Simultaneous AVA/AVO Inversion
Since its introduction in the fall of 2000, RockTrace® has significantly impacted the way the industry uses and incorporates PSTM seismic. It is the only technology that quantitatively integrates well log elastic rock properties and AVA seismic to produce calibrated quantitative 3D volumes of rock properties.
RockTrace builds on the InverTrace®Plus technology by extending it to the AVO domain. In InverTracePlus, the constraints applied are in terms of acoustic impedance (Zp). In RockTrace, the objective is to solve for shear impedance (Zs) and density in addition to acoustic impedance, so the constraints are set for all three parameters independently. The parameterization can be in terms of triplets of elastic parameters:
- Zp Zs and density
- Zp, Vp/Vs and density
- P-Sonic S-Sonic and density
- Vp, Vs and density
When applied in global mode, just like in InverTracePlus, a spatial control term is added to the objective function and large subsets of traces are inverted globally at the same time. The RockTrace seismic inversion algorithm takes multiple angle-stacked seismic data sets and generates three elastic parameter volumes as output. The algorithm is an extension to the global multi-trace seismic inversion algorithms in the InverTracePlus program, which takes a single seismic data volume as input and produces a single impedance volume (one of the sets above) as output. While the RockTrace algorithm is generalized for three elastic parameters, many of the principals and constraints of the other programs remain.
This is unique in the industry and brings the following significant advantages:
- The elastic parameters are real rock properties that can be directly related to reservoir properties.
- No approximations are made as the full Knott-Zoeppritz equations are used.
Full allowance for amplitude and phase variations with offset. This is done by deriving unique wavelets for each input partial stack.
- The elastic parameters themselves can be directly constrained during the seismic inversion.
- Rock physics relationships can be applied constraining pairs of elastic parameters to each other.
- Enhanced robustness against noise as all input data must conform to a single output model.
- Final elastic parameter models optimally reproduce the input seismic, as this is part of the seismic inversion optimization.
- Analysis of QC issues easier and more precise because process is integrated with all data conforming to a single model.
Components within RockTrace
There are three modules in the RockTrace product:
- P-P and P-S Elastic Impedance Log Curve Generator
- Simultaneous AVA Constrained Sparse Spike Inversion
- Vertical Data Alignment
In addition, in the Wavelets™ application, a multiple angle stack wavelet estimation tool is available with a RockTrace license.
Reflectivity logs are computed for estimating wavelets from the input partial angle stack data, this ensures AVA effects are properly incorporated into the estimated wavelet. However, RockTrace includes specific additional functionality in the Wavelets program of the Jason™ suite. A near stack wavelet can be used as the starting point for estimating the far angle or offset wavelet. The effects of increasing angle on the wavelet are modeled in a controlled way using parameters such as Q, phase rotation, and shifting. These parameters are estimated during the wavelet estimation process.
Vertical Data Alignment
The RockTrace simultaneous inversion cannot cope with excessively large time shifts between the partial stacks, which is caused by (among other things) residual NMO. The RockTrace Vertical Data Alignment module aligns the data by stretch and squeeze of one data set relative to another on a trace-by-trace basis. (The calculation is made by cross correlation, so it is often better to calculate the vertical alignment using band limited impedance data sets rather than seismic data sets, as the side lobe and tuning effects of the wavelet have been reduced.)