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ABSTRACT The ability to monitor seismicity and structural integrity of a mine using seismic noise can have great implication for detecting and managing ground-control hazards. The noise wavefield, however, is complicated by induced seismicity and heavy machinery associated with mining operations. In this study, we investigate the nature of time-dependent noise cross-correlations functions (CCFs) across an active underground longwall coal mine. We analyze one month of continuous data recorded by a surface 17 geophone array with an average station spacing of ∼200 m. To extract coherent seismic signals, we calculate CCFs between all stations for each 5-min window. Close inspection of all 5-min CCFs reveals waveforms that can be categorically separated into two groups, one with strong and coherent 1–5 Hz signals and one without. Using a reference station pair, we statistically isolate time windows within each group based on the correlation coefficient between each 5-min CCF and the monthly stacked CCF. The daily stacked CCFs associated with a high correlation coefficient show a clear temporal variation that is consistent with the progression of mining activity. In contrast, the daily stacked CCFs associated with a low correlation coefficient remain stationary throughout the recording period in line with the expected persistent background noise. To further understand the nature of the high correlation coefficient CCFs, we perform 2D and 3D back projection to determine and track the dominant noise source location. Excellent agreement is observed on both short (5-min) and long (daily) time scales between the CCF determined source locations, the overall migration of the active mining operation, and cataloged seismic event locations. The workflow presented in this study demonstrates an effective way to identify and track mining induced signals, in which CCFs associated with background noise can be isolated and used for further temporal structural integrity investigation. 

Steamboat Geyser in Yellowstone National Park’s Norris Geyser Basin began a prolific sequence of eruptions in March 2018 after 34 y of sporadic activity. We analyze a wide range of datasets to explore triggering mechanisms for Steamboat’s reactivation and controls on eruption intervals and height. Prior to Steamboat’s renewed activity, Norris Geyser Basin experienced uplift, a slight increase in radiant temperature, and increased regional seismicity, which may indicate that magmatic processes promoted reactivation. However, because the geothermal reservoir temperature did not change, no other dormant geysers became active, and previous periods with greater seismic moment release did not reawaken Steamboat, the reason for reactivation remains ambiguous. Eruption intervals since 2018 (3.16 to 35.45 d) modulate seasonally, with shorter intervals in the summer. Abnormally long intervals coincide with weakening of a shallow seismic source in the geyser basin’s hydrothermal system. We find no relation between interval and erupted volume, implying unsteady heat and mass discharge. Finally, using data from geysers worldwide, we find a correlation between eruption height and inferred depth to the shallow reservoir supplying water to eruptions. Steamboat is taller because water is stored deeper there than at other geysers, and, hence, more energy is available to power the eruptions. 

Abstract Doublet Pool is an active hydrothermal feature in Yellowstone National Park, USA. Approximately every half hour, it thumps for about 10 min due to bubbles collapsing at the base of the pool. To understand its thermodynamics and sensitivity to external factors, we performed a recurring multiple‐year passive seismic experiment. By linking recorded hydrothermal tremor with active thumping, we determine the onset and end of thumping, and the duration of silence between each thumping cycle. The silence interval decreased from around 30 min before November 2016 to around 13 min in September 2018. This change followed unusual thermal activity on the surrounding Geyser Hill. On a shorter time scale, wind‐driven evaporative cooling can lengthen the pre‐thumping silence interval. Based on energy conservation, we determine the heating rate and heat needed to initiate thumping to be 3–7 MW and ∼6 GJ, respectively. 

Abstract Steamboat Geyser in Yellowstone National Park is the tallest active geyser on Earth and is believed to have hydrologic connection to Cistern Spring, a hydrothermal pool ∼100 m southwest from the geyser vent. Despite broad scientific interest, rare episodic Steamboat eruptions have made it difficult to study its eruption dynamics and underground plumbing architecture. In response to the recent reactivation of Steamboat, which has produced more than 130 eruptions since March 2018, we deployed a dense seismic nodal array surrounding the enigmatic geyser in the summer of 2019. The array recorded abundant 1–5 Hz hydrothermal tremor originating from phase‐transition events within both Steamboat Geyser and Cistern Spring. To constrain the spatiotemporal distribution of the tremor sources, an interferometric‐based polarization analysis was developed. The observed tremor locations indicate that the conduit beneath Steamboat is vertical and extends down to ∼120 m depth and the plumbing of Cistern includes a shallow vertical conduit connecting with a deep, large, and laterally offset reservoir ∼60 m southeast of the surface pool. No direct connection between Steamboat and Cistern plumbing structures is found. The temporal variation of tremor combined within situtemperature and water depth measurements of Cistern reveals interaction between Steamboat and Cistern throughout the eruption/recharge cycles. The observed delayed responses of Cistern Spring in reaction to Steamboat eruptions and recharge suggest that the two plumbing structures may be connected through a fractured/porous medium instead of a direct open channel, consistent with our inferred plumbing structure. 

Abstract Old Faithful Geyser in Yellowstone is one of the most well‐known hydrothermal features in the world. Despite abundant geophysical studies, the structure of Old Faithful's plumbing system beneath ~20‐m depth remained largely elusive. By deploying a temporary dense three‐component geophone array, we observe 1–5 Hz low‐frequency hydrothermal tremor originating from Old Faithful's deeper conduit. By applying seismic interferometry and polarization analyses, we track seismic tremor source migration throughout the eruption/recharge cycle. The tremor source drops rapidly to ~80‐m depth right after the eruption and gradually ascends vertically back to ~20‐m depth, coinciding with the previously inferred bubble trap location. Likely excited by the liquid/steam phase transition, the observed tremor source migration can provide new constraints on the recharge process and deeper conduit geometry. Combined with the shallow conduit structure from previous studies, these results provide constraints on the major fluid pathway down to 80‐m depth.