When oil-based mud invades into a formation,the resistivity distribution and response characteristics of the invaded zone are complex and cannot be represented by a simple radial two-layer step model.Considering the actual structure and dimensions of the array induction logging tool,this study derives the expressions for the induced electromotive force and effective tool constant of the three-coil measurement structure with a built-in copper tube.The outer diameter of the copper tube is adjusted depending on the derived expression of the effec-tive tool constant and the effective tool constant of the actual tool is simulated to approximate the radial detection characteristics of the actual tool.Subsequently,a forward model for the radial layered medium model of array in-duction logging is constructed.Based on the analysis of invasion response characteristics under oil-based mud conditions in the fractured tight sandstone formation of the Cretaceous Bashijiqike Formation in the Dabei and Keshen areas of the Kuqa Depression in the Tarim Basin,the radial step invasion model is selected for forward modeling of the complex invasion response characteristics of array induction logging in oil-based mud.The study shows that the radial three-layer step model can reasonably represent the complex response characteristics of for-mation invaded by oil-based mud,laying a foundation for the subsequent inversion application of array induction logging data.
The actual underground medium is very complex. There are inevitable errors in the numerical forward modeling results by employing computer software, and it is difficult to simulate the complex underground medium. The dynamic photoelasticity method can obtain the propagation characteristics of wave fields at the seismic scale in the laboratory, and can observe the wave field characteristics in real time. The wave field characteristics in solid media are obtained by dynamic photoelastic physical simulation device experiment, and the propagation law of ultrasonic pulse waves in media is observed. By carrying out the sample model pixel calibration of the instantaneous wave field image and longitudinal pixel sampling of the wave field image, the simulated single shot seismic record in solid media can be obtained. Comparison of the simulated single shot seismic records of the non-hole medium model and containing-hole medium model shows that there are obvious differences in waveform characteristics between the two models. By comparing the simulated single shot seismic record with the actual field single shot seismic record, it is found that the two have sound correspondence, which shows that the laboratory high-frequency small-scale and field low-frequency large-scale can also reflect the same internal physical essence without meeting the fixed ratio observation theory.
In order to study the propagation law of seismic waves and the generation mechanism of noise in the loess plateau,this paper establishes 3D realistic models(velocity and quality factor Q)of the loess plateau in Or-dos basin according to actual surface elevation and velocity structure of the Qingcheng North 3D work area.Then under the condition of undulating land surface,3D numerical modelling using the coordinate transforma-tion method is accomplished based on a viscoacoustic wave equation with explicitly expressed quality factor.This method can simulate the strong energy noise near the offset caused by complex near surface and strong at-tenuation effects,and obtain simulation results similar to the real seismic data.Through wave field analysis of the numerical modelling results of the models with different complexity,this paper gradually clarifies the genera-tion mechanism of near-offset strong energy noise,multiple reflection and multiple refraction.Finally,this paper analyzes the differences in wave field characteristics at different locations(plateau,ridge,hillside,ditch)and at different source depth in the loess plateau area based on the numerical modelling and actual data.Analysis shows that when excited in the ditch,the near-offset strong energy noise is weaker,and the data quality is rela-tively good;As the source depth increases,the high-frequency component of the signal gradually increases;For real seismic data acquiring in the loess plateau,it is necessary to avoid excitation in the dry loess layer and try to excite in the clay layer or deeper layer.The conclusion of wave field analysis has positive guiding significance for real data acquiring and processing in the loess plateau.
Full waveform inversion(FWI)can establish high-precision velocity models of subsurface media.Due to geophone limitations,towed-streamer data often lacks reliable low-frequency components.Conse-quently,FWI based on towed streamer data often encounters the cycle skipping problem,resulting in the objec-tive function converging to a local extremum.Ocean bottom node(OBN)seismic data contains more low-fre-quency components compared to towed-streamer data.Combining OBN data with towed-streamer data in wave-form inversion can compensate for the low-frequency deficiency in towed-streamer data.However,the source wavelet of towed-streamer data is different from that of OBN data,and the joint waveform inversion is more se-verely affected by the source wavelet.This paper proposes a joint waveform inversion method for towed-streamer and OBN data that does not depend on source wavelets.When constructing the objective function,the simulated and observed data are respectively convolved with the average traces of the other,solving the problem of wavelet error in joint waveform inversion and achieving the effect of not relying on the source wavelet.The feasibility and effectiveness of the proposed method are verified through model testing using the Marmousi model synthesized towed-streamer data and OBN data.
The high-resolution and small trace marine seismic exploration method has the characteristics of high dominant frequency and wide frequency band, which is an important means for the study of near-seabed stratigraphic sequence and fine structure, gas hydrate exploration and so on. The particularity of acquisition and the inherent problems of equipment cause unequal floating effect of the cable, resulting in unevenness, even bending and shaking of the in-phase axis on the gathers after NMO correction，which affects the effect of in-phase stacking and reduces the resolution and imaging accuracy of seismic data processing. The conventional residual moveout correction methods such as: interactive pick-up seabed reflection and ghost reflection traveltime fitting method, gather residual moveout correction method, etc., due to the large amount of pick-up, high quality requirements of gathers, and the residual moveout is a comprehensive reflection of the cable unequal floating, inaccurate speed and anisotropy, it is difficult to accurately identify whether residual moveout is caused by the unequal floating of the cable, which will cause errors in the correction results. This paper proposes a cable floating correction method based on ghost reflection traveltime controlled by coherent analysis. Firstly, the ghost reflection traveltime of the seabed reflection and receiver points are interactively picked up on the shot gathers in intervals, the information of all shots is obtained through fitting interpolation, and the real-time depth of the cable is obtained based on the moveout, and the initial moveout correction amount is calculated; Then the reference moveout correction amount is obtained through a two-dimensional coherent algorithm based on model trace constraints. Finally, based on the cross-correlation coefficient, a weighted selection is made to obtain an accurate residual moveout of unequal cable floating, and the moveout is applied after smoothing, the effect of in-phase stacking is achieved, thereby improving the imaging quality of seismic data. The test analysis combining synthetic data and actual processing shows that, compared with the conventional correction methods, the method proposed in this paper is more accurate and effective.
