The role of ground penetrating radar and geostatistics in reservoir description

Interpreters are routinely required to develop a continuous model of the subsurface through the integration of available data. One of the issues commonly encountered is how best to “fill in” the region between data points, or describe the region at a scale less than the scale of the sampling. We take this opportunity, as part of the Near-Surface Geophysics issue, to describe research currently underway that can assist in the development of geologic models. A near-surface geophysical method, ground penetrating radar (GPR), provides high resolution images of near-surface sedimentary packages that can be used to obtain an improved understanding of both the large-scale and sub-meter scale architecture found in a variety of depositional environments. In addition, the GPR images can be analyzed to obtain a geostatistical representation of the depositional environment that could be used in generating stochastic models of the subsurface. In this paper we present GPR images from selected deltaic, coastal, and fluvial environments and show both the detailed sedimentology that is imaged and the information that is both captured and lost through the geostatistical characterization. GPR images of selected depositional environments. GPR studies of both modern and ancient systems can provide a large amount of information that can improve the understanding of the lithologic variation and internal structure of ancient reservoirs. A number of recent studies have used 2-D and 3-D GPR data to characterize sedimentary units and different depositional environments. In a GPR survey, electromagnetic energy in the frequency range 11000 MHz is transmitted into the ground. Changes in the dielectric properties of the subsurface cause reflections of energy which are detected on the surface. The result of a GPR survey, the radar image, is a map of reflections marking interfaces across which there are changes in dielectric properties. GPR data are commonly collected by using a single transmitter antenna and a single receiver antenna and moving these two, at a constant offset, along the survey line. Station spacing (i.e., trace spacing on the final GPR section) is usually on the order of tens of centimeters to a meter, depending on the survey’s objective. GPR data provide remarkably good images of coarse sedimentary packages. An excellent example of GPR data collected over a deltaic environment is shown in Figure 1. These data were collected in the Brigham City Sand and Gravel Company pit floor and show the late Pleistocene Box Elder Creek delta. A picture of a nearby outcrop is shown on the cover of this issue. This is a classic Gilbert-type fan-foreset delta dominated by steeply inclined beds of sand and gravel. The dipping reflections seen in the GPR data exhibit a high degree of continuity and are interpreted as steeply dipping strata. As GPR records changes in the dielectric properties of the subsurface, it is most likely in this environment that we are seeing the boundaries across which there are changes in grain size. Figure 2 is an example of GPR data collected over a sandy coastal barrier spit, a regressive modern barrier spit at Willapa Bay, Washington which is 38 km long and up to 5 km wide. The dipping reflections in this section, are interpreted as beachface/upper shoreface beds indicating a shingle-like accretionary depositional pattern. In the strike profile shown in Figure 3, these same boundaries are seen as a subhorizontal, nearly continuous bedding pattern. Figure 4 shows an example of GPR data from a Late Pleistocene braided fluvial deposit from the Embarras Airfield, northeastern