Seismic Data Processing Lab

LAB 2: Seismic Data QC

Basically after seismic data had been acquired and recorded, few steps of quality control (QC) are necessary before carrying out the remaining seismic data processing forward and ultimately gained an accurate seismic range of subsurface structures. The QC process involves a step-to-step to condition the data and prepare the data for further QC and processing

1. De-multiplexing: The data is transferred from the recording mode, where each record contains the same time sample from all receivers, to the trace mode where each record contains all time samples from one receiver. De-multiplexing usually was performed in the field.

2. Reformatting: From one seismic digital format (SEG-Y for example), the data is converted to another format so that the processing software can easily use the data throughout the processing (e.g., MATLAB).

3. Setup of field geometry: The field geometry is written into the data (trace headers) in order to associate each trace with its respective shot, offset, channel and CMP.

4. Trace editing: Bad traces or parts of traces are muted or deleted from the data during this step. Problems regarding polarity are also fixed.

5. Gain application: Amplitude corrections are applied to compensate for losses of amplitude due to spherical divergence and absorption of the seismic waves.

6. Application of field statics: In land surveys, elevation statics are applied to bring the sources and receivers to a single planar reference level known as the datum. This step can be delayed until the static correction process where better near surface velocities might be available.

Existing data for this lab session had already took care step 1-3, thus only step 4 and forward needs to be executed. Basically we are still using the east Texas seismic dataset in Lab1 that comprises some high noisy amplitudes.

Figure shows one way of muting at shot gather number 16, the recorded trace must be muted due to the use of bad geophones in land data. The code below was used to execute the muting

[I,j] = find (D==max(max(D)));

D(:,j)=0;

Figure: Seismic data shot gather
number 16 before muting

Figure: After muting

Correction to Amplitude Losses

Raw seismic data usually characterized by a noticeable decrease in the amplitudes of its recorded traces with time. It includes geometric divergence effects as waves spread out from a source as well as conversion of the seismic energy into heat besides other factors such as transmission losses.

Amplitude correction or gain must be applied to seismic data at various stages. The data-independent scheme corrects the amplitudes using a common scaling function to all the traces such as gain through multiplication by a power of time using:

where f(t) is the seismic trace amplitude to be corrected,t is the independent variable, and α is the power of time which controls the change in the amplitude of f(t). Another commonly used function is the exponential gain function correction:

where θ is the parameter which controls the exponential function. We can apply amplitude correction using the function iac.m. In this example, we display amplitude correction at shot gather number 8 by executing command

1. Shot_num = 8;

2. P=0;

3. [Dshot,dt,dx,t,offset] = extracting_shots(D,H,shot_num,p);

4. Pow=2;

5. T=0;

6. Dg=iac(Dshot,t,pow,T);