3/2/2016 Statics ­ Spectrum Geo

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StaticsThe weathering layer may be the most variable of all layers yet in seismic processing it is taken to be either uniform inthickness or velocity. Datuming through an incorrect weathering model can corrupt the stack and can introduce falsestructure into deep reflectors. It is important to correct for the effect of variable thickness and lateral velocity variation of theweathering layer. The main methods used to correct for these effects are given below.

Uphole SurveysAn uphole survey is used to determine the weathering layer velocity. A borehole is drilled that penetrates below theweathering layer; several geophones are placed at various known depths within the hole. The geophone locations must spanthe weathering and sub­weathering layers. A shot is fired at the surface near the hole and the direct travel times to thegeophones are recorded.

A plot of the direct travel times versus the geophone depths can be used to compute the velocities of the weathering layerand sub­weathering layers along with the thickness of the weathering layer at that point.

This method attempts to construct a model of the weathering layer by estimating the velocity and thickness of the weatheringlayer at several locations and interpolating between these locations.

Refraction StaticsThe purpose of refraction statics is to compute weathering statics corrections during the processing of reflection seismic databy using the travel times of critically refracted seismic energy (first breaks). There are different techniques in the applicationof refraction statics one of which is the General Linear Inversion method (GLI).

In order to incorporate low velocity layers into the statics solution, accurate uphole information is required. By including theuphole information we have a good estimate of the weathering layer velocity which allows us to build a good initial near­surface geological model.

Spectrum has licences for Hampson­Russell GLI3D, Green Mountain Geophysics Millennium Series and Sigma ’s SeismicStudio .

GLI3D relies on user interpretation of layers/velocities (i.e. slope/intercept) at discrete control points which can be used tobuild the initial depth/velocity model. This is then used as the input to the first run of GLI or Tomography to obtain updates.(This sequence can be iterated as necessary to achieve a stable result).

In contrast, Seismic Studio utilizes user picked offset ranges for each refractor to build a velocity/depth model. Delay timesare calculated for each defined layer via various methods, or tomography.

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3/2/2016 Statics ­ Spectrum Geo

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Initial Model (LEFT) Tomographic Model Update (CENTER) GLI Model Update (RIGHT)

Seismic Studio is an interactive software package which offers a full geometry build and QC, functions to pick first breaksmanually and automatically and can derive full 2D/3D refraction statics.

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Residual StaticsSpectrum has a number of residual static routines, both surface consistent and CDP consistent. Highly flexible iterativetechniques are often used to compute static corrections. As well as computing shot and receiver statics the user has optionsto compute the reflection structure, residual normal moveout (RNMO) and perform a residual phase calculation.

GLI static effect on shot gathers – example from onshore Croatia

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Straight Ray DatumingThe conventional “statics correction” method to correct for time distortions introduced by the weathering layer has itslimitations. It relies on assumed surface consistency, requires a vertical ray path and needs ray theory to be an acceptableapproximation of near surface wave propagation.

In the presence of rapid lateral variations of theweathering layer, one or more of these conditions arenot met. However, more advanced methods such asKirchhoff Pre­Stack Re­datuming or finite difference forsuch corrections can also often be unfeasible. Thesemethods can have a high computational cost and canalso require highly rigorous, time consuming processing,and considerable sampling of the sources and receivers,not seen in conventional 3D land acquisition.

Spectrum offers an innovative solution to bridge the gap between these methods. Straight Ray Datuming (SRD) is anadvance in statics modelling developed for commercial use by Tariq Ali Alhalifah.

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Marine – Water Column StaticsThe variation in water velocity due to temperature and salinity changes can be compensated for by applying an appropriate

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static to the shot records. The way this static is calculated is to accurately measure the velocity of the seabed reflection usingnear offset data from one of the inner cables. The difference in the picked velocity from a reference velocity of 1480 ms­1 ismapped for each sail line and a static is worked out using the following formula:

Static = Twb(Vwater/1480)­1

Where Twb and Vwater are the picked times and velocity of the water bottom.

The raw static values are applied to near trace crossline stacks to check for sail line to sail line continuity and adjusted bymanual editing. The final statics are then applied to the data