An onshore and offshore seismic investigation across a creek

A seismic survey across a river with refraction and multichannel analysis of surface waves (MASW) methods was carried out to investigate the ground condition for design of a bridge across Iron Creek near Hobart, Tasmania. The survey had onshore and offshore components. Therefore it was necessary to use a hydrophone cable as well as land geophones. A sledge hammer was used as an onshore seismic source and a small airgun across the creek. The result is presented as P-wave velocity section from the refraction analysis and S-wave velocity section from MASW. Two boreholes onshore indicated the depth of basalt with very high strength at 8 metres on west bank and 3 metres on east bank. These depths correspond to P-wave velocity about 1400 m/s and S-wave velocity about 600 m/s. The sections showed the depth of this strong basalt increases in the creek up to about 10 metres, and it is the deepest in the eastern side of the creek. With this information, necessity of expensive offshore drilling was eliminated.