Passive seismic survey of sediment thickness, Dasht-e-Nawar basin, eastern Afghanistan

Abstract

Exploration of water resources is needed for public supply, extraction of mineral resources, and economic development in Afghanistan. Remotely-sensed data are useful for identifying the general nature of surface sediments, however, “boots on the ground” geophysical surveys or drilling programs are needed to quantify the thickness of sediments or aquifers. The nature of such investigations presents a risk to field crews that may prohibit exploration of potentially valuable aquifers or mineral resources. The Dasht-e-Nawar basin, in east-central Afghanistan, contains a 400 km2 playa that includes evaporative mineral deposits, particularly lithium, which has been of interest since the 1970s. However, exploration of the basin, as with many areas of Afghanistan, has been hampered by decades of conflict. In 2014, an investigation of the basin was conducted by the U.S. Department of Defense Task Force for Business and Stability Operations (TFBSO), and their contractor, in cooperation with the U.S. Geological Survey (USGS). For this investigation the USGS compared the results of a rapid passive seismic survey of basin sediment thickness to the results of an independently conducted gravity survey of the same area. Each point measurement for the passive seismic method requires less than 30 minutes in the field by one person. The technique utilizes ambient seismic noise without an external sound source such as required by traditional seismic surveys. Additionally, the technique does not require external sensor arrays, which can be kilometers long for some geophysical techniques. The passive seismic equipment used in this study weighs approximately 1 kilogram and is about 10 cm3 in size. Although relatively new for assessment of sediment thickness, several investigations have found this method to be capable of estimating sediment thicknesses, in the 10’s to 1000 meter range, in settings with unconsolidated sediment over bedrock and a contrast in acoustic impedance. In this investigation, the gravity survey was conducted during a period of 3 weeks by an experienced field crew; required a detailed, centimeter-scale land elevation survey; and required laboratory analyses of sediment and rock densities to interpret the gravity data. In contrast, the passive seismic survey was collected by two inexperienced operators over a period of 8 days and required no additional data to interpret. Due to security restrictions, USGS personnel could not visit the site and the seismic operator was trained immediately prior to the field work. Although the quality of the seismic survey was affected by strong afternoon winds, and by the inexperience of the field operator, the results were fairly comparable to the gravity survey. Similar basin sediment thicknesses and patterns in sediment thickness were identified by both surveys in the basin with an estimated maximum thickness of approximately 170 m. The passive seismic technique required substantially less field resources and time than would be required by other geophysical surveys. Although this method will not be effective in all geologic settings, it may be a valuable assessment tool for use before conducting other, more intensive, geophysical efforts or drilling programs, especially in regions with elevated security risks such as Afghanistan.