HOW PETROPHYSICS CAN CONTRIBUTE TO SEDIMENTOLOGICAL AND STRATIGRAPHICAL GROUND PENETRATING RADAR STUDIES

Over the last two decades ground penetrating radar (GPR) has emerged rapidly as a popular geophysical method for near-surface sedimentological and stratigraphical studies. GPR reflections are commonly triggered by changes in water content and associated variations in dielectric properties. Because water content is in part controlled by the sediment characteristics, GPR allows for imaging sedimentary features. Although the underlying theory of electromagnetic wave propagation and reflection is well established, for practical matters in the field there is a lack of quantitative understanding of physical relationships between sediment characteristics and dielectric properties.

Petrophysics is the study of physical properties of rocks. In the oil industry petrophysics-based models are often used for predicting seismic impedance or velocity values from core-plug porosity and material properties. In GPR studies one commonly deals with a 3-phase medium of solids, water, and air. For this situation, the petrophysical relationships between textural characteristics on one side and water retention characteristics and dielectric properties on the other side are of greatest importance. Measurements of textural characteristics can focus on grain size, porosity, diagenetic features and organic matter content. Both water content and dielectric properties can be measured directly or one can be derived from the other using Topp’s model or dielectric mixing models.

Several complementary methods of performing petrophysical measurements are useful in combination with GPR studies. The pore size distribution (PSD) of sediment controls the capillary forces and, thus, water content. In most cases the grain size can be used as a proxy for the PSD. Time domain reflectometry can be used to make detailed measurements of the dielectric properties, both in the field along vertical transects or in the laboratory. Hydraulic head measurements of undisturbed samples allow for the prediction of soil moisture induced variations in reflection strength. A number of modeling examples for diagenetic features, soil horizons, and sedimentary structures demonstrate how petrophysics can be used to improve understanding of ground penetrating radar reflections.