Internal structure of a pacific coast barrier spit using ground penetrating radar

Ground penetrating radar (GPR) field experiments were carried out on the modem barrier spit at Willapa Bay, Washington (north ofthe Columbia River mouth). The barrier spit is influenced by a 3.6 m tidal range (Spring), as well as high wave energy and longshore transport depositional processes. The spit is 36 kIn long by 2-5 kIn wide and is composed of fine-grained beach and eolian sand. The spit has a freshwater aquif~r recharged by rainfall. To determine paleodepositional processes of the barrier spit, radar facies (reflection patterns) which correspond with sedimentary facies and depositional' processes were identified. Next, interpretations of depositional processes using the geomorphic association between facies and process were made. The objective was to reconstruct the overall sequence of paleo-depositional patterns and processes which have formed the Willapa barrier spit during the late Holocene (last 5000 years). A puiseEKKO™ IV radar system was used with 25, 50, 100 and 200 MHz antennae. GPR transects were carried out along beaches and roadways across the spit and where necessary were corrected for topography. Processing and plotting of the radar profiles was carried out using puiseEKKO™ IV software packages. GPR surveys indicate a shingle-like accretionary depositional pattern of beach and shoreface reflections which dip towards the ocean at about 1 degree. At the distal portion of the barrier reflections dip in the direction of longshore transport (north). Nearby radiocarbon-dated wood in beach sediment suggests that the spit began to form 4000 years ago. The loss ofradar signal from below 10-15 m deep may indicate a major lithofacies change. Very difficult penetration by vibracoring of spit sediment suggests that GPR reflections may be partially attributed to the tight packing of the individual sediment particles along nearly straight inclined reflections, as well as slight changes in sediment grain size at bedding planes deposited by major winter storm events.