- Geometry definition
- First break picking
- Antialias filter
- Gain recovery
- Statics Elevation and refraction statics correction
- F–K or apparent velocity filter
- Velocity analysis and NMO Correction
- Dip moveout (DMO) correction
- Common midpoint (CMP) stack
- CDP stack and residual statics
Post-stack main procedures:
- Poststack filter
- Migration Velocitypreparation
- Time and Depth Migrations
- Multiarrival amplitude-preserving prestack 3D depth migration
- Presudo width-azimuth 3D/3C seismic acquisition
- Azimuthal AVO analysis
The name given to sorting the traces from time ordered storage (all receiver stations at a given time) to receiver ordered format (all times for a given receiver) or trace sequential format. Many modern instruments do this in the field, but much data still comes in from the field multiplexed. SEG A and SEG B formats are multiplexed, SEG Y is a trace sequential format, and SEG D can be either way.
The process of flagging traces or pieces of traces to be ignored for one reason or another
The association by unique identifier of each recorded trace with shot and receiver locations.
First break picking
The earliest arrival of energy propagated from the
energy source at the surface to the geophone in the wellbore in vertical
seismic profiles and check-shot surveys, or the first indication of seismic
energy on a trace. On land, first breaks commonly represent the base of
weathering and are useful in making static corrections.
"Mute" zeroes the beginning of traces of seismic records. The purpose is to eliminate the noises preceding the first arrivals. An example would be zeroing the sections of traces before the first reflection from the water bottom in marine seismic data. A mute is simply an area of data that is zero'd; it might be based on a line (with data above or below the line being muted), or even a polygon, with data inside the polygon being muted.
A low pass filter applied before resampling the data to a coarser time scale to prevent aliasing. Aliasing is a phenomenon in which high frequency data masquerades as low frequency energy as a result of undersampling. To sample a signal properly, there must be at least two samples within the shortest period of interest. Antialias filters remove frequencies above the sampling limit (Nyquist frequency) of the new sampling time. The operation is performed before the sampling is reduced.
The correction for the loss in amplitude of a signal as it travels through the earth and spreads its energy over a larger surface area. This involves multiplication of the signal by a number that increases with time. The exact time variant multiplier can be based on the theoretical concept of spherical spreading (related to the square of the distance traveled), can be based on measurements of amplitude decay with time made on the data itself, or can be entirely arbitrary.
The removal of the frequency-dependent response of the source and the instrument. The instrument response is normally known and can be removed exactly. The source shape is not usually known but can be measured directly (marine air gun signatures) or estimated from the signal itself under certain assumptions. Signature deconvolution, wavelet deconvolution, spiking deconvolution, gapped deconvolution, predictive deconvolution, maximum entropy deconvolution, and surface consistent deconvolution are various manifestations of the attempt to remove the source width from the observed reflections. The resulting reflection sequence always has some smoothing function left, usually called the residual wavelet. Attempting to be too exact about deconvolution usually results in a very noisy section.
Statics Elevation and refraction statics correction
The removal of traveltime artifacts relating to the placement of the source and receiver at or near the earth's surface. Differences in traveltime to the same reflector which result from elevation differences and near-surface velocity changes at different source and receiver stations must be removed. The relative elevation of each shot and receiver location and the near surface velocity must be known to make these corrections. An elevation datum is chosen, and the distance above or below that datum is measured for each source and receiver. The difficulty is in knowing what velocity to use to convert this elevation difference to a time correction to be added to or subtracted from the entire trace (hence the term statics). Refraction statics, surface consistent statics, and residual statics are all techniques used to estimate and apply the appropriate velocity and time corrections
Strong reflections can act as a secondary source of seismic energy that will interfere with the primary reflections and confuse the interpretation. Such secondary reflections are called multiples. The most common are water bottom multiples, but interbed multiples also exist. The demultiple process attempts to remove these
F–K or apparent velocity filter
Acoustic signals that are not reflections from subsurface layers appear in shot records (Figure 1) as straight lines rather than hyperbolic curves. These events have a constant “apparent velocity” as they travel along the receiver cable. This simple organization allows them to be isolated from the reflection signal and to be removed from the record. A common way to do this is with the FK (sometimes called pie slice) filter. Judicious selection of the range of apparent velocities to be removed can eliminate linear noise. Too wide a filter can remove too much information from the section and causes serious interpretation problems
Velocity analysis and NMO Correction
Dip moveout (DMO) correction
NMO corrections are made under the assumption of horizontal planar reflectors. If the reflector has appreciable dip, then the actual movement will be slightly different. The DMO correction is a method for estimating the effect of dip on moveout and removing it from the records as well.
