineate the voids located at the shallow depth. The magnetic dipole anomaly with its positive part located north of the negative part is typical of anomalies of a concealed void. We have been studying the feasibility of methods mapping the sites with sinkholes occurring in alluvial fans and mud flats along the Dead Sea shore since 1999. About 20 thousand gravity stations were measured at eleven sites and micromagnetic measurements were taken at five sites. The results suggest: - The microgravity and micromagnetic data indicate a subsurface mass deficit beneath some of the areas where open sinkholes are observed, suggesting that a mass deficit is presented and additional sinkhole development can be expected. Indeed, new sinkholes opened up at several anomalous locations. - Moreover, the absence of negative residual gravity anomalies and typical magnetic dipole anomalies at the sites with open and/or filled sinkholes, suggests that the subsurface mass deficit was recently compensated and these sinkholes are not presently developing. - The methods can be an effective tool for monitoring the subsurface mass redistribution that may signal impending collapse. - The elongated anomalies observed in several sites indicate that most collapse is related to tectonic lineaments. - The possible wavelength and magnitude of the effects of structural variations could be similar to concealed hollows. This means that the microgravity and micromagnetic surveys should be accompanied by a seismic refraction study.
DE: 1219 Local gravity anomalies and crustal structure
DE: 1517 Magnetic anomaly modeling
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2005 Joint Assembly