Reflection seismograms Each reflection will follow a moveout trajectory according to the moveout equation

Reflection data processing A collection of shot gathers from around the world (from Yilmaz, 2001)

Reflection data processing Even for simple models, reflection shot gathers can be very complex Reflections, refractions, ground roll, direct waves are all present in the data Mode conversions, diffractions, multiples also complicate the reflection record Nevertheless, for many decades the reflection method has dominated Part of this success is due to the development of a powerful approach for processing shot gathers and producing reflection sections

Reflection data processing A few shot gathers from a marine seismic survey

Reflection data processing The final stacked, migrated seismic section

Common Midpoint (CMP) method The basic objective is to sample each subsurface point more than once For horizontal reflectors, the reflection point is halfway between shot and receiver (at the “midpoint”) Survey is organized to sample midpoints repeatedly

Common Midpoint (CMP) method The first step in the processing is to re-sort the original data (the shot gathers) in “CMP” gathers, in which all traces in each gather share common midpoints Note that each shot-receiver pair comes from a different physical shot point The number of traces in a CMP gather is known as the “fold” of the survey: N is the number of receivers, n is the “move-up” rate The fold of modern surveys may be as large as several hundred

Common Midpoint (CMP) method Each reflection will have a distinct “moveout” on the CMP gather, related to the RMS velocity of the overlying layers The essence of CMP processing is: Resorting into CMP gathers Correction for moveout Summation, or “stacking” Result is the enhancement of reflected signal, and the discrimination against non-signal events

Reflection data processing The essence of seismic data processing is simple: Sort into CMP gathers Correct for NMO Stack The chart on the right shows some of the details used in practice In the following slides this flowchart will be described in more detail, with a sample dataset from the Caspian Sea, Turkey

Raw shot gathers The data shown in the top of Figure 6 Raw shot gathers The data shown in the top of Figure 6.41 are the data as they are originally recorded (after de-multiplexing). The few shot gathers shown here are only a tiny subset of the full dataset. There are several points to note at this stage: The data are of very high quality, with clear hyperbolic arrivals even at this stage There is a signicant change in the arrival time of the rst reection, due to the bathymetry There is a low frequency (1-2 Hz) signal running along the cable - this is wave swell Raw shot gathers

Low cut ltered data The same data following removal of the wave swell by low cut frequency ltering are shown in the bottom of Figure 6.41. Low cut filtered data

Amplitude correction The amplitudes of seismic data generally decrease with time, as the initial energy is spread over an ever-increasing wavefront area. This \geometrical divergence" can be approximately corrected by boosting the signal using a gain function that increases with time. This form of amplitude correction has been applied to the data in the top Figure 6.42. Amplitude correction

Deconvolution Seismic reections will carry with them the full \signature" of the downgoing signal: if this tends to oscillate, or \ring" then the signal quality of the individual reections will be downgraded. Deconvolution is a signal-processing lter that is applied to each trace in the seismic data to tighten the energy packet associated with each reection. The data following \spiking-deconvolution" are shown in the bottom of Figure 6.42. “Deconvolution”

CMP sorting Following these initial shot-gather processes, the data are now re-sorted from shot-gathers into CMP (Common Midpoint) gathers. There will be thousands of gathers; the gathers shown in top of Figure 6.43 are only a tiny subset of the data volume. CMP sorting

CMP sorting Following these initial shot-gather processes, the data are now re-sorted from shot-gathers into CMP (Common Midpoint) gathers. There will be thousands of gathers; the gathers shown in top of Figure 6.43 are only a tiny subset of the data volume. CMP sorting

NMO correction A variable time shift is now applied to each trace in order to atten each reection. If properly applied the reections will now all occur at the equivalent, zero-oset times no matter what the original oset of the recorded trace. The result of the NMO correction process is shown in the bottom of Figure 6.436. NMO correction

Reflection velocity analysis In order to carry out the “Normal Moveout Correction”, we make use of The rms velocity to each reflector is required We generate the required velocities using “reflection velocity analysis” (see later)

Reflection velocity analysis

CMP sorting Following these initial shot-gather processes, the data are now re-sorted from shot-gathers into CMP (Common Midpoint) gathers. There will be thousands of gathers; the gathers shown in top of Figure 6.43 are only a tiny subset of the data volume. CMP sorting

NMO correction A variable time shift is now applied to each trace in order to atten each reection. If properly applied the reections will now all occur at the equivalent, zero-oset times no matter what the original oset of the recorded trace. The result of the NMO correction process is shown in the bottom of Figure 6.436. NMO correction

Muting The distortion introduced by NMO correction (Figure 6 Muting The distortion introduced by NMO correction (Figure 6.43) is most severe at large osets and/or early times. The worst of this distortion is simply digitally removed from the records in a process known as a \mute" (top of Figure 6.44). Muting

Stacking Stacking Following alignment of the reections by NMO correction, the CMP gathers are simply summed, or \stacked". This creates a single trace at each CMP location with enhanced primary reection energy. The full set of CMP traces is displayed as a \stacked section" in the bottom of Figure 6.44.

Amplitude scaling Amplitude scaling The stack section shown in Figure 6.44 may often require further processing to further enhance the image; particularly at late times, the stacking process itself may not have been entirely successful and some amplitude recovery may be required. The stack following amplitude recovery is shown in the top of Figure 6.45.

Seismic “migration” Seismic migration It will be appreciated that the stack section, even after all the processing that has been applied, will only be an imperfect image of the subsurface. In particular, we have made a number of assumptions regarding the subsurface (especially when forming CMP gathers and carrying out the NMO correction). Since these are most in error when the layering is non-horizontal, we may expect problems near signicant structures. Correcting the stacked section for the true nature of the subsurface is a process known as \seismic migration", and we shall have more to say about this process in section 6.13. The image following seismic migration is shown in the bottom of Figure 6.45.

Next lecture: Reflection velocity analysis In order to carry out the “Normal Moveout Correction”, we make use of The rms velocity to each reflector is required We generate the required velocities using “reflection velocity analysis”