Comparative Ground-Penetrating Radar Stratigraphy: Differentiation of Lithologies and Depositional Environments Using Bedform Geometry and Radar Facies Analysis

Ground-penetrating radar (GPR) profiles are valuable in the investigation of terrestrial subsurface stratigraphy at scales of cm to m, below the resolution of commonly-used acoustic profiling. One of the major benefits of GPR is its ability to characterize deposits non-invasively (without coring or trenching). Like nearly all efforts to invert true geology from geophysical data, attempts to derive lithology and depositional environment from radar-imaged layering provide non-unique solutions. However, we point out that qualitative and quantitative radar stratigraphy, including radar facies analysis, provides valuable information concerning depositional history in the shallow subsurface. Basic characteristics of any sedimentary deposit are generated by a combination of the depositional process and the nature of the particles themselves. For example, bedform height depends on the depth of the fluid in which it is created. Bedform steepness depends upon grain size, sorting, water velocity, and antecedent topography. Development of bedding depends upon grain size, sorting, and fluctuations in the transport capacity of the system. These characteristics can be understood from radar profiles in the absence of outcrop or core data. For example, properties of some clays cause energy absorption which decreases the depth of penetration. Extremely large grains, lateral heterogeneities, and irregular lithologic contacts cause EM energy to scatter, resulting in poorly resolved subsurface interfaces or chaotic reflector patterns. Analyses of radar wave velocity also yield information about water content, which is a function of grain size, sorting, and composition. Bedform geometry is often clearly visible in GPR profiles, and can be quantified after appropriate processing. GPR profiles from five different deposits accumulated during the past 18 ka in New Jersey are compared: fluvial deposits of the Delaware River; glacial outwash and an end moraine from the Wisconsinan glacial retreat; eolian sand deposited shortly after glacial retreat; and a migrating shoreline sand spit at Sandy Hook. Profiles from four of these settings show similarities in internal architecture due to comparable depositional processes in a moving fluid. The scale and shape of the bedforms seen in profiles varies as a function of grain size, velocity, and paleo-water depth. Differences in grain size, degree of sorting, and degree of heterogeneity of the deposits lead to markedly different radar facies. We describe and compare the radar facies and bedform geometries and scales of each environment to assist in the identification of ancient deposits of unknown origin using GPR profiling.