Common midpoint (CMP) stack
This is the single most effective step for noise reduction in the processing flow. The shooting procedure results in many traces being acquired with the point midway between source and receiver (called the midpoint) being coincident on the earths surface. The only difference between the traces is the distance between source and receiver (offset). Once these traces have been NMO (and DMO) corrected, they are really redundant samples of the same reflection. Adding them together increases the signal to random noise ratio by the square root of the number of redundant samples. The process reduces the field data to a stacked section consisting of one trace for each midpoint location, assumed to have been recorded with a shot and receiver coincident at the midpoint location
CDP stack and residual statics
To improve stacking quality, residual statics corrections are performed on the moveout-corrected CMP gathers. Static shifts introduced by topographic variations fall under the class of field statics, and those due to near-surface lithological variations that occur within a cable length fall under the class of residual statics. Correcting for static shifts in the traces can make a significant difference in the quality of a migrated or stacked image. It is easy to determine elevation static corrections, but not so easy to find the residual static corrections. One means is to determine the near-surface velocity distribution by refraction tomography.
Usually a band pass filter, this process excludes frequencies above a certain value (high cut) and below a lower value (low cut) to retain that part of the signal with the highest signal to noise ratio. The values are usually set by trial and error and judged by a visual comparison of sections. The values may be different for different time gates of the section. Typically, the deeper reflections (later time) have less signal at high frequencies because these frequencies are absorbed or scattered more readily in the earth. Consequently, a lower value for the high cut frequency must be used as the bandpass is applied to later times on the trace.
Migration Velocity Field preparation
Migration needs an accurate velocity model to fully focus reflections and correctly position reflectors in space. The determination of an accurate migration velocity is a crucial step in seismic imaging. Migration Velocity Analysis (MVA) is the process of estimating interval velocity in the image domain by performing several iterations of the following three-steps process: 1) the data are migrated with the current best estimate of interval velocity, 2) the prestack images are analyzed for kinematic errors, 3) the measured kinematic errors are inverted into interval velocity updates by a tomographic process.
Time and Depth Migrations
- Moveout corrections by different waves types and VSP data.
- 2D models physical parameters determine into the depth converted or vice versa.
- 2D seismic model of average velocities conversions into interval velocities or vice versa.
Range of subroutines had been developed obtaining three-component seismic records of P-, S-waves and to determine following moveout attributes and elastic parameters:
• Full waves vector;
• Waves polarization;
• Three-component records orientations;
• Phase of wave record;
- CDP record definition of P- and S-wave on certain times discreet of seismic horizons.
–2D model subtraction based on analysis of P- & S- seismic attributes, physical parameters of alternative geo-models.
- Thin-layer 2D models calibration based on P- and S- the velocities processing of real seismic data.
- Thin-layer 2D models calibration of physical parameters, including the fracturing, obtained by real seismic data and GWL.
- Poisson’s ratio and Young modulus definition with various stress statements consideration and visualization of obtained parameters demonstrated of non-linear impact and to calculated elastic attributes of the third order.
- Detection and partial traces erasing of 2D models are not corresponded to the acoustic interfaces.
- Observed depths adjustment of 1D models (determined by GWL data) for deviated wells.
- Pore-pressure 2D model recalculation base on specified 2D thin-layer model of rocks density.
- Individual poor-quality layers screening and layers separation based on physical parameters matching to the specified criteria.
- Depth layers models' adjustment for deep deviated well.
- Similar lithological properties separations based on 1D velocities and densities models and new models generation. Subroutine can compile 1D model of elastic attributes of clastic rocks by various lithologies: clay, siltstone, sand, etc.
- 2D model of formation pressure definition in the vertical and lateral dimension based on densities and Vp and Vs parameters.
- AVO attributes detection by standard approaches and seismic properties prediction based on analyses of P- and S-wave seismic data. Subroutine can operate with far-angles partial stack data.- Thin-layer models of physical parameters smoothing defined on real seismic data within certain window of analysis (or horizon). The module can reduce the dispersion of seismic data.