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

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