Estimating the Seismic Quality Factor (Q) in the Western Izmit Bay, NW Turkey

The seismic Q (quality factor) parameter, the inverse of attenuation, is a petrophysical parameter being more sensitive to lithology and physical properties of the medium (pressure, temperature, saturation to fluid and gas, etc) than the velocity and it can be used as an indicator to determine the physical changes along the fault zones. The multi-channel seismic reflection data of the Western Izmit Bay, collected after 17 August 1999 Izmit Earthquake (Mw=7.6) by Mineral Research and Exploration Institute of Turkey is used for the Q estimation to investigate the lateral and vertical variations of the Q parameter along the seismic profiles. The Q value is determined from the reflection seismograms, and is an average of the quality factors of the layers passed by the reflected wavelet. Since it is difficult and less reliable to estimate the interval-Q values from surface seismic reflection data, instead of estimating the interval-Q values for lithological correlation, the variations of the Q parameter along and around the North Anatolian Fault zone is investigated. In this study, the constant-Q wavelet modeling method, based on the comparison of the attenuated synthetic wavelets by the observed wavelets, is used. The measure of agreement of the wavelets are provided by two methods: the correlation coefficient and L2 norm. The prominent, isolated reflected waves are determined from the raw shot data of the seismic reflection profile and used for Q estimation. In a former study it is observed that the correlation coefficient method has a limitation for the estimation of the Q value higher than 100. Therefore the Q parameter is estimated by two different methods along four seismic lines and the results are compared. It is seen that the L2 norm method gives more reliable results when compared to the correlation coefficient method. It is also observed that low Q values are observed in shallow sediments and around the North Anatolian Fault.