Shallow Structure of the Broadlands-Ohaaki Geothermal Field (NZ): Analysis of Seismic Refracted Arrivals by Iterative Ray Tracing

Abstract

The shallow near-surface structure of the Broadlands-Ohaaki geothermal field has been studied by analysis of near offset, refracted seismic arrivals. By using a combination of a surface-consistent delay time method and reduced time plots, travel time delays between the surface and a datum below the water table can be mapped. Large time delays (>30 msec), are confined to the region defined by the resistivity low, and occur beneath geothermally active areas. The origin of these anomalies is attributed to localised low velocity zones. The observed decrease in P-wave velocity and t h e occurrence of low frequency arrivals can be explained by the presence of hot water and gases at temperatures and pore pressures near the water-steam transition. Iterative ray trace modelling of head waves is very effectivein reconstructing the velocity structure of rocks beneath the Ohaaki Pool down to 200 m depth.