Localized Ambient Noise Tomography Over the FORGE Utah Site

In December 2016 and August 2017 two geophone arrays recorded 30 days of seismic data near Milford, Utah, to perform ambient noise tomography, centered on the Frontier Observatory for Research in Geothermal Energy (FORGE) Utah project site. The initial deployment in December 2016 consisted of 93 5-Hz three-component geophones roughly centered on the FORGE project area but spanning across the Mineral Mountains and into the Milford Valley. The second deployment in August 2017 consisted of 49 densely spaced geophones centered about the FORGE deep test well site. Ambient noise cross-correlations were calculated using 10-minute windows for all station pairs. The individual correlation functions were then stacked in order to estimate the Green’s functions for virtual source and receiver paths. For the initial 93-station deployment, clear Rayleigh wave signals were observed in the 3 to 10 second period range. During the August 2017 dense deployment, signals were observed between 0.5 and 1 seconds. Frequency-time analysis was used to obtain surface wave dispersion measurements from the cross-correlation functions for both arrays. Dispersion measurements were inverted for Rayleigh wave group velocity maps at 3, 5, and 7 second periods for the initial 93-station array. Sensitivity kernels indicate that these periods are sensitive to depths from 2.5 to 8 km. Group velocity maps show a general trend of higher velocities in the core of the Mineral Mountains with lower velocities in the alluvium of the Milford Valley. For the 3 to 7 second periods, group velocities range from 2 to 3.5 km/s, but show the top of a low-velocity layer beginning to develop at 5 to 8 km depth. This low-velocity zone is consistent with work from a previous teleseismic tomography study. For the dense array, a group velocity map was determined for the 0.67-second period, which roughly correlates to the upper 500 m. This map shows higher velocities (600 m/s) at the eastern portion of the array, closer to the Mineral Mountains consistent with the shallowing of a granitic reservoir unit and thinner alluvial cover. INTRODUCTION As part of site characterization efforts for the Utah Frontier Observatory for Research in Geothermal Energy (FORGE) project, two geophone seismic monitoring arrays were deployed over the FORGE project area in order to record both microseismicity and ambient noise for seismic imaging (Trow et al., 2018). The instruments are compact 5-Hz three-component Nodal geophones. Other studies using this instrumentation have proven its applicability to earthquake detection and crustal imaging studies (Bowden et al., 2015; Li et al., 2015, 2018; Hansen and Schmandt, 2015; Wang et al., 2017). Specifically, these geophone arrays facilitate high-resolution crustal imaging at periods below 10 seconds (e.g., Wang et al., 2017; Ward and Lin, 2017; Wu et al., 2017). For imaging we use ambient noise cross-correlation records to estimate the Green’s function between a virtual source station and receiver (Shapiro and Campillo, 2004). Primarily, we extracted Rayleigh wave signals from the vertical-vertical noise cross-correlations. The study area is located in southwestern Utah to the northeast of Milford, Utah, on the western flank of the Mineral Mountains (Figure 1). This area has undergone many investigations to understand its geothermal potential since the late 1970’s (e.g., Ward et al., 1978; Robinson and Iyer, 1981), and geologic and thermal conditions have been summarized in more recent reports (Hardwick et al., 2016; Simmons et al., 2016; Allis et al., 2017). Overall, the region is characterized by high heat flow and Quaternary volcanism (Blackwell, 1983; Edwards and Chapman, 2013) with crystalline basement rock (Moore and Nielson, 1994). The high heat flow can be attributed to thinning crust in the Basin and Range Province along with an active volcanic history (e.g., Benz et al., 1990). Recent volcanism began in the Oligocene (Faulder, 1991) with emplacement of the granitic rocks beginning about 25 Ma, with the most recent rhyolite flows and domes occurring about 0.5 to 0.8 Ma (Robinson and Iyer, 1982; Faulder, 1991). Structural interpretations (Smith and Bruhn, 1984; Barker, 1986) of seismic profiles across the project site, extending from the Mineral Mountains into the Milford Valley, highlight a major reflector dipping westward into the basin that could be a lowangle listric fault or a basement rock-sediment contact. In addition, teleseismic P-wave tomography indicates a low-velocity zone beneath the western portion of the Mineral Mountains (Robinson and Iyer, 1981). This zone has been identified in the upper crust below the study area at depths between 5 and 15 km (Robinson and Iyer, 1982; Barker, 1986) with up to a 7% reduced P wave seismic velocity indicating higher basement temperatures and the possibility of partial melt. Low-velocity by Andy J. Trow, Kristine L. Pankow, Yadong Wang, and Fan-Chi Lin Geothermal characteristics of the Roosevelt Hot Springs system and adjacent FORGE EGS site J2 anomalies and areas of elevated crustal temperatures are important for understanding the heat source of the Roosevelt Hot Spring System. We propose the use of ambient noise seismic tomography to look for and better constrain these along with other structures in the FORGE project area. To map seismic velocities, continuous seismic data is used to perform noise cross-correlations between geophone stations. This allows an estimation of the Green’s functions along the path between station pairs (Shapiro et al., 2005; Yao et al., 2008). Surface wave tomography is performed on a grid by inverting travel-time measurements (Barmin et al., 2001; Lin et al., 2008, 2013; Ward, 2015). This technique has been applied to the high-permeability zone in a geothermal reservoir in portions of Europe (Calò et al., 2013; Lehujeur et al., 2017) and has identified low-velocity regions in known geothermal fields that have been interpreted to be geothermal reservoirs (Calò et al., 2013). Repeated experiments that show mapped velocity changes can relate to changes in the reservoir of the geothermal system. Group velocity measurements from this study will serve as a base model to establish reservoir changes as the geothermal field evolves with development and production during the Utah FORGE project (Simmons et al., 2018). Figure 1. Overview Map of Utah FORGE project area and Roosevelt Known Geothermal Resource Area (KGRA). Inset map shows the Roosevelt KGRA (blue box) and FORGE site within the state of Utah. The detailed map shows the footprint of the proposed research facility, the deep well location, and location of the Blundell geothermal power plant. N