Ground penetrating radar: 2-D and 3-D subsurface imaging of a coastal barrier spit, Long Beach, WA, USA

Abstract

The ability to effectively interpret and reconstruct geomorphic environments has been significantly aided by the subsurface imaging capabilities of ground penetrating radar (GPR). The GPR method, which is based on the propagation and reflection of pulsed high frequency electromagnetic energy, provides high resolution (cm to m scale) and shallow subsurface (0–60 m), near continuous profiles of many coarser-grained deposits (sediments of low electrical conductivity). This paper presents 2-D and 3-D GPR results from an experiment on a regressive modern barrier spit at Willapa Bay, WA, USA. The medium-grained sand spit is 38 km long, up to 2–3.5 km wide, and is influenced by a 3.7-m tidal range (spring) as well as high energy longshore transport and high wave energy depositional processes. The spit has a freshwater aquifer recharged by rainfall.The GPR acquisition system used for the test was a portable, digital pulseEKKO™ system with antennae frequency ranging from 25 to 200 MHz and transmitter voltages ranging from 400 to 1000 V. Step sizes and antennae separation varied depending on the test requirements. In addition, 100-MHz antennae were used for conducting antennae orientation tests and collecting a detailed grid of data (50×50 m sampled every meter). The 2-D digital profiles were processed and plotted using pulseEKKO™ software. The 3-D datasets, after initial processing, were entered into a LANDMARK™ workstation that allowed for unique 3-D perspectives of the subsurface. To provide depth, near-surface velocity measurements were calculated from common midpoint (CMP) surveys.Results from the present study demonstrate higher resolution from the 200-MHz antennae for the top 5–6 m, whereas the 25- and 50-MHz antennae show deeper penetration to >10 m. For the study site, 100-MHz antennae provided acceptable resolution, continuity of reflections, and penetration. The dip profiles show a shingle-like accretionary depositional pattern, whereas strike profiles show a horizontal and subhorizontal, nearly continuous reflection pattern. Results from the GPR experiment reveal upper shoreface reflections with dip towards the ocean at about 1–2°. The loss of signal from below a depth of 6–8 m indicates a lithofacies change because of the storm wave base.The parallel broadside and perpendicular broadside antennae orientation tests show detailed stratigraphy, continuity, and depth of penetration. The cross-polarization test exhibits reduced continuity of reflections and less depth of penetration, but dipping reflections are apparent.The grid pattern data provided a detailed view of 3-D geometry of individual reflections. High quality data were obtained, processed, and directly exported into a LANDMARK™ workstation for interpretation. The resulting interpretations of the upper shoreface beds from the test cube (50×50 m; total 2600 traces) are shown as vertical sections (slices), horizontal sections (time slices), contour maps, 3-D representations of individual beds, and an isopach map. The 3-D depositional framework allows a more detailed interpretation than widely spaced 2-D profiles.