Geophysical Tools and Applications, April 19th by James Callegary

 [Presentation]

This presentation offers a variety of geophysical methods as tools for  geophysical prospecting and evaluation of groundwater (GW) (seismic refraction, DC resistivity, FDEM, CSAMT and gravity). These methods allow to study the physical properties of rocks and to detect potential aquifers. The seismic refraction is used specially when a layered structural rock exists, and when different velocities of path allow to identify clearly the bed rock (basament), from the upper sequence rock. It is also recommended for regional studies to prospect deep and extended aquifers. The resistivity method allows to identify subsurficial lithologies and structural paths of the rocks. It can be applied to regional studies to obtain cross sections depending on its arrangements  and may reach up to 3500 ms deep.  It also may be used with success to local overviews.  It is applied with good result in the study of hardrock aquifers for the location of fractures. The application of the Inductive Electromagnetic method allows to identify moisture and salt content in GW, lithological distributions (clay, sand, gravel), potential recharge in shallow aquifers and to penetrate successfully from 60 m to 3000 m in deep investigation.

The new Audio Frequency Magnetic (Teluric) method offers very good results in the study of deep and large aquifers, allowing to reach depths of sounding greater to 3000 m. The Gravity method (Temporary, Bouguer and Modelling) is used to calculate the depth of the basement (elevation). Recently microgravity measures are performed with success in monitoring GW level, based in an empirical  equation which relates GW (volume contain) to conductivity. This allows to know volumetric water content in the aquifer and, in consequence, the variability of the piezometric level. Combinations of these Electromagnetic methods are used in airborne  identification of electrical conductors like silt and clay, resulting in  maps of isoconductivity and depth.  Complements of gravity and electromagnetic methods constitute very good tools in the interpretation of geological structures in rocks (bedrock depth, faults, grabens, horsts, rifts and others). Finally these methods may be applicated in wells to obtain borehole logging. The most used are Nuclear logs (gamma and neutron) and spontaneous potential (continuous, light resolution, integrated to other data).   

                                                                                                                                                                                                                                                                                                                                                                                                                                   Blog reporter: Juan J. Ledesma MSc, Alternate National coordinate Of Uruguay
Tucson, 20th April 2007