Seismic Data Processing

Recording Media

Reflection Seismic

SEISMIC DATA PROCESSIING

A set of logical operations on the input data aimed at reducing the unwanted components and gathering the wanted components.

SEISMIC DATA PROCESSIING1.Demultiplexing / Format Conversion2.Trace header generation <…observers’ data3.Spherical divergence correction4.Deconvolution before stack5.Band pass filter6.Trace normalization7.Velocity Analysis8. Normal Move Out Correction9. CMP Stack

10. Residual statics estimation & application11. Dip Move Out Correction12. Velocity analysis13. DMO stack14. Random noise attenuation15. Decon after stack16. Time Variant Filter17. Migration18. Scaling

SEISMIC DATA PROCESSIING

Demultiplexing

Required if the Seismic Data is recorded inmultiplexed formatConversion of scan sequential mode to

trace sequential mode.Essentially a Matrix transposition (rows to

columns and vice versa)

Multiplexing of data

Multiplexed Data are written in Sample order

Demultiplexing of data

Demultiplexing of data

Trace Headers Generation

Generation of addresses to the traces

Geographical positioning

Facilitates for the unique identification

Sorting with respect to a common group (common shot, common receiver, common midpoint & common offset)

Trace Headers Generation

Different Types of Gathers

Common Mid Point gatherCommon Receiver gather

Spherical Divergence Correction

Seismic Amplitude decays as a function of time due to spherical spreading and inelastic attenuation.

Compensation is done using a gain function that is inverse of the decay curve.

Objective is to see that nearly same amount of energy is reaching at every layer of the subsurface.

Amplitude Decay

Normal Moveout (NMO)

• NMO correction is ‘Dynamic’; it is a function of time (To), source to receiver offset, and Velocity.

Pre-Stack Analysis In the case of marine data, pre-stack analysis includes selection

of velocity analysis locations, identifying records and traces that require editing, and determining deconvolution parameters.

In case of land data processing, especially for noisy data, prestack analysis is also done to analyze frequency content of the data and to choose parameters for processes that enhance signal-to noise ratio.

Band-pass filter, velocity filter, and deconvolution tests are run for this purpose. Band-pass filtering discriminates between signal and noise on the basis of frequency.

Velocity filters discriminate between signal and noise on the basis of apparent velocity.

Pre-Stack Analysis Decon can attenuate undesirable events such as short period multiples

and enhance the vertical resolution, by collapsing wavelets. Marine data normally have less random and coherent noise than land

data, but they suffer from a higher degree of multiple reflections in many offshore areas.

A common midpoint sort is generated for conducting many processing steps, such as applying elevation statics, velocity analysis, residual statics, and stack to name few.

Figure shows the flow chart of pre-stack analysis. Distances and angle changes on the seismic line must be taken in

consideration to obtain the correct distance from the source to receiver for each trace.

Field statics (that account for trace to trace elevations differences) are usually applied before NMO to derive velocity analysis from seismic data.

Pre-Stack Analysis

Pre-Stack Analysis

Static Corrections

• The elevation differences among the traces of a CMP gather cause delays.

• The Low Velocity Layer (LVL) near the surface also introduces delays in the observed travel times.

• The data has to be corrected to a reference surface (Datum) removing these differences.

• These corrections are static; they don’t change with time; hence the name ‘Static correction’.

Static CorrectionsIn order to obtain a seismic section that accurately shows the subsurface structure, the datum plane at a known elevation above mean sea level and below the base of the variable-velocity weathered layer

The value of the total statics (ΔT) depends on the following factors:1) The perpendicular distance from the source to the datum plane.2) Surface topography; that is, the perpendicular distance from the

geophone to the datum plane.3) Velocity variations in the surface layer along the seismic line. 4) Irregularities in thickness of the near-surface layer. In computing ΔT it is usually assumed that the reflection ray path in the

vicinity of the surface is vertical.The total correction is:ΔT = Δts+ΔtrWhere Δts = The source correction, in ms & Δtr = The receiver

correction, in ms

Static Corrections

Static Corrections• Figure shows common shot gathers from a seismic land line,

where statics (due to near-surface formation irregularities) caused the departure from hyperbolic travel times on the gathers at the right side of the display.

Deconvolution

• A seismic trace can modeled as the convolution of the input signature with the reflectivity function of the earth impulse response, including source signature, recording filter, surface reflections, and geophone response.

• It is also has primary reflections (reflectivity series), multiples, and all types of noise.

• If decon were completely successful in compressing the wavelet components and attenuating multiples it would leave only the reflectivity of the earth on the seismic trace.

• In so doing, vertical resolution is increased and earth impulse response or reflectivity is approximately recovered.

Deconvolution Deconvolution is a process that improves the vertical resolution of

seismic data by compressing the basic wavelet, which also increases bandwidth of the wavelet.

In addition to compressing or shortening reflection wavelets deconvolution can also be used to attenuate ghosts, instrument effects, reverberations and multiple reflections.

The earth is composed of layers of rocks with different lithologies and physical properties.

In seismic exploration, their densities and the velocities at which the seismic waves propagate through them define rock layers.

The product of density and velocity is called acoustic impedance. It is the impedance contrast between layers that causes the reflections

that are recorded along a surface profile Decon is normally applied before stack (DBS). But it is sometimes is

applied after stack (DAS)

Deconvolution

Mute This is the process of excluding parts of the traces that contain

only noise or more noise than signal. The two types of mute used are front-end mute and surgical mute.

Front-End Mute In modern seismic work, the far geophone groups are quite

distant from the energy source. On the traces from these receivers, refractions may cross and

interfere with reflection information from shallow reflectors. However, the nearer traces are not so affected.

When the data are stacked, the far traces are muted (zeroed) down to a time at which reflections are free of refractions.

The mute schedule is a set of time, trace pairs that define the end of the muting.

Mute changes the relative contribution of the components of the stack as a function of record time.

Front-End Mute In the early part of the record, the long

offset may be muted from the stack because the first arrivals are disturbed by refraction arrivals, or because of the change in their frequency content after applying normal moveout

The transition where the long offsets begin to contribute may be either gradual or abrupt.

However, an abrupt change may introduce frequencies that will distort the design of the deconvolution operator.

Surgical Mute Muting may be over a certain

time interval to keep ground roll, airwave, or noise patterns out of the stack.

This is especially applicable if the noise patterns are in the same frequency range as the desired signal.

Convolution to filter out the noise may also attenuate the desired signal.

Figure illustrates the surgical mute approach.

Velocity Analysis

• The word velocity seldom appears alone in seismic literature. Instead it will occur in combinations such as instantaneous velocity, interval velocity, average velocity, RMS velocity, NMO velocity, stacking velocity, migration velocity, apparent velocity, etc.

• Figure can be used to illustrate some of these

Velocity AnalysisInstantaneous Velocity The velocity indicated by V(xa, za) is the velocity that would be

measured at a point a distance xa from the left of the Figure and at a depth za is an example of instantaneous velocity.

Average velocityThe velocities indicated as V1,V2, V3, etc. are interval velocities. They are the average velocity through an interval of depth or record time

and equal the thickness of the depth interval divided by vertical time through the interval.

Average velocity to a particular depth is simply the depth divided by the time it takes a seismic wave to propagate vertically to that depth.

Since seismic wave propagation times are usually measured as two-way times, the average velocity to, say, za, in Fig. is (2za/Ta), where Ta is the two-way time to depth za.

Average velocity is required to convert time to depth.

Velocity AnalysisRoot Mean Square or RMS velocity at a particular record time, Tn,

is calculated as follows:1. Determine what interval times sum to the value Tn2. Square the corresponding interval velocities3. Multiply the squared interval velocities by their interval times4. Sum the products obtained in step 35. Divide the sum obtained in step 4 by Tn6. Take the square root of the value resulting from step 5, This is the

RMS velocity at time Tn

If all reflectors are flat or nearly flat, RMS velocity is the same as NMO velocity.

NMO velocity is the velocity used to correct for NMO. If NMO corrections are correct, and no other factors are involved, all primary reflections on CMP gather records occur at the same time on all traces.

Velocity Analysis Stacking velocity is the velocity that gives the optimum common midpoint

(CMP) stack output when it is used for NMO corrections. It may be the same as NMO velocity but if there is significant dip on

reflectors, it probably will not be the same. Migration velocity is the velocity that optimizes the output of a migration

algorithm, i.e. – moves the reflected energy to the correct times and places. Apparent velocity is determined by dividing a horizontal distance by the

time a seismic signal appears to propagate across it. For source-generated noise, apparent velocity and propagation velocity are

equal but for reflections, it is much faster than NMO or stacking velocity. Apparent velocity is important in designing velocity or F-K filters. Stacking velocity, migration velocity and average velocity are the most

important to seismic data processing.