Fault detection in reflection seismic data by common diffraction surface stack, a case study in Rheine Graben, Germany

^*نگارنده رابط:     تلفن: 09133070411      دورنگار: 3395509-0273                  E-mail:msoleimani@shahroodut.ac.ir

 





 



A seismic survey in a geothermal site near the Karlsruhe city in Rheine river graben in Germany was performed to provide a detail map of structures for geothermal usage of that area. The geothermal site uses the enhanced geothermal system (EGS) for power generation. The EGS system needs a hot dry rock (HDR) in depths with a spread system of faults and cracks. An injection well, injects the surface water to the fractured hot rock and water pass through the fractures and faults and becomes hot. Then two pumping wells will pumps up the hot water to the surface. One of the pumping well in that site did not perform with efficiency that was planned for. The problem might be due to the impermeable rock between injection well and pumping well in that part of the hot rock. Therefore mapping the faults, major or minor ones, and the boundary of the layers is an important task in that project. It should be mentioned that before the surface seismic project, a vertical seismic profiling (VSP) was performed in that well that due to the high temperature in well, the receiver sounding devise was failed and no data was gathered. The second VSP project even with thermal resistant devise was also failed. Therefore obtaining a reasonable seismic image of that part was so critical for stopping or continuing the geothermal activities there. Then a surface seismic project was planned with two 2D seismic lines. However, seismic imaging in such faulted regions is among the crucial task by conventional methods. The CMP stack followed by DMO and post stack migration and prestack depth migration (PSDM) were both performed on the data. Result of the conventional post stack time and depth migration was not so ideal that interpreter could trace the minor faults on the final section. PSDM method also needs an accurate velocity model that would be obtained by so much effort. However, the prestack depth migration was carried out and faults could be traced in that section. However, the seismic image has some ambiguities in interpretation and the velocity model was not fully trustable. Therefore it was needed to use new imaging method for seismic imaging in such media. The common reflection surface (CRS) stack is among the new methods for seismic imaging. It has some advantages that make it usable for imaging in such regions. It is also independent from macro velocity model. Therefore, accuracy of the velocity model is not a big concern here. The CRS stack method also gives three kinematic wavefield attributes that could be used for further interpretation. These attributes relates to the dip, curvature and depth of the reflector. The best advantage of the method is that it gives an enhanced section for migration correction. The CRS stack method use a higher order of traveltime equation rather than the CMP stack method. Therefore the CRS stack operator works on a surface in time domain rather than on a trend. These properties make the CMP stack method a special case of the CRS stack method. Moving to a higher order traveltime equation will dramatically increase the signal to noise ratio in the final section. Höcht (1998) derived equations that give the traveltime of ray by kinematic wavefield attributes, known as CRS equation. In his calculation, the second order of travel time for t², is known as CRS stacking operator:
                                                              (1)
where R_NIP, R_N and α are CRS attributes, V₀ is the near surface velocity and X₀ is the point of the emergence of the central ray. As it could be seen from the equation, this equation does not need any information about the velocity model of the subsurface, and only knowing the near surface velocity would be enough. Then the CRS stack method was performed on the data. The result of the CRS stack method was comparable to the result of PSDM. It should be noted that in the CRS stack method, an accurate velocity model is not necessary and the final section has better quality. However, in the CRS stack processing chain, mapping the position of faults was difficult due to the problem of conflicting dips that exists at the end points of faults. Therefore a new method was developed here designed especially for imaging the faults or any type of discontinuity in the reflectors, while preserves advantages of the CRS stack method. To achieve this goal, the idea of diffraction stack migration in Kirchhoff migration was used to modify the CRS stack operator. To have an idea about how the CDS operator works, consider a segment of a reflector in a predefined position in zero offset (ZO) section. This segment would be defined by its related wavefield attributes on that point. Now consider a hypothetic diffraction point exactly at the same point (in the middle of the segment). The ray path for diffraction and reflection in that point would be the same for both situation and the traveltime and emergence angle are the same, too. In the literature of the CRS stack method; we can see that the parameter R_NIP is independent from the curvature of the reflector in the point of imaging. Soleimani and Mann (2008) proved that the Kirchhoff migration operator is a special case of the CRS operator with R_NIP=R_N. In other words, if the CRS stack operator for an arbitrary reflector segment is known, an approximation of the associated Kirchhoff migration operator is readily available by substituting R_NIP for R_N in equation. To perform diffraction stack migration on the CRS stack method, the operator should switch from reflection response to diffraction ones. Therefore the method could be called common diffraction surface (CDS) stack method. The new introduced CDS operator will gather all of the diffracted energy in the data. The CRS stack method gives the priority to the most coherent event for producing the operator in (x_m,t,h) space and will have only one stacking surface, while the CDS stack, does not put any criteria for selecting the surfaces. Otherwise there is a risk to lose a relatively weak diffraction that is masked by strong reflection. Thus, there would be no image of the geological phenomena that was responsible for that lost diffraction in final migrated section. It is the reason of not imaging faults or discontinuity in the layers in most of the seismic imaging method. To preserve all diffraction shown in (x_m,t,h) space, there would be as many as operators according to the number of diffraction in that space. To switch CRS stack traveltime to the diffraction CDS stack, the following combination between two kinematic wavefield attributes was considered and the new attribute (R_CDS) was derived:
                                                     (2)
where . The new introduced method was applied on the data. Like as the conventional method, the first section obtained in the processing chain was the stacked section by CDS method. In the CDS stacked section, the quality of the section was not improved well. However, as it was mentioned, the advantage of this method would be clearer in the migrated section. In the next step, all the stacked sections obtained by the CMP, CRS and the CDS methods have been migrated by the Kirchhoff post stack depth migration. In the migration section that was obtained by migration correction on the CDS stacked section, more reflection events and more faults were imaged with better quality. In comparison to the post stack time migration on conventional method and CRS stack method, the CDS stack operator even with a smooth velocity model could gather more diffracted energy for imaging than the other migration operators. The final CDS migrated section is of a better quality and contains more detail about the location of the minor faults. The result of this migration is also comparable with the PSDM result. In some cases, the new method could image faults better than the PSDM result. It should be mentioned that the post stack depth migration applied on the CDS stacked section, needs only a smooth velocity model that would be easily obtained. With detail geological interpretation on this section, the results show that the region contains many small faults that transfer the pumped water from injection well through the hot rock to the steam extraction well.