The modular neural network (MNN) inversion method has been used for inversion of self-potential (SP) data anomalies caused by 2D inclined sheets of infinite horizontal extent. The analysed parameters are the depth (h), the half-width (a), the inclination (α), the zero distance from the origin (x o) and the polarization amplitude (k). The MNN inversion has been first tested on a synthetic example and then applied to two field examples from the Surda area of Rakha mines, India, and Kalava fault zone, India. The effect of random noise has been studied, and the technique showed satisfactory results. The inversion results show good agreement with the measured field data compared with other inversion techniques in use.

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The electromagnetic loop‐loop response of a uniaxial conducting half‐space is derived. The solution is presented in terms of the Geonics EM34 apparent conductivity, which is based on an underlying isotropic half‐space model of earth conductivity. The uniaxial medium generates an apparent conductivity that depends on the angle that the line joining the transmitter and receiver loops makes with respect to the strike of the anisotropy. Furthermore, a “paradox of anisotropy” occurs in which apparent conductivity is higher in the resistive across‐strike direction and lower in the conductive along‐strike direction. The paradox is resolved once it is recognized that the receiver loop response is controlled by the conductivity in the direction of the local induced electric current flow. Accurate geological interpretation of electromagnetic loop‐loop data in fractured rock terrains depends on the ability of the geophysicist to recognize the counterintuitive effects of anisotropy.

Magnetic and seismic methods have been used in this study as complementary methods to each other to construct a geologic hazard map for Wadi Thuwal area. Magnetic interpretation for deep-seated geologic structures has involved reduction to pole algorithm and downward continuation techniques. It showed that there are three major fault trends: NE-SW and NNE-SSW, NW-SE, and N-S. Furthermore, shear zone has been found close to Harrat Thuwal, which was confirmed by the seismic method. Seismic method revealed three lithologic layers where the depth of the bedrock was found to be ranging between 9 m at the southeastern part of the study area and 24 m at its northern part. It showed also five major fault trends: NW-SE, ENE-WSW, NE-SW, and nearly E-W. Supported by the surface geology, magnetic and seismic results showed that the Wadi Thuwal area can be divided into three zones on the basis of geologic hazards, depending on the presence of geologic features such as faults. It is recommended that before any development plan in Wadi Thuwal area, the delineated hazard zonation should be taken into account.

In the coastal western part of Saudi Arabia at Thuwal area located close from the Red Sea, the shallow groundwater specific electrical conductivities measured at the drill holes range from 6 to 13 mS/cm. In order to study the origin of this salinity, a good knowledge is required of the aquifer geometry with depth. Ninety nine transient electromagnetic (TEM) soundings were carried out over an area of about 100 km 2 . From the TEM profiles, a conductive substratum with a resistivity of 1-13 Ωm was identified at most of the sites at depth ranging from 50 to 150 m. This substratum is related to Oligocene-Miocene sediments (Shumaysi Formation) which are mainly red clay-rich formation containing brines at coastal zones. Clayey sediments are more likely present in the southeastern part and along the faults that run NE-SW across the study area and parallel to the Quaternary volcanic which runs NW-SE. The study demonstrated the effectiveness of the TEM sounding method to map conductive zones.

Subsurface structures associated with hard rocks are very important for groundwater. Wadi Fatima runs through the volcanic and metamorphic rocks of the Arabian Shield which are characterized by higher magnetization than the overlaying alluvium sediments. Magnetic and direct current (DC) resistivity methods have been used for groundwater exploration in the northern part of Wadi Fatima. The magnetic survey was used mainly to map the subsurface structures, using analytic signal algorithm, of the study area. The DC resistivity method was applied to describe the lithologic domain as a function of depth, depending on their electrical property contrasts where it provided a good indication for water bearing formations. The magnetic and DC resistivity interpretations were confirmed by drilling which have provided a clear idea about the hydrogeological regime of the study area. The selected drilled well is successfully productive and it produces 30 m 3 /h.

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