Surface wave methods are non-invasive, low-cost, and robust approaches to image near-surface S-wave velocity (Vs) structure. In terms of the energy source types, they can be classified in two groups: active-source surface wave methods and passive-source surface wave methods. A clean and high-resolution dispersion image is critical for the subsequent dispersion curve picking as well as Vs inversion for either the active-source surface wave methods or the passive-source surface wave methods. However, aliasing or other artifacts are almost inevitable in surface wave dispersion measurements in practice, and they can seriously pollute the measured dispersion spectra. It is significant to figure out how they are generated, how they affect the dispersion measurement, and how they can be attenuated. We provide the first comprehensive review on artifacts that are frequently observed in surface wave dispersion measurements, and summary them into three general types, including artifacts from spare spatial sampling, artifacts from array response, and artifacts from weak coherent signals. Both numerical and field examples, as well as mathematic derivations, are presented to help reader understand the generations of the various types artifacts and the way to attenuate them. This work will help us understand the complex components on the measured surface wave dispersion spectra, and lead to potential improvements on dispersion measurements.

Tomographic imaging based on long-term ambient seismic noise measurements, mainly the phase information from surface waves, has been shown to be a powerful tool for geothermal reservoir imaging and monitoring. In this study, we utilize seismic noise data from a dense nodal array (192 3C nodes within 20km2) over a ultra-short observation period (4.7 days) to reconstruct surface waves and determine the high-resolution (0.2km) three-dimensional (3-D) S wave velocity structure beneath a rural town in Zhejiang, China. We report the advantage of cross-coherence over cross-correlation in suppressing pseudo-arrivals caused by persistent sources. We use ambient noise interferometry to retrieve high quality Rayleigh waves and Love waves. Body waves are also observed on the R-R component interferograms. We apply phase velocity dispersion measurements on both Rayleigh waves and Love waves and automatically pick more than 23,000 dispersion curves by using a Machine Learning technique. 3-D surface wave tomographic results after depth inversion indicate low-velocity anomalies (between -1% and -4%) from the surface to 2km depth in the central area. Combined with the conductive characteristics observed on resistivity profile, the low-velocity anomalies are inferred to be a fluid saturated zone of highly fractured rock. Joint interpretation based on HVSR measurements, and existing temperature and fluid resistivity records observed in a nearby well, suggests the existence of the high-temperature geothermal field through the fracture channel. Strong correlation between HVSR measurements and S wave velocity model sheds light on the potential of extraction of both amplitude and phase information from ambient noise.

The Imperial Valley, CA, is a tectonically active transtensional basin located south of the Salton Sea; the area hosts numerous geothermal fields, including significant hidden hydrothermal resources without surface manifestations. Development of inexpensive, rugged, and highly-sensitive exploration techniques for undiscovered geothermal systems is critical for accelerating geothermal power deployment as well as unlocking a low-carbon energy future. We present a case study utilizing distributed acoustic sensing (DAS) and ambient noise interferometry for geothermal reservoir imaging utilizing an unlit fiber-optic telecommunication infrastructure (dark fiber). The study utilizes passive DAS data acquired from early November 2020 over a ~28-kilometer section of fiber from Calipatria, CA to Imperial, CA. We apply ambient noise interferometry to retrieve coherent signals from DAS records, and develop a spatial stacking technique to attenuate effects from persistent localized noise sources and to enhance retrieval of coherent surface waves. As a result, we are able to obtain high-resolution two-dimensional (2D) S wave velocity (Vs) structure to 3 km depth based on joint inversion of both the fundamental and higher overtones. We observe a previously unmapped high Vs and low Vp/Vs ratio feature beneath the Brawley geothermal system that we interpret to be a zone of hydrothermal mineralization and lower porosity. This interpretation is consistent with a host of other measurements including surface heat flow, gravity anomalies, and available borehole wireline data. These results demonstrate the potential utility of DAS deployed on dark fiber for geothermal system exploration and characterization in the appropriate contexts.