if we use the intercept points of these branches to estimate the layer thickness beneath the shot.
The intercept point of shot #108 should give a reasonable estimate of the layer thickness for the hanging wall side however.
For shots on the hanging wall side of the fault, such as #110 to #113, the first arrival branches exhibit a normal cross-over point (E1-E3), but at a point (X) just to the left of the fault, the travel time curves of all the shots converge to each other and adopt a low apparent velocity—a velocity that approaches asymptotically to the velocity of the layer on the left side. Eventually at about the crossover distance of the left side interface away from the shot, the headwave phase travelling along the left side half-space boundary appears as first arrival and the travel time curve adopts the half-space velocity. The relatively slow diffracted phase between the two fast phases is the third characteristic of faults (F2) and it is due to the change of ray paths from headwaves along the right interface to a diffracted phase from the point U in the model. We can use the true crossover points like E1 to E3 to get a correct estimate of the layer thickness of the hanging wall if they exist.
The ray paths that give rise to the above three features are shown as arrowed lines in the velocity model in Figure 2.5. The middle panel shows the ray paths for the negative-slope segment ACM(F1). The bottom panel shows the reciprocal ray paths for shots from the footwall and the hanging wall. They give rise to the unusual low velocity segment feature(F2) and the systematically increasing intercept time head wave branches(F3).
The important points in the model are the layer piercing points (U for the hanging wall side, and K for the footwall side), which are closely related to the dip direction of the fault. The discontinuity of the interface in the velocity structure gives rise to the features in the first arrival data described above, and these break points control the location of the three features. Feature F1 depends on K, while features F2 and F3 depend on U. When the fault dips, these two points move laterally relative to one another, their corresponding features move with them. For the vertical fault described above (Figure 2.5), the kink point A of F1 is slightly to the hanging wall side of the fault, and the kink point X of F2 ishtanθaway from the fault, with a distance between them being in the range ofhtanθ < D < 2htanθ. Here h is the thickness for the hanging wall, andθis the critical angle.
For a steep normal faulted structure, the kink point A of F1 is on the footwall side, i.e., on the same side as U. This makes the distance between A and X much smaller than the vertical
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Figure A.1 Velocity models and synthetic first arrival picks for normal faulted and thrust faulted structures. On both plots, the solid line represents synthetic first arrivals from finite-difference modelling and the plus signs are the calculated travel times from ray theory. The ray paths for the calculation are shown the lower two panels. F1 and F2 in the upper panel represent the two diagnostic features discussed in the text. The middle panel shows the ray paths for different phases that appear as first arrival for a shot on the hanging wall, while the bottom panel shows those for a shot on the footwall. The top layer thickness of the hanging wall is designated as h. Note that the location of F1 (point A) is normally controlled by point K in the model, while F2 (point X) is controlled by point U. On the page is for a normal fault with a dip angle of45◦. Note that the distance between point A and X is much smaller that a vertical fault situation.
Compare it with Figure 2.5. Here,D < htanθ.
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Figure A.1(continued)On the page is for a steep thrust faulted structure (dip angle is45◦), the distance between point A and X isD > 2htanθ. Relative position of A and X does not vary much with the dip angle. For normal fault situation, D varies with the dip angle, the smaller the dip angle, the smaller the distance. A and X might be aligned or reverse relative position for a shallow dipping normal fault.
situation (compare Figure 2.5 and Figure A.1a). The two points might be aligned to the same ground point as is the case for the Pelona Fault or even reverse relative position when the dip angle of the fault gets shallower.
If the fault is a thrust fault (Figure A.1b), the kink point A of F1 is located considerably (at leasthtanθ) to the hanging wall side of the fault. The position of point A in the arrival data is also controlled by point U in the model and the distance between the two kink points (X and A) is almost independent of the dip angle of the fault, with a value bigger than2htanθ. As the dip angle gets smaller, the amplitude of the pulses consist of feature F2 in the waveform data gets smaller and almost invisible for a 30 dipping fault. From our finite difference modelling, amplitudes of the segment F2 are generally much smaller for a thrust faulted structure than that of a vertical or normal fault situation. Two synthetic seismograms for a thrust fault and normal fault are shown in Figure A.2.
In Figure A.1, the plus signs (+) denote travel time computed using the rays in the lower panels of the figures. They compare well with the travel time denoted by the solid lines, which are determined by finite-difference modelling.
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Figure A.2 Amplitude comparison of the segment F2 for the two faulted structure in Figure A.1.
Synthetic seismogram for the normal fault is shown in (a), while one synthetic shot gather from the thrust faulted structure is shown in (b). Note the amplitude difference for the segment F2.
The amplitude for the normal case is much higher than that for the thrust fault. This amplitude difference is helpful in estimating the dip directions of faults for seismic surveys.