Statics Modelling
Spectrum fully recognises the considerable influence a robust static model solution plays in the precise imaging of land data. Experience of a wide range of topographic conditions and the use of effective 3rd party software enable our geophysical staff to provide customers with the correct solutions for various survey conditions.
Refraction StaticsSpectrum has licences for Hampson-Russell GLI3D and Renegade Seismic Studio
Both are interactive programs designed to interpret first breaks from 2-D and 3-D seismic data. They derive a near-surface geological model from which static corrections are calculated for application to the seismic data. 2-D, 3-D, or crooked lines are all handled with the same algorithm with no restrictive assumptions concerning the acquisition geometry or ordering of the data.
For the GLI 3D software, first breaks are picked interactively and the initial geological model is then specified at selected control points. Once the initial model had been formed, GLI3D uses Generalised Linear Inversion or Tomographic Inversion to modify the model. By analysing the error or misfit between the real first breaks and the predicted values, the model is iteratively updated.
The Linear Inversion method makes a number of assumptions e.g. constant velocity weathering layers. The tomographic inversion method uses a turning-ray model in which first arrivals represent a wavefront travelling through a continuous velocity medium with vertical and horizontal velocity gradients. The algorithm uses a finite difference solution for forward modelling of travel times and an inversion procedure that calculates the back propagation of errors and updates the model velocity.
From the final geological model, static corrections are calculated and these are then written to the seismic database for later application to the data.
The Renegade ‘Seismic Studio’ package offers an alternative to the above. It features full geometry build and QC, manual and automated first break picking, and full 2D / 3D refraction statics modelling. Models can be derived using delay time, or tomographic inversion techniques and there is a comprehensive set of interactive tools for QC at every stage.
Seismic Studio is an fully interactive software package. Where necessary first break picks can be monitored in shot, detector or offset domain, and appropriate offset ranges assigned. Picks within these offset ranges can be used to derive an initial crude estimate of refractor velocity and delay time.
After first breaks are established, Seismic Studio uses a technique called “Residual Velocity Correction” (RVC) to derive delay-time values, refractor velocity values, and pick quality for each source and detector point. Further QC can be done at this stage to identify points with high pick error values, which might need re-picking.
The next step is to use the 3-D modelling tool to derive a near-surface model. An estimate of the velocity between surface and refractor is entered, and the elevation of the refractor is calculated from delay times and velocities. The shape of the refractor is usually more rugged than is likely, and options exist to smooth it, and to put the variation into the weathering velocity. Finally, the datum and replacement velocity are inserted and statics calculated.
Spectrum has a number of residual static routines, both surface consistent and CDP consistent. Highly flexible iterative techniques are often used to compute static corrections. As well as computing shot and receiver statics the user has options to compute the reflection structure, residual normal moveout (RNMO) and perform a residual phase calculation.
Residual statics can also be calculated by cross-correlating traces. This is done by automatically picking static shifts using a statistical method and computing the relative static shift by cross-correlating the current trace with a number of traces within a co-ordinate and offset range. The relative static shift is defined by the peak of the cross-correlation function. Three peaks on the cross-correlation function and other information are passed to a second module which uses a conjugate gradient method to solve for surface consistent shot and geophone statics, RNMO and dip.
Water Column Statics (Marine)The variation in water velocity due to temperature and salinity changes can be compensated for by applying an appropriate static to the shot records. The way this static is calculated is to accurately measure the velocity of the seabed reflection using near offset data from one of the inner cables. The difference in the picked velocity from a reference velocity of 1480 ms - ¹ is mapped 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 cross line stacks to check for sail line to sail line continuity and adjusted by manual editing. The final statics are then applied to the data
