OpendTect dGB Plugins User Documentation version 4.2
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2.4. Benchmark Steering Cube Creation

In this chapter the quality and speed of the available steering algorithms are compared in order to guide the choices for the right SteeringCube algorithm. First, the results of a test on a standard data set are presented to give an indication of the speed of SteeringCube calculation versus the chosen algorithm and calculation cube size parameter (where applicable). Then, a visual quality check on one of the inlines is presented. This is done by checking the crossline dip, which is directly extracted from the SteeringCube, and by reviewing the steered similarity, which uses information derived from the SteeringCube.

The final output quality is determined not only according to the steering algorithm. The size of the calculation cube also plays an important role. Next, the application of additional dip-steered median filter influences on the output quality are presented and finally, the use of dip limit.

2.4.1. Speed vs. algorithm and calculation cube size

In Figure 2-1 the relative calculation speed is displayed for the different algorithms. The test was done on a cluster of 6 computers using distributed computing. The input cube has 817811 traces. Each trace has 1551 samples with step of 4 ms, making a total of 6200 ms. Calculation speed is measured in traces per second.

Figure 2-1. Comparison of relative speed of the different steering algorithms.

2.4.2. Visual quality check

In the following sections, the crossline dip component of a SteeringCube created with the different algorithms is presented. The figures contain the results from the Precise FFT algorithm, the Combined FFT algorithm, the Standard FFT algorithm and the BG Fast Steering algorithm (for technical details see Section 2.2.2) for a cube size of 3. Notice that the nomenclature convention used is the following:

The FFT precise is called FFT+.
The FFT combined is called FFT++.
The standard steering algorithm is called FFT+++.
The calculation cube size is specified after the algorithm (e.g. FFT7, BG5, etc.).
The limit of dip is called maxdipXXX, where XXX is the limit in µs/m.
Additional filtering are called medXYZ, where X is the inline stepout, Y the crossline stepout and Z the sample stepout.

The inline itself is displayed in Figure 2-2 for reference. The inline was selected because many geological and seismic features are visible, which enable evaluation of the performance of the algorithm in different environments.

Figure 2-2. Seismic data of the test inline.

2.4.3. Crossline dip attribute

The crossline dip is one of the two SteeringCube components (together with inline dip) and is related to the dips projected in the crossline direction. In Figure 2-3 to Figure 2-5 the crossline dip is displayed for the algorithms mentioned in Figure 2-1 with cube sizes of 3, 5, and 7.

Figure 2-3. Crossline dip with (calculation cube size=3): precise steering (A), combined steering (B), standard steering (C) and BG steering (D).

Figure 2-4. Crossline dip with (calculation cube size=5): precise steering (A), combined steering (B), standard steering (C) and BG steering (D).

Figure 2-5. Crossline dip with (calculation cube size=7): precise steering (A), combined steering (B), standard steering (C) and BG steering (D).

From Figure 2-3 to Figure 2-5 it is notable that there is no significant difference between the combined steering algorithm- (FFT++) and precise steering algorithm (FFT+)speed and accuracy. The standard steering algorithm (FFT+++) is fast but apparently often produces erroneous results in high dip areas. In order to avoid getting a noisy steering cube the calculation cube-size of the FFT algorithms has to be at least 5 or 7. The latter is the default setting. The BG algorithm has a different behaviour: a cube size of 3 seems to be sufficient, but the raw steering cube is useless and a median filtering appears to be mandatory.

2.4.4. Filtering of the steering cubes

Figure 2-6 and Figure 2-7 show the results after applying a median filter with different step-outs to the steering cubes. It is apparent that no lateral filtering and only a small vertical filtering, gives the best result with the FFT algorithm.

The BG steering needs to be filtered in the lateral and vertical direction. The best results were obtained with median filter with step-outs 1 1 3. After filtering, the outputs of the precise steering with cube size 7 and median filtered with step-out of 2 only in the vertical direction (FFT7+ med002) and Fast BG steering median filtered with the step-outs 1 1 and 3 (BG3 med113) are very similar in accuracy. However, the latter is produced 10 times faster.

Figure 2-6. Filtering of FFT+ using calculation cube size = 7: raw (A), median filter with step outs 0 0 2 (B), median filter with step-outs 0 0 4 (C), median filter with step outs 1 1 2 (D).

Figure 2-7. Filtering of BG steering using calculation cube size=3: raw (A), median filter with step-outs 0 0 2 (B), median filter with step outs 1 1 1 (C), median filter with step outs 1 1 3 (D).

Figure 2-8 shows that adding a dip limit during the processing does not affect the speed of the algorithms. The final result is strictly identical when the dip is lower than the limit L, and the extreme values are rounded toward L. Using a limit requires a priori knowledge and is in the end a choice of the interpreter.

Figure 2-8. Filtering of FFT+ using calculation cube size=7: median filter with step-outs 0 0 2 (A) median filter with step-outs 0 0 2 and maximum dip of 300 (B), and filtering of BG steering using calculation cube size=3: median filter with step-outs 1 1 3 (C), median filter with step-outs 1 1 3 and maximum dip of 300 (D).

2.4.5. Steered Similarity attribute

Figure 2-9 displays the Similarity attribute for the algorithms FFT+ and BG. As an extra reference, the non-steered similarity is added. All figures were calculated with the time gate [-32,32] ms.

Figure 2-9. Positive curvature using perfect FFT (time gate from[-32,32]ms): no steering (A), precise steering with cube size =7 (B), BG steering with cube size =3 (C).

2.4.6. Choosing a steering algorithm

Different Steering algorithms are available. The precise FFT algorithm yields an almost perfect SteeringCube at the costs of considerably longer calculation times. The BG Fast Steering algorithm seems to fit 95% of the situations and is very fast. dGB recommends the use of this algorithