Bruce ZR, Cameron H. 2021. SLIDE (Wellington): seismic profiling of Camrose Grove, Wellington. Lower Hutt (NZ): GNS Science. 31 p. (GNS Science report; 2019/60). doi:10.21420/Y2TS-TD08.
Abstract
This report represents an aspect of the ground truthing methods used to confirm the geomorphic mapping carried out as part of the Stability of Land In Dynamic Environments (SLIDE) project within the Wellington Region. A simple hammer plate seismic survey was completed in Camrose Grove, Kingston, Wellington, on an area of fill identified from historical records of US Marine Corps construction in Camrose Grove during WW2. The purpose of the survey is to confirm the geometry of the fill body and the depth to the historical ground surface calculated from the geomorphic mapping based on differencing of digital surface models derived from photogrammetry of historical air photos with modern LiDAR derived digital elevation data. Field survey of the site at the Camrose Grove sports field was carried out with a hammer and plate source and a StratavisorTMNX data recorder with 48 single-component geophones spaced at 4 m along a 175-m-long survey line crossing the deepest portion of the infilled valley. In addition to the pressure-wave vertical hammer and plate shots every 8 m, horizontal shear-wave shots on a weighted beam were also recorded at three sites to test the site for shear-wave velocity information. Data processing was carried out with Globe ClaritasTM processing software. The aim of the processing is to define seismic-wave velocities of both fill material and underlying greywacke to identify any variations within the fill and to determine the geometry of the base of the fill layer. Seismic refraction velocities were able to be modelled and the geometry of the refraction modelled greywacke contact agreed well with the modelled depths from the historical and LiDAR differenced ground model of the fill site, proving the usefulness of the differencing technique for geomorphic mapping. Seismic reflections were not observed in the field data. The poor quality of the reflection data is likely to be due to the sparse shot and receiver spacing used. The intense ground roll was spatially under-sampled preventing its removal from the reflection records. Similarly, as the survey was limited to single-axis geophones, together with the poor quality of the surface waves, this prevented spectral analysis of surface waves from being used to determine shear-wave velocities. (auth)
