Comparing Theoretical With Measured Seismic Anisotropy in the Central Taupo Volcanic Zone

Abstract

<p dir="ltr"><b>Geothermal resources using high-enthalpy supercritical fluids (374◦C and 22.1 MPa for pure water) have the potential to generate up to ten times more electricity than conventional high-temperature geothermal resources. In the Taupo Volcanic Zone (TVZ), supercritical geothermal reservoirs may exist at 4 km in greywacke basement rocks, deeper than currently drilled depths (3.2 km). Finding permeable fractured zones is crucial for exploring supercritical resources.</b></p><p dir="ltr">Shear wave splitting (SWS) is proposed as a supporting method to map deep fractured zones. SWS has been previously used to determine the dominant fracture alignments, fluid saturation and anisotropy response to stress variations with the fluid injections at the Rotokawa and Ngatamariki geothermal fields (TVZ), but uncertainties remain on the fractured rock parameters used in the inversion of earthquake data. This thesis combines forward modelling of elastic media (fractured rocks) and synthetic SWS using the MATLAB Seismic Anisotropy Toolkit (MSAT) to test the reliability of SWS observations in predicting fractured rock parameters and to validate SWS as a geothermal exploration method.</p><p dir="ltr">Firstly, we investigate the SWS of different types of fracture infills and rock parameters in numerical fracture networks. From ten parameters, only crack density per unit volume and the orientations of the fractures (strike and dip magnitude) significantly impact S-wave anisotropy. The difference in anisotropy caused by fractures filled with conventional or supercritical geothermal fluids is negligible and would not be discernible in real data.</p><p dir="ltr">Secondly, we seek the best rock parameters that fit the SWS observations at the Rotokawa and Ngatamariki geothermal fields measured at 42 stations. We model the effect of two fracture sets steeply dipping and striking NE-SW and N-S as documented from borehole images. The best fits to Shear wave polarisation observations are obtained with: </p><p dir="ltr">• At Ngatamariki, dominant fracture set 205/NW/60 (strike/dip direction/dip) i and secondary fracture set 000/E/60, total crack density of 4.5%; </p><p dir="ltr">• At Rotokawa, dominant fracture set 065/SE/70 and secondary set 000/E/60, total crack density of 2.5%.</p><p dir="ltr">This work provides a framework to map fracture orientations and densities using shear-wave splitting of natural seismicity. This non-invasive method can help mapping zones that are sufficiently fractured to host geothermal resources prior to drilling. Identifying zones where fluids are at supercritical conditions still requires other methods.</p>