GEOPHYSICS LETTERS

6 Delineation of the structural patterncorrelation factor, which is higher than 0.7 indicates a good similaritybetween th(.,maps.These methods give satisfactory results with the second orderresidual map, so as the second order can be selected as the optimumorder as described below.Figures 9 and 10 show the least-squares second order regionaland residual maps respectively. The second order residual gravity map(Fig. 10) shows three major anomalies in addition to some local onesthat have negative and positive nature trending NW-SE, NE-SW andNS. The first anomaly (a) is located at the northwestern corner of thearea. It has a positive nature, oval shape and large size. It trends NE-SWdirection. The second anomaly (b) is located to the right of the first one,it has a negative value and trends N-S. It is a large anomaly with contourvalues ranging from 0 to -13 mgal. The third one (c) is located East ofanomaly (b) with a positive nature and trending in NW-SE direction.The second anomaly (b) may reflect a sedimentary basin or synclinebetween two uplifted blocks (a and c). There are other small anomaliesof negative and positive nature located at the south, southeastern andnortheastern parts.3- Depth estimation methodsa) ModelingTen profiles were selected for modeling using GM-SYS prog~am,which is produced by Northwest Geophysical Associates Inc., USA(1995). The location of these profiles is indicated in Figure (11). Theinput data are the gravity values along the profile and the densitycontrast between the sedimentary and basement rocks (~p=O.25gm1cm3).The calculations of the models for these profiles producesatisfactory results. Figure (12) represent one example for the modelingtechnique.Profile P3: This profile runs in the NW-SE direction (Fig.12-a) andpasses near the Khatatba No.1 well. The depth to the basement is about1.1 km near the well and increases toward the south eastern part to about3 km.b) Spectral analysisSpectral analysis has proved to be a suitable tool for the directinterpretation of gravity and magnetic data. It gives sufficientinformation about the mean depth of the causative bodies. In thismethod, the potential anomalies in the space domain are transformedinto frequency domain and the vari0us parameters of the anomaly"