HR: 18:05h
AN: NS34A-07    [Abstracts]
TI: The VLF EM Method Used for Verification of Fracture/ Shear Zone Aquifers in the Hyper-arid Eastern Desert, Egypt
AU: * Sauck, W A
EM: sauck@wmich.edu
AF: Department of Geosciences, Western Michigan University, Kalamazoo, MI 49008, United States
AU: Sultan, M
EM: mohamed.sultan@wmich.edu
AF: Department of Geosciences, Western Michigan University, Kalamazoo, MI 49008, United States
AU: Wagdy, A
EM: awagdya@yahoo.com
AF: Dept. of Irrigation & Hydraulics Engineering, Cairo University, Giza, Egypt
AU: Roouf, O A
EM: ossama_elmorsy@yahoo.com
AF: National Water Research Center, Water Resources Research Institute, El-Qanatir, Egypt
AB: An integrated program using Landsat remote sensing and ground follow-up with the Very Low Frequency (VLF) geophysical method was applied to the basement rocks of the Red Sea Hills (Eastern Desert) to locate fracture and shear-system aquifers. This part of the Nubian Shield was formed by accretion of a complex of ensimatic and ensialic island arcs and interleaving oceanic basins that were later accreted against the old African continent. Hence, melange and ophiolite sequences are common. This basement complex was intensely fractured (630- 530 Mybp) by the Najd transcurrent shear system (NSS) along a NW-SE trend that is up to 350 km wide, and finally the ocean-arc complex was intruded (~550 Mybp) by anorogenic K-granites. A false-color composite image was created, from Landsat thematic mapper band ratio images that are sensitive to the Fe-bearing aluminosilicate, hydroxyl, and opaque phase content of rocks. On these images mafic rocks (e.g., gabbro and mafic volcanics) rich in Fe-bearing aluminosilicates appear in shades of blue, ultramafics (e.g., serpentinites) rich in hydroxyl-bearing phases and opaque phases appear in shades of red, and granitoid rock units poor in the above phases show as green areas. Using this base map (effectively a pseudo geologic map) and a co-registered DEM, locations of potential shallow water occurrence were plotted based on the following criteria: 1) intersection of the NSS system with transverse faults defining wadis, 2) intersection of two or more fault zones, 3) within highly deformed melange units, especially their internal lithologic contacts and their crossings of wadis, 4) relatively unfractured younger dikes and their intersections with wadis. The VLF instrument was first used to make profiles at a number of existing water wells located at the structural intersections described above, to verify that sub-vertical sheet-like electrical conductors (water-filled fissures) could be successfully located with this instrument. Then, other sites that had been marked as having high potential for water using one or more of the above criteria were visited and profiled with the VLF. Many (but not all) of these sites gave anomalies characteristic of conductive fissures. Finally, several sites not chosen by the GIS interpreters were selected at random and profiled with the VLF. One of these (albeit adjacent to a melange area) did show a good conductor. Field navigation was directly on the scrolling false-color image on a laptop computer, linked to an active GPS receiver. VLF transmitters used were mainly those in Europe. Fourteen sites were visited in five days and nineteen total profiles were surveyed, ranging from 160 to 1200 meters in length. In summary, this methodology, beginning with satellite imagery and GIS, and ending with transects on foot with a VLF, proved to be a useful technique in such desert landscapes where fractured bedrock aquifers occur and the discovery of even a low-yield local aquifer is very important.
DE: 0925 Magnetic and electrical methods (5109)
DE: 1819 Geographic Information Systems (GIS)
DE: 1829 Groundwater hydrology
DE: 1835 Hydrogeophysics
SC: Near-Surface Geophysics [NS]
MN: 2007 Joint Assembly