Improving detection of P-wave first arrivals using supervirtual interferometry
Refraction profiles usually need to be between five and ten times as long as the required depth of investigation (Keary, 2002). By increasing the investigation depth seismic source waves travel long trajectory and because of attenuation cannot record strong head-waves. Sometimes it is not possible to use the appropriate source according to profile length. Whenever the seismic source energy is underestimated for long seismic refraction profiles/deeper targets or in case of highly absorptive media because of wave attenuation, the identification of first breaks becomes complicated particularly at far offsets. This can lead to inaccurate estimates of the deeper velocity distribution.Â Supervirtual refraction interferometry (SVI) based method is proposed to improve first arrival detection. The benefits are that traveltime picking errors can be greatly reduced for noisy head-wave arrivals, velocity and depth calculations will be correct subsequently. Application of the method will improve the signal to noise ratio of the first arrivals by a factor of ,where Â is the number of postcritical source points used in SVI. In this study, supervirtual interferometry is applied to a numerical example and the field data from the south-west of Iran and its efficiency in enhancing of signal-to-noise ratio (SNR) of the first arrivals is presented. Â Â Â Interferometry formula in x-Ï domain in crroscorrelation method, based on wavefield reciprocity and time reverse invariance principle (Wapenaar and Fokkema, 2006), is: where Â is position vector of source/receiver in Cartesian coordinates, A and B are sources in the media surrounded by Â and Â are Greenâs function between the receiver A and the source B, Greenâs function between receivers AB and source x with angular frequency Ï respectively. C, Ï, Â and Â denote compressional wave velocity, density, real part and complex conjugate, respectively. Â Â Â The product of Greenâs function complex conjugate at Â and Greenâs function at Â in frequency domain is equivalent to correlation in time domain. Thus by integrating cross correlation of recorded wavefield at Â and Â around the closed surface , which the sources are located on it, Â becomes recoverable. There are two stages to apply SVI: Correlation every receiver record with the first to generate virtual refracted traces (correlations gather) and stacking the result for all post critical sources to enhance their quality. Â Â Â Convoluting the first receiver data with the virtual traces to create supervirtual head- wave arrivals for each source. Since the arrival time as travel geometry of virtual head-waves for all post critical sources is the same, stacking can improve them and attenuate other waves. Because of random nature of incoherent noise, they are reduced by summation of virtual refracted traces. The difference in arrival times makes coherent noise weaker and migrates. Â Thecomparison the SNR graphs of artificial and real data before and after using SVI provides the evidence for the success of the method. All the supervirtual traces SNRs are bigger than one and have sensible difference with initial traces especially for far offsets receivers. Â Â Â In the interferometry formula the sources/receivers take place all around the closed surface , but in this work the sources are linear. The linear sources arrangement and their limited number cause unwanted effect on supervirtual records.
Iranian Journal of Geophysics, Volume:9 Issue:4, 2016
82 - 93
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