Seismic Anisotropy at the Rotokawa and Ngatamariki Geothermal Fields in the Taupo Volcanic Zone

Abstract

<p>In order to investigate the cracks/fractures in the geothermal fields of Rotokawa and Ngatamariki, we measure seismic anisotropy across both fields and interpret the results in the context of stress aligned microcracks. Cracks aligned perpendicular to the direction of maximum horizontal stress close and their fluid is forced into cracks aligned with maximum horizontal stress (SHmax). Seismic anisotropy is the directional dependence of a seismic wave's velocity and provides a measure of crack orientation and density. To measure seismic anisotropy we conduct shear wave splitting measurements on 52,000 station-earthquake pairs across both Rotokawa and Ngatamariki from earthquakes recorded during 2015. Both fields are the subject of other geophysical and geological studies. Thus they are excellent subjects for studying seismic anisotropy. We cluster our measurements by their station-event path and fit the parameters from these clusters to those from theoretical crack planes. We also apply 2-D tomography to shear wave splitting time delays (𝛿t) and spatial averaging to shear wave splitting fast polarisations (∅). In addition, we compare time delays with P-wave to S-wave velocity ratios (νP / vS). Local measurements of stress within Rotokawa and regional measures of stress within the Taupo Volcanic Zone provide a comparison for the shear wave splitting measurements. We measure ∅ which agrees with the NE-SW regional direction of SHmax across Ngatamariki and parts of Rotokawa. Within Rotokawa, we observe a rotation of ∅ away from NE-SW toward N-S that agrees with borehole measurements of direction of SHmax of 023° and 030°. Spatial averaging of ∅ reveals mean orientations close to the strike of nearby active faults. The theoretical crack planes, that fit best to the shear wave splitting measurements, correspond to aligned cracks striking 045° outside of both fields, 035° within Ngatamariki, and 035° through to 0° within Rotokawa. The average percent anisotropy for the full dataset, approximately 4%, is close to the upper bound for an intact rock. Delay time tomography shows regions of higher delay time per kilometre of path length (s=km) within both fields and possibly associated with the production field fault in Rotokawa. vP =vS shows a wide range of normally distributed values, from 1.1 through to 2.4 with a mean of 1.6, indicating a mixture of gas filled and saturated cracks. A positive correlation between delay time per kilometre (𝛿tpkm) and νP /νS indicates that the majority of the cracks are saturated.</p>

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

“…This thesis study area contains the Ngatamariki and Rotokawa geothermal fields (Figure 2.2). We use the earthquake data and previous shear wave anisotropy anal-BACKGROUND ysis done in these areas (Mroczek, 2017;Mroczek et al, 2020). Ngatamariki and Rotokawa geothermal fields have been studied using drilling data (stratigraphy and large faults), geophysical surveys and surface mapping.…”

“…Mroczek. The file includes events which were grouped and paired with the seismic stations at Rotokawa and Ngatamariki Geothermal Fields (Mroczek, 2017;Mroczek et al, 2020). When referring to events, or models at Rotokawa or Ngatamariki, we mean the collection of earthquakes paired with stations within either Ngatamariki or Rotokawa.…”

“…The majority of stations are part of Mercury's ongoing seismic monitoring of both geothermal fields, with additional GeoNet station data. Mroczek (2017) and Mroczek et al (2020) provide a detailed description of their methods and data processing to obtain the catalogue used for this work. The GNS earthquake catalogue used by Mroczek (2017) was provided by Steven Sherburn Sherburn, Bromley, et al, 2015) and supplemented with detections described by the methods in Hopp et al (2016), Hopp et al (2019) and Mroczek et al (2020).…”

“…3 -10 km depth (McNamara et al, 2014;Stern & Benson, 2011). A density value of 2700 kg/m 3 of the host rock or main rock medium (RHM) is chosen, since the interest is in the deeper rocks, and it is the same as Mroczek (2017) used in his crack plane fitting calculations.…”

“…For the P-wave velocity in the fracture infill material (VPC), a conventional geothermal setting is used as an assumption. The default value is the sound velocity in hot water, approximately 1.5 km/s, similar to METHODOLOGY AND DATA (Mroczek, 2017). S-wave velocity (VSC) for a fluid is 0.00 km/s, whether it is gas or liquid -this is used for all the fluid-filled fracture models throughout the whole analysis.…”