HIGH RESOLUTION GROUND PENETRATING RADAR MAPPING OF THE PERMIAN GARBER-WELLINGTON AQUIFER IN CENTRAL OKLAHOMA

The Garber Sandstone and the underlying Wellington Formation are important sources of drinking water in Oklahoma and Cleveland counties in central Oklahoma. The aquifer package consists of interbedded sandstone, shale, and siltstone deposited under fluvial conditions. The formations outcrop in central Oklahoma. Because of similar hydrological and petrophysical properties, the Garber Sandstone and the Wellington Formation are not easily distinguishable in outcrops. Previous geological work done on the Garber-Wellington aquifer suggests that water produced from sand bodies isolated by fine-grained sediments contains high levels of naturally occurring arsenic. Therefore, an understanding of the paleogeographic distribution and internal architecture of the sandstone bodies within the Garber-Wellington is critical to the placement of water wells.

This paper demonstrates the application of Ground Penetrating Radar (GPR) in imaging the Garber-Wellington. A PulseEkko100 GPR system with center frequencies of 50 MHz, 100 MHz, and 200 MHz is used to image outcrops of the Garber-Wellington. The 200 MHz data gives much better resolution than the 50 MHz or the 100 MHz data in identifying channelized features, resolving the basal erosional contact of the sandstone with the underlying shale, imaging faults, and defining the margins of the alluvial system. As expected, the 50 MHz data shows greater penetration. However, the depth of penetration of the GPR signal is limited in this area because of shales that underlie the imaged sandstones. Our preliminary conclusions from this work are that 1) GPR can be used to map and better define the external and internal architecture of the Garber-Wellington, 2) the Garber-Wellington is highly faulted which explains the isolated nature of some of the sand bodies, 3) a combination of antenna frequencies is desirable for better resolution and depth penetration in the Garber-Wellington, and 4) sand bodies that appear horizontally continuous are actually broken up into several smaller depositional units. This last finding is revealed only in GPR data acquired with a small transmitter-receiver step size.