HYDRO-GEOPHYSICAL INVESTIGATIONS ON THE ALLUVIAL FANS INSIDE THE DANAKIL DEPRESSION, DALLOL AREA, AFAR, ETHIOPIA

Geophysical application mainly Transient Electromagnetic (TEM) surveys have been taken place as an important tool to evaluate the groundwater potential on the alluvial fans inside the Danakil depression of Dallol area, Afar. The survey was intended to locate potential fresh groundwater well sites and to delineate the depth and lateral extent of fresh, brackish and saltwater interfaces.

Accordingly, 21 TEM sounding points of nearly 500m intervals along 4 different profile lines were surveyed with the help of GEONIX PROTEM of 200m by 200m transmitter loop size on the ground. The field TEM data are processed with a three sweeps at different frequencies and Tx loop size are set on the TEM2IX1D software package. The output data are then processed using the IX1D software package.

Apparent resistivity versus time models are displayed at the start of the processing incorporated with initial models which contain the apparent resistivity and thicknesses. At first forward model is run and then inversion is applied to obtain the best fit of the observed and calculated curves.

In general, the model curves illustrate the presence of depleted apparent resistivity responses at the top and bottom depths, whereas , at the intermediate depths there are enhanced electrical parameters which are indicatives of the presence of clay at the top, fresh groundwater in coarse alluvial deposit at the middle and saline groundwater at depth in evaporates and clay beds. The results of geo-electric sections also reveal the lateral extent of fresh and brackish groundwater in the area.

The TEM results calibrated with test boreholes indicates that the fresh water and salt water interface might goes from 120 to 200m depth in the major fan areas of Gahartu and Mosley, and decreases to 50-60m depths eastwards.