Early-arrival waveform tomography(EWT) is one of the most promising techniques for building near-surface velocity model. Based on finite-frequency wave equation, EWT estimates velocities by matching calculated early-arrival waveforms with the observed ones. However, the objective function of EWT can easily converge to local minimum because of the cycle-skipping phenomenon. In order to reduce the cycle-skipping problem, a hybrid-domain early-arrival waveform tomography(HEWT) is proposed in this paper. The forward modeling of HEWT is realized in the time domain where early-arrival waveforms are easier to be selected from seismic data and less memory is needed than they are in the frequency domain. The inversion is implemented in the frequency domain where multi-scale strategy is more convenient to be realized than that in the time domain. Discrete Fourier transformation(DFT) is used to transform the time-domain wavefield to the frequency-domain wavefield. Test results show that HEWT is more competitive than EWT in both accuracy and computational time.
A velocity model is an important factor influencing microseismic event locations.We review the velocity modeling and inversion techniques for locating microseismic events in exploration for unconventional oil and gas reservoirs.We first describe the geological and geophysical characteristics of reservoir formations related to hydraulic fracturing in heterogeneity,anisotropy,and variability,then discuss the influences of velocity estimation,anisotropy model,and their time-lapse changes on the accuracy in determining microseismic event locations,and then survey some typical methods for building velocity models in locating event locations.We conclude that the three tangled physical attributes of reservoirs make microseismic monitoring very challenging.The uncertainties in velocity model and ignoring its anisotropies and its variations in hydraulic fracturing can cause systematic mislocations of microseismic events which are unacceptable in microseismic monitoring.So,we propose some potential ways for building accurate velocity models.
In order to improve the efficiency of 3D near-surface velocity model building,we develop a layer-stripping method using seismic first-arrival times.The velocity model within a Common Mid-Point(CMP) gather is assumed to be stratified into thin layers,and the velocity of each layer varies linearly with depth.The thickness and velocity of the top layer are estimated using minimum-offset first-arrival data in a CMP gather.Then the top layer is stripped and the second layer becomes a new top layer.After removing the effect of the top layer from the former first-arrival data,the new first-arrival data are obtained and then used to estimate the parameters of the second layer.In this manner,the velocity model,being regarded as that at a CMP location,is built layer-by-layer from the top to the bottom.A 3D near-surface velocity model is then formed using the velocity models at all CMP locations.The tests on synthetic and observed seismic data show that the layer-stripping method can be used to build good near-surface velocity models for static correction,and its computation speed is approximately hundred times faster than that of grid tomography.
Taikun ShiJianzhong ZhangZhonglai HuangChangkun Jin
Reverberation is significant in shallow water and produces obvious notches in OBC spectra.It also degrades the quality of sections and increases the difficulty of processing and interpretation.This article presents the relationship between notch,shooting depth,and seabed depth based on the seismic convolution model.Forward modelling based on wave equation theory is used to verify this relationship.Dual-sensor summation is applied to suppress receiver-side multiples and remove notches according to the opposite response of geophones and hydrophones to down-going wave fields based on a detailed analysis of the OBC technique.The good results obtained in practical applications reveal the effectiveness of this method.
