Contribution of geophysical studies on detection of the Petrified Frost Qattamiya, Cairo, Egypt

摘要

Different geophysical tools such as resistivity, seismic refraction, and magnetic survey have been applied to delineate the subsurface stratigraphy and structural elements, which controlled the distribution and origin of the Petrified wood in Qattamiya, Cairo, Egypt. Land magnetic survey was carried out in the study area through two stages, the first stage includes all area by measuring 11,674 stations and the second stage was carried out in the detailed area that was located at the southeastern part of the all area including 9441 stations. All measurements have been corrected for diurnal variation and reduced to the north magnetic pole. The results of magnetic interpretation indicated that the area dissected by different structural elements trend toward NE–SW, NW–SE, N–S and E–W directions. Twenty-eight samples have been collected from the detailed area to analyze for magnetic susceptibility values. Four electrical resistivity tomography (ERT) profiles were measured by using dipole–dipole configuration to estimate the vertical and lateral variation of the subsurface sequence. Results of quantitative interpretation of the ERT data indicate that the subsurface sequence consists of different geoelectric units; the first unit is characterized by high resistivity values upto 1000ohmm corresponding to sand, gravel and Petrified wood at the surface and extends to a depth of a few meters. The second geoelectric unit is corresponding to sandy clay which exhibits moderate resistivity (few hundred ohmm) values with thickness ranging from 6 to10m. The third geoelectric unit is characterized by very low resistivity corresponding to clay of depth ranging from 10 to 30m overlaying the fourth unit which reached to a depth ranging from 30 to 56m and characterized by very high resistivity (8000ohmm) corresponding to limestone. Three shallow seismic refraction spreads of geophone spacing 7.5m were measured to investigate the subsurface sequence, where the results of interpretation indicate that the subsurface section consists of three units of average velocity 500, 2000 and 4000m/s, respectively corresponding to sand, gravel and Petrified wood, sandy clay, clay and limestone.