Reverse time migration (RTM) is an accurate migration method, which is based on two-way wave equation, eliminates angle limitation, and can be capable for imaging of reverse branch and multiple. In the development of an oilfield in Pearl River Mouth (PRM) Basin, RTM has been applied to solve the problems of fault shadow and structure in distinction and the drilling has proven that the RTM result is reliable. This paper introduces the theory of RTM and emphatically discusses parameter selection. The results of RTM showed that it had advantages in resolving fault shadow and the imaging of steep dip structure, and could be utilized in the oilfield.
At the present stage of seismic prospecting, the most widespread pre-stack depth migration (PSDM) method is the Kirchhoff integration migration based on ray theory and the wave equation migration based on one-way wave equation. The Kirchhoff PSDM is an efficient and practical technique, and possesses the features of high offset angle, no dispersion, less resource occupancy and high efficiency. However, since the application of high frequency approximation and single travel time hypothesis, the Kirchhoff PSDM is not suitable for the imaging of complex structure or drastic variable velocity condition. The one-way wave equation PSDM can resolve the complex wave in complex media and multipath problem, and has the advantage of amplitude preservation. Whereas, this method utilizes the one-way wave approximation and the simplified quasi differential equation, there are certain problems in the precise imaging of the steep-dip interface. The RTM based on two-way equation is equipped with the advantages of the above-mentioned two methods, adopts the source and detector wave field continued by full acoustic wave equation, avoids the separation of upgoing and downgoing wave, less approximates wave equation, as a consequence, overcomes the limitation of migration dip angle and migration aperture, and is appropriate for steep-dip imaging and arbitrary velocity-variation medium.
Whitmore [
RTM is a depth migration method which is on the base of wave equation. RTM applies full acoustic wave equation by simulating two-way wave equation and using appropriate imaging condition to acquire image points. It does not possess the insufficient of losing correct amplitude information in integration migration and wide-angle imaging in one-way wave migration. RTM is one of the most precise migration methods in the current.
RTM had been primitively proposed by Whitmore [
On account of the above thought, reverse-time extrapolation dose not calculate along depth, instead along time to solve the acoustic or elastic wave equation. Therefore, the known conditions are the wave field value recorded in the receiver points. For 2D seismic, the recorded wave field value is P(x, z = 0, t); meanwhile, when t > T (T is the maximum record time of receiver points), assuming P(x, z, t = 0). The detailed steps of RTM are that defining the wave field when t = T the initial value, extrapolating along the direction of the time decreased, calculating each the wave field P(x, z, t − Δt) by the step size of Δt. Until the wave field, when t = 0, has been calculated.
After RTM, the 3D seismic result P(x, z, t) has been obtained. Through applying appropriate imaging condition and stacking each shot record, the migration sections would be gained, as shown in
Imaging condition application is the critical step in RTM. Appropriate imaging condition can effectively image, even cancel noise. The widespread imaging condition at present contains excitation moment imaging condition, cross-correlation imaging condition and deconvolution imaging condition [
of excitation moment imaging condition is the small storage space and its disadvantage is the limitation of multipath propagation and feeblish flexibility [
In conclusion, the concrete realization steps of RTM is divided into following four stages:
1) Forward calculation: Forward extrapolating the wave field of shot points by two-way wave equation, calculating the wave field of the wavelet generated by shot point along timer shaft, and saving the wave field information in each sampling site;
2) Reverse-time extrapolation: Backward extrapolating the wave field of receiver points along timer shaft, and saving the wave field information in each sampling site;
3) Imaging: Taking the saved source wave field and receiver wave field, analyzing imaging by appropriate imaging condition, and then finishing the RTM process of one shot;
4) Finally, repeating the above steps in each depth point of all shots, stacking each migration result of one shot, conducting necessary noise reduction, and then finishing the RTM process of the entire work area.
The research area is the offshore block of a certain oilfield, located in Pear River Mouth Basin of the South China Sea, and its depth of water is about 105 m. According to the previous research results and the drilling data, it’s known that reservoir-controlling fracture develops in the work area and the region entrapment is fault trap based. Committing breakpoints and sections of faults is the crux of implementing oil reservoir, confirming oil domain and deploying well positions. Whereas, in prospecting, the problems of vague fault location and steep dip imaging are always perplexing researchers, and the inaccurate explanation often brings about faulty judgment of fault sealing model and misplaced drilling (as shown in
During the exploration in the oilfield, two wells, well 1 and well 2, has been drilled, and they are apart from 1.7 km. In the process of drilling, the two wells both contain oil layers in log interpretation, but the oil testing results are not the same. In the same oil layer, there is oil and gas shows in well 1, however, there is no in well 2. Analysis suggests that the inaccuracy of fault imaging causes the uncertainty of structure, therefore, the result of well 2 became a failure. In consequence, it has positive sense for utilizing RTM; besides, this application performs the guiding effect for popularizing new technique in the oilfield.
The selection for migration parameter plays a crucial role in RTM [
Velocity model is the core factor of RTM [
The selection of imaging condition is the important step in RTM [
Migration aperture is the imaging space of underground information, and its size decides the seismic trace range of effective imaging points [
Forward continuation of seismic wave is a vital step in RTM [
Frequency band is the frequency domain of input data in RTM, and it’s the output frequency band of RTM result. Frequency band is wider, then the frequency band of result is wider and resolution is higher, but computing costs are bigger. The selection of this parameter should consider frequency range of actual data and migration computing costs.
Due to the high costs of CRP gather in RTM, mute parameter cannot be defined on migrated CRP gather, consequently, angle mute parameter should be defined in the process of migrating. The value of this parameter is the angle of reserved data, namely, the included angle between mute line and perpendicular line.
After applying RTM in the seismic data of the work area, single-shot migration section and 3D migration stack section have been obtained. From the result of migration, the distribution of reflection layers is reasonable, waveform is well and SNR is high. The stack process of RTM is as shown in
Compared with the achieved results of routine PSDM, the result of RTM has the following advantage: better continuity of reflection events in primary purpose layers, and more obvious reflection character; clean reflection background, well waveform, and stable power of wave group. The more important is that unambiguous fault imaging, clear fault points, distinct fault position and lucid occurrence features all effectively decrease the multiple solutions in interpretation, reduce the risk of drilling; the observable geologic phenomena, such as crack, pinching and overlap, is in favor of the recognition and judgment on some certain geologic phenomena, and provides good achieved data for interpreting. It should be noted that the frequency band of data has been decreased in RTM, hence, the resolution of section also has been reduced.
RTM is based on two-way wave equation, rids the angle limitation in one-way wave migration imaging, and can image reverse branch and high steep structure. The result of RTM maintains the feature of wave mechanics. RTM is the better migration imaging method at present.
The utilization of RTM in the research area has realized the imaging of high steep faults, the homing of minor fault, steady wave group power and superior imaging precision. The result of RTM precedes PSDM’s. According to the verification of drilling, the processing results tally with the actual structure situation, provide strong and firm support for later-developed exploitation. The technique of RTM can be spread in high-steep structures and subsalt structure in future.
Run He,Yuan Fang,Yanchun Wang,Lifang Cheng,Xi Zheng, (2016) The Application of Reverse-Time Migration in the Z Area of PRM Basin. International Journal of Geosciences,07,785-791. doi: 10.4236/ijg.2016.76060