Year: 1993
Abstract:
Although in its infancy, ground penetrating radar (GPR) is rapidly emerging as a geophysical technology with many possible applications. To assess whether GPR could become a useful tool to investigate internal sedimentary structures, GPR field experiments were conducted on several modern, Holocene and Pleistocene lacustrine deltas to better understand the internal stratigraphy.
A pulseEKKO$\sp{\rm TM}$ IV radar system was used with 25, 50, 100 and 200 MHz frequency antennae and 400 and 1000 V transmitter power levels. For most surveys one metre steps (station spacing) were used to provide detailed horizontal resolution of the sedimentologic structures. The profiles were processed and plotted (wiggle trace format) using pulseEKKO$\sp{\rm TM}$ IV software. Depth of reflections was determined from using the common midpoint (CMP) method. Experiments with different antennae frequencies and transmitter powers showed significant variations in vertical resolution, depth of penetration and continuity of reflections.
Radar stratigraphic (facies) analysis of GPR profiles, evolved as a secondary development in this dissertation, provided identification of three deltaic types: (1) fan-foreset, (2) wave, and (3) braid. These deltaic types are in general agreement with existing concepts. Fan-foreset deltas are dominated by steeply dipping (25$\sp\circ$) reflections, sandwiched by surface and basal radar facies which have slightly inclined or horizontal reflections. Wave deltas have low-angle inclined reflections capped by discontinuous, wavy reflections. Braid deltas have distinct, continuous and semi-continuous, wavy reflections, often overlying a basal, horizontally continuous reflection, below which an abrupt signal loss occurs. As well, two post depositional features found in deltaic environments were investigated: (1) a potential failure plane and (2) a peatland. The insights gained from the radar stratigraphic analysis of deltaic environments may be extended to the interpretation of seismic records and ancient deltaic systems. More importantly, this information will provide a better understanding of the internal structure which will aid earth scientists in the interpretation of deltaic sequences from drill cores.
GPR was found to be most effective (resolution and depth of penetration) in dry and/or wet (freshwater), quartzose-rich, clean (no clay) sand and gravel deposits. The technique does not work well in sediments with silt, clay, caliche (CaCO$\sb3$, calcrete), or saline ground water which attenuate the electromagnetic signal.
