Lab 5: Sorting, Velocity Analysis, Stacking

After performing various seismic data processing steps such as data quality control, frequency filtering and deconvolution on the east Texas real seismic data, the objective of today's lab session is to display the true image of the subsurface which required compressing the seismic data to infer a first approximation for such image. This includes: 

1. Sorting the shot gathered data into common mid-point gathers (CMP)
2. Picking appropiate stacking velocities and applying accordingly normal move-out corrections (NMO). 
3. Stacking all the CMPs such as average each CMP to form stacked CMP and then concatenate all stacked CMP traces together. 

1. Common Midpoint Sorting 

Figure 1: CMP Sorting 

Seismic data acquisition with multifold coverage is done in shot-receiver (s, g) coordinates. Figure 1  depicts the geometry of a CMP gather and raypaths associated with a flat reflector. Note that CDP gather is equivalent to a CMP gather only when reflectors are horizontal and velocities do not vary horizontally. The surface seismic reflection surveys are commonly acquired using the CMP method. In this method, points in the subsurface are covered more than once by primaty reflection from different shot-receiver pair. 

Seismic data is acquired in the shot gather mode while most seismic data processing is performed in the CMP-offset mode. Therefore, we need to sort the traces between these modes. For this purpose, the need stacking chart which indicates the geophone location (x-axis) vs source location (y-axis). This is used to sort traces into various modes ot gathers such as shot, receiver, offset, or CMP. 

Figure 2: The fold (number of traces per CMP) vs the CMP numbers. 

2. Velocity Analysis 

The objective of velocity analysis is to determine the seismic velocities of layers in the subsurface. It is used in many processing and interpretation stages such as: spherical divergence correction, NMO correction and stacking, interval velocity determination, migration, and time to depth conversion. 

2.1 The Velocity Spectrum

This method attempts to find the stacking velocity to each reflector. It maps the time-space data of a single CMP gather onto a velocity-spectrum plane. In the velocity-spectrum plane, the vertical axis is t0 and the horizontal axis is Vs.

a. Selecting CMP gather that has a relatively high SNR ratio. The CMP gather should be sorted in offset as shown in the figure below. 

Figure 3: CMP gathers number 250 

b. Velocity Picking and NMO Correction

Normal Moveout (NMO) Correction is defined as the time difference between the two-way travel time, t(x), and the two-way travel time at zero-offset. NMO will apply the picked stacking velocities to CMP traces and corrects with the nonzero offset travel times for their additional travel from source to receiver. The objectives of NMO correction includes: 

- To prepare data stacking 

- To estimate the NMO velocity function

Figure 4: Top: After NMO-correction. Bottom: Its semblance (Velocity Picking)

Figure 5: Top: After NMO-correction. Bottom: Its semblance (Velocity Picking)

Figure 6: Top: After NMO-correction. Bottom: Its semblance (Velocity Picking)

Figure 7: Top: After NMO-correction. Bottom: Its semblance (Velocity Picking)

3. Stacking

Stacking is the processed seismic record which contains traces that have been added together from different records to reduce noise and improve overall data quality. The objective of stacking is to enhance the SNR ratio by eliminating coherent and incoherent noise in the data and to reveal a first subsurface image approximation. The traces in the NMO-corrected CMP gather are stacked (summed up) to produce one stacked trace that represents that CMP. The amplitude of the stacked trace can be the sum or average of the amplitudes of traces in the CMP gather.

Figure 8: Stacked wiggle 

Figure 9: Stacked Density Gray

Figure 10: Stacked Color