Seasonal Variations of Seismic Travel-Time Changes in Binchuan, Southwestern China, Inferred from Large Volume Air-Gun Source Array Data

Active sources, especially air-gun sources in the water reservoir, have proven to be powerful tools for detecting regional scale velocity changes. However, the water level change affects the repeatability of the air-gun waveform and, thus, affects the stability of the detection of the velocity changes. This article explores how to make full use of the air-gun signals excited at different water levels from analyzing three years of air - gun data recorded by 20 stations deployed from - 50 m to - 25 km from the source. At the same time, by utilizing the poroelastic model, we quantify both vertical and hori- zontal distances affected by the water level change. More important, supported by the strain data from one borehole strainmeter station, the influence mechanisms of the sea- sonal variation derived from the three years of air-gun data are also discussed. Results indicate the water level affects the main frequency of the air gun and has a strong influ- ence on the coda wave. When the water level of the reservoir changes abruptly, the dominant effect on the derived delay time change is from the water level change. In this case, the deconvolution method can hardly eliminate the influence of the abrupt water level change. However, when the reservoir's water level changes gently, the delay time varies accordingly rather than inversely with the water level. Other reasons affect the material properties and, thus, influence the derived delay time. The modeled vertical component of poroelastic strain caused by the reservoir water level change is 1 x 10-7. The observed strain (4 x 10-7) from the strainmeter is likely associated with thermoelastic strain induced by temperature change. Our results show that although the long-term air-gun signal is affected by water level, there is still much information about changes in the subsurface that is worth mining.