GPR STRATIGRAPHY - PHYSICAL MECHANISMS

Since the earliest recordings of GPR data, the presence of responses from fine scale structures in unconsolidated geologic materials has always been observed. GPR records from “uniform” sands were far from “uniform” in character. After 10 to 15 years, the geological community and the GPR community came to realize the valuable stratigraphic insight contained in GPR records.

Both communities now routinely accept that the texture of GPR records clearly reflect depositional history. However, the physical mechanisms which explain the response are still not well understood.

GPR reflections require a change in electromagnetic impedance. Impedance changes can be caused by changes in mineralogy, by changes in compaction (density) and by changes in texture (grain size, shape orientation). These material changes and also have a major impact on pore water distribution. Water distribution and degree of saturation in turn have a large impact on electromagnetic impedance. As a result, the question as to whether a direct material change or an indirect change in pore water content gives rise to observed GPR events is always in debate.

The purpose of this presentation is to provide a basis for discussion of these issues and point to paths of needed research efforts to improve our understanding.