Investigation and Identification of Subsidence Problems at Hilo Harbor, Hawaii, Using Geophysical Methods

A geophysical survey was conducted of Hilo Harbor where persistent subsidence problems have been reported. The purpose was to identify subsurface conditions that may be causing the problems. Ground penetrating radar and seismic refraction/reflection with closely spaced geophones were conducted above fill areas that have been settling for nearly 100 years. The results confirm a very non-uniform subsurface with buried concrete structures, loose fill, and areas that have been remedied in the past. Major areas of subsidence are seen to correspond with concave reflectors in GPR profiles and low-velocity zones in refraction profiles. Previous borings made during prior investigations of the problems suggest that these are zones of very low density with the granular fill settling as a result of ever-heavier cargo machinery and associated vibration from traffic and ships.