Nobukazu Soma

Reflection technique in time-frequency domain using multicomponent acoustic emission signals and application to geothermal reservoirs

We have developed a reflection technique for estimating deep geothermal reservoir structures using acoustic emission signals as a source, which is useful when there is no proper estimating technique because of high temperature, high pressure, and great depth. Because its resolution is not high enough for comparison with methods such as well logging, we have enhanced the technique by developing a time–frequency‐domain analysis of multicomponent acoustic emission signals using a wavelet transform. The reflected wave is separated from an incoherent coda by analyzing the shape of a 3‐D hodogram: a linear shape indicates the arrival of a coherent signal such as a reflected wave, and an incoherent signal such as a coda makes a spherical shape. We construct a spectral matrix of 3‐D particle motion using a wavelet transform, as is done in a time–frequency domain. We evaluate the linearity of the 3‐D hodogram for each time and frequency by using the eigenvalues of the spectral matrix. Three‐dimensional inversion o...

Passive seismic monitoring of a stimulation of HDR geothermal reservoir at Cooper Basin, Australia

A team of Japanese researchers, with long experience in passive seismic monitoring in the area of development of hot dry rock (HDR) and hot wet rock (HWR) geothermal systems, carried out seismic monitoring at the Australian HDR site being developed by Geodynamics Limited in the Cooper Basin. The seismic network consisting of 4 near-surface stations and 4 downhole stations detected approximately 32,000 triggers during injection of 20,000 m3 of liquid into granitic basement over 3 weeks. The authors located the events on a semi-realtime basis and the locations were fed back to the pumping side. The locations of seismic events showed sub-horizontal extension of the reservoir to 1,800 m away from the injection well at a depth of around 4,500 m. The heterogeneous source migration suggest that the reservoir was close to a critically stressed state.

Reflection imaging using microseismic multiplets as a source

Summary The passive seismic reflection method can be effectively used to delineate 3D structures inside deep basement or for time-lapse surveys of fluid injection/production associated with microseismic activities. We have derived a variation of these passive reflection imaging techniques, where microseismic multiplets are used as a source. Close similarity in their trace enables us to identify absolute and relative times of arrival and the state of polarity of the reflected phase with high accuracy. With more precisely determined relative times of arrival of the reflected phase we achieve clearer imaging of the reflector compared to previously developed microseismic reflection methods. Here we describe the principles behind the microseismic multiplet reflection technique and demonstrate its advantages through synthetic studies.