| Title | IMAGING THE ALTERATION AND DEFORMATION HALO ABOVE THE DIORITE TONALITE INTRUSIVE AT THE NGATAMARIKI GEOTHERMAL FIELD |
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| Authors | S. Sewell, M. Savage, J. Townend, C. Hopp, S. Mrocek and K. Graham |
| Year | 2021 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | Ngatamariki, Rotokawa, seismic tomography |
| Abstract | An expanded seismic array of ~55 seismometers deployed across the Rotokawa and Ngatamariki geothermal fields during 2017 and 2018 was used to derive 3D seismic velocity models via local earthquake tomography using P and S wave traveltimes from a set of 302 microseismic events. These events were mostly from three clusters of seismic activity around injection wells within the fields. The P and S arrival times used in the inversions were from both manual picking (90 events) and high-quality automatic picking (261 events) with estimated arrival times accurate to approximately ± 0.05 s for P and ± 0.1 s for S. The tomography code tomoDD was used for the inversions that progressed from 2D to coarse to fine inversion grids in order to examine the improvement in traveltime residuals with increasing model discretization. A 1D constant-velocity starting model previously determined using Monte Carlo VELEST was used for most models. In addition to this, a further fine inversion grid was constructed with a 3D starting velocity model based on available well-logging data (checkshot and sonic logs) and geological information. Model solution robustness and spatial resolution were assessed using derivative weight sum (DWS) values that provide a measure of ray-path coverage in conjunction with synthetic recovery tests (spike test and interpretation model test). The inversion models strongly indicate that a west to east, high to low-velocity variation of at least ±10% Vp and Vs exists across northern Ngatamariki. The high velocity in the west is interpreted to be due predominantly to the very low porosity that formed within the Tahorakuri Formation due to high temperature alteration and ductile deformation during intrusion of a diorite-tonalite magma approximately 600 thousand years ago. The location of the transition from high to low velocity agrees well with a transition from high to low gravity from the publicly available gravity data in the area and a transition from high to low resistivity from a 3D inversion of magnetotelluric (MT) data. Relatively low Vp and Vs is imaged in the east of Ngatamariki and is most likely due to either a greater proportion of porous volcaniclastics and sediments and less rhyolite lava in the upper 1 km and/or deeper smectite and smectite-illite clay alteration in that area, as suggested by magnetotelluric data. High Vs is observed at Rotokawa, which is interpreted as being due to the shallower contacts of the andesite and greywacke. An alternative explanation is that there is less volcaniclastics/sediments and more rhyolite lava in the upper 1 km. Since the magmatic alteration and deformation in the north of Ngatamariki is associated with low-permeability wells, the tomography results can be used to inform future well targeting and numerical modelling of the field. |