Geologic controls on seismic velocity from well logs and seismic tomography at the Rotokawa and Ngatamariki Geothermal Fields, New Zealand
Authors:   , Sewell Steven 
Published: May 2022

<p><b>In this thesis, the factors controlling seismic velocity at the log scale (less than a metre) were explored and this knowledge used to interpret seismic velocity images at the field scale (greater than 200 m) obtained via seismic tomography at two geothermal fields in the Taupō Volcanic Zone in New Zealand; the Ngatamariki and Rotokawa fields. Seismic velocity imaging has the potential to provide valuable insights into geothermal reservoirs that can be used for well targeting, resource management and conceptual and numerical models. However, the potential factors influencing seismic velocity in geothermal reservoirs are many. In this thesis, the factors controlling seismic velocity were explored by pairing geophysical logging data, which included P-wave velocity, S-wave velocity (for two wells), neutron porosity, density, gamma, resistivity and formation image logs with newly acquired geochemical (via portable XRF) and mineralogical (via quantitative XRD and automated mineral scanning) data from drill cuttings in three deep wells in the Ngatamariki field. The combined analysis of these datasets demonstrates that the largest factor controlling seismic velocity in the two fields is porosity, which in turn can be influenced by both primary lithology, alteration and ductile deformation above the magmatic intrusions that provide the heat in high-temperature, volcanic, geothermal systems. The results also show that the physical and chemical properties of the tuff-dominated Tahorakuri Formation in the north of the Ngatamariki field were dramatically altered by high-temperature (>375°C) alteration and ductile-deformation processes during an intrusion event that occurred approximately 65 kya. Wide-spread quartz deposition in the Tahorakuri Formation in the north of the field due to the intrusion event appears to have decreased porosity and increased velocity relative to the same formation in the south of the field (quartz abundance of 58% in the north cf. 38% in the south, Vp of 4.34 km/s in the north cf. 3.78 km/s in the south). Ductile deformation that occurred when temperatures in the past were above the brittle-ductile transition (>375°C) has resulted in closing of pore space and a zone of very low porosity (mean of 4%) and consequent high seismic velocity (mean of 4.89 km/s) that extends approximately 400 m above the intrusion in NM9. The knowledge gained from this work provided the framework for interpreting the results of the seismic tomography.</b></p>
<p>To improve the resolution and robustness of the seismic tomography, 30 seismometers were deployed across the Ngatamariki and Rotokawa fields, to complement the existing seismic network of 22 seismometers, for approximately one year. From this deployment, a sub-set of 351 of high-quality earthquakes were used to perform the tomography analysis. A 1D Monte Carlo VELEST analysis was performed using this dataset by randomly generating 1000 different 1D starting models with velocity distributions based on measured velocity data and knowledge of rock types across the two fields. This yielded an average 1D model for the two fields which was then used as the basis for 3D inversion work. VELEST also inverts for station correction terms that can be used to provide an indication of the spatial variation in velocity for Vp and Vs with negative station corrections indicating faster velocity and positive station corrections indicating slower velocity. Three areas of similar station correction terms were identified; an area of negative Vp and Vs station corrections in the northwest of Ngatamariki, an area of positive Vs station corrections to the east of Ngatamariki and positive Vp and Vs station correction terms at Rotokawa. The spatial pattern of the station correction terms agrees well with the spatial patterns of velocity observed in the 3D inversions.</p>
<p>3D inversion of the same earthquake dataset was performed using the program tomoDD. Inversions were performed using three different inversion grids and two different starting velocity models. Model solution robustness and uncertainty were assessed using both ray-path coverage measures and synthetic testing. In all of the inversions performed, a west to east, high (Vp up to 4.8 km/s, Vs up to 2.7 km/s) to low velocity (Vp up to 3.8 km/s, Vs up to 2.2 km/s) transition was observed in the north of the Ngatamariki field for both Vp and Vs. High velocity in the northwest of Ngatamariki was observed over the depth range of 1-3 km bsl (below sea level), corresponding mostly to the Tahorakuri Formation tuffs and volcaniclastics above the intrusion in the north of the field. As was demonstrated with the logging, geochemical and mineralogical datasets, high velocity in this area is likely associated with reduced porosity due to alteration (particularly quartz deposition) and ductile-deformation that occurred during an intrusion event. The area of high velocity in the northwest of Ngatamariki aligns well with high gravity and high resistivity at 1.5 km bsl from a 3D inversion of magnetotelluric data. The transition between high to low velocity, gravity and resistivity all occur approximately across the NM9 well in the north of the field, suggesting that drilling in the northeast of the Ngatamariki field is unlikely to encounter the low permeability that was observed in the NM4 and NM8 wells. </p>
<p>The findings of this thesis highlight that alteration and deformation above magmatic intrusions in geothermal fields can cause pronounced changes in the physical, chemical and mineralogical properties of the rock and that these changes may be imaged by seismic tomography methods. This has potential implications for well targeting in geothermal fields and wider implications for seismic imaging of magmatic systems.</p>