Gravity Survey, Salar Basin, Central Afghanistan

Objective

To perform a gravity survey on an area of 22km x 7km over a mostly dry saline lake

The aim was to image the lake sediment thickness in order to target deeper areas for sonic drill testing of brines. The survey did not directly target the brines, as this method is not capable of differentiating such features at this scale. By highlighting areas where the sediment package is at its thickest, it was possible to prioritize drilling targets

  Digital terrain model generated by the differential GPS survey


Digital terrain model generated by the differential GPS survey

 Residual gravity after linear trend (regional) removal

Residual gravity after linear trend (regional) removal

Survey Details

The gravity survey was completed using 1km line spacing and a 250m station spacing. The area lies within a mountain chain comprised of mostly andesites, tuffs, Proterozoic sediments and metamorphic rocks; recent alluvial fans and sediments are also present. For the purposes of the gravity survey, it was assumed that the country rocks would have a density of around 2.8 g/cc and that the much lighter lake sediments should have a density of approximately 1.8-2 g/cc. This density contrast would of course be fundamental to the gravity survey being successful in mapping out the areas where the lake sediments are thickest.

Equipment Used

A Scintrex CG5 semi-automated gravimeter was used for the survey. 
A Leica Real-Time Kinetic (RTK) GPS unit was used to survey the precise elevation at each station. 

Results

The survey was successful in imaging the lake sediments. Two types of data were obtained from the survey: topographic data (from the GPS), and the gravity data itself. The lake is exceptionally flat with the elevation change over the 20km length of 1m. The purpose of accurately measuring the topography is to remove the effect of elevation change from the gravity data in order to find anomalies regardless of topography. The Bouguer gravity results are plotted and topography and regional residual gravity are removed, this is shown in Figure 2. The results found gravity lows where the sediments are at their thickest, shown in dark blue. There were two areas of gravity lows identified. 

Interpretational Work

Individual lines were forward-modeled, based on an estimate density contract of -0.8 g/cc using the Grav2DC gravity modelling software. The modeling showed the thickest sediments are located in the south at ~160m thick while in the north they appear to be ~120m thick. The central area of the lake appears to have a much lower sediment thickness of 50-80m.

Conclusions

The successful survey was completed well within the set timeframe of 20 days. Results were interpreted and modeled to give drilling targets fulfilling the main objective.