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Abstract: We report on the application of seismic noise investigations, including H/V (horizontal to vertical) spectral ratio and array techniques, to a shallow gas-rich geothermal reservoir in Heybeli, southwestern Turkey. Fundamental resonant frequencies were determined to estimate the sediment thickness. Using small-scale seismic arrays, phase velocity dispersion curves were derived by correlating noise recordings according to the extended spatial autocorrelation method. Improved shear wave velocity profiles were estimated by combining Rayleigh wave dispersion curves and horizontal to vertical spectral ratios in a joint inversion. We found that the velocities obtained for the reservoir site are higher than those for a location outside the reservoir. In addition to the fundamental resonant peaks in the spectra, a clear 6-Hz-signal could be identified originating from the center of the geothermal field, repeatedly observed in 2010 and 2011. It had been claimed that low frequency (1–10Hz) seismic signal anomalies were correlated with the occurrence of hydrocarbons. One of the physical mechanisms under consideration to explain these tremor-like signals above such reservoirs is resonant amplification due to the oscillation of bubbles. Based on the signal similarity with volcanic tremors, it is not a priori given that the liquid phase must be oil for resonance effects to occur. We therefore applied array techniques to identify potential noise originating from the Heybeli reservoir. In fact, the frequency–wavenumber (f–k) method clearly indicated a noise source coming from the main production well of the reservoir. In 2011, as part of our assessment, the operators of the spa facility stopped the extraction of thermal water for 2h: the 6-Hz-signal disappeared after the pump had been stopped and reappeared after the pump began operating again. Thus, the 6-Hz-signal is likely of artificial origin. In addition, no natural noise source inside the reservoir could be identified. [Copyright &y& Elsevier]

The western coastal plain of Taiwan borders the Western Foothills, which is a fold-and-thrust zone and one of Taiwan's major seismic zones. Earth quakes from the Western Foothills are a frequent cause of disaster in the western coastal plain. In this study, array measurements of microtremors were conducted at seven sites throughout the western coastal plain. At each site, four arrays (S, M, L, and XL arrays) of different sizes were considered. The largest radius of each array ranged from 32 m to 1 km. The Maximum Likelihood Method of Frequency-Wavenumber (F-K) analyses was used to obtain phase velocity dispersion curves. After the inversion of the dispersion curves with the initial models decided by Genetic Algorithm (GA) searching, shallow S-wave velocity structures for the western coastal plain were estimated. The S-wave velocity structures were estimated from the surface to a depth of 3 km. Above a depth of 0.4 km, they increased gradually under all sites. The first main interfaces were between 0.4 to 0.75 km in depth, and the average velocity at all sites increased sharply from 0.808 to 1.289 km sec-1. This interface seems to be the lower boundary of the thick unconsolidated sediments. The second main interfaces were between 0.9 to 1.75 km and the average velocity increases from 1.289 to 1.703 km sec-1. The third main interfaces were between 1.15 to 2.55 km and the average velocity in creased from 1.703 to 2.45 km sec-1. The depths of the last two main interfaces corresponded to the tops of the Pliocene and upper Miocene formations, respectively, as identified by seismic surveys and exploration wells. Further, the average S-wave velocities of the Plio-Pleistocene, Pliocene, and Miocene formations were estimated in this study. The existence of the Peikang Basement High, which is a prominent barrier in the area between the pre-Miocene and Neogene affects the features of these two interfaces. The depths vary apparently with the distances between the sites and the top of the Peking Basement High; i.e., Penghu Island. Furthermore, the complex structures near the Yichu hinge fault make the S-wave velocities of deeper structures under the Yijhu site higher than those under other sites based on the plane-layer assumption of the F-K analysis. [ABSTRACT FROM AUTHOR]