A Detailed Ground Penetrating Radar Investigation Of A Coastal Barrier Spit, Long Beach, Washington, U.S.A.

Abstract

A regressive modern barrier spit at Willapa Bay, Washington (north of the Columbia River mouth) is influenced by a 4 m tidal range (spring), high energy, longshore transport and high wave energy depositional processes. The spit is 38 km long, up to 5 km wide and is composed of medium-grained beach sand. The spit has a freshwater aquifer recharged by rainfall. Results from a ground penetrating radar (GPR) test indicate a shingle-like accretionary depositional pattern of beach and upper shoreface reflections which dip towards the ocean at about one degree. The loss of radar signal from below 7-11 m deep may indicate either a brackish/saline water zone or a major lithofacies change. PulseEKKO™ IV GPR reflection profiles, with antennae frequencies of 25, 50, 100 and 200 Mhz, were obtained along a main test line. Both 400 v and 1000 v transmitters were used. In addition, 100 MHz antennae were used for collecting a detailed grid of data (50 x 50 m, sampled every metre), a multifold dataset and antennae orientation tests. The grid pattern data provides a detailed view of the three-dimensional geometry of individual facies units. Results from the present study demonstrate that the 200 MHz antennae showed high resolution within the top 5-6 m, while the 25 and 50 MHz antennae showed deeper penetration to 12-13 m. For this study site, the 100 MHz antennae provided acceptable resolution, continuity of reflections and penetrated (7-8 m) to the base of the dipping beach beds. The dip profiles show a shingle-like accretionary depositional pattern while the strike profiles show a horizontal and subhorizontal, nearly continuous reflection patterns. The parallel broadside and perpendicular broadside antennae orientation tests showed detailed stratigraphy, continuity, and depth of penetration, while cross polarization tests exhibited reduced continuity of reflections and less depth of penetration, but dipping reflections were apparent.