Integration of seismic refraction tomography and electrical resistivity tomography in engineering geophysics for soil characterization

Abstract

The use of both seismic refraction tomography (SRT) and electrical resistivity tomography (ERT) techniques have commonly been used to detect physical properties in the subsurface materials in order to map the subsurface geological features and soil characterization in the site investigations. Using of both techniques increases confidence in interpretation to limit inaccurate interpretation due to the large amount of heterogeneity in the near surface. Twelve shallow seismic refractions and ten profiles for electrical resistivity were conducted in the Teaching Hospital Project site in Mosul University, Iraq. The linear arrays by using 12 geophones with 10 Hz frequency are used to SRT while ERT traverses were conducted with a total length (280 m) by using Wenner array with an initial electrode spacing of 3 m. Both SRT and ERT data were acquire and interpreted using SRT method and ERT methods to create tomogram velocity models and two dimension resistivity images by using the SeisImager/2D and RES2DINV software, respectively. The seismic velocity values show that the site has three layers, just as follows: the first one corresponded to recent superficial deposits and is characterized by low velocity ranging from about 340 to about 700 m/s. The second one corresponded mostly to the river deposits composed especially of river terraces and clays are characterized by relatively highest velocity values ranging from about 840 m/s to about 1700 m/s. The third one corresponded to the upper part of Fat’ha (Lower Fars) formation which is mostly composed of marl layers and is characterized by high velocity ranging from about 1900 m/s to about 2800 m/s. Interpretations of 2D resistivity profiles indicated alteration zones at depth. It was determined that the material could be classified into two main zones. The first zone has a relatively high true resistivity value ranging from about 80 to about 320 Ω m with a thickness ranging from about 1 to about 25 m, which is mainly consisted of conglomerate, gravel and sand (weathered layer). The second zone has a relatively low true resistivity value reached to 80 Ω m which represents upper part of Fat’ha (Lower Fars) formation deposits which mainly consisted of clays. The two geophysical methods were used to be enjoined the better way to aid the interpretation and evaluate the significance and reliability of the results obtained in each single method. It is demonstrated that engineering geophysics is able to provide solutions for determining subsurface properties and that different prospection techniques are necessary for developing a reasonable model of the subsurface structure. Hence, the third layer is suggesting for engineering and foundation purposes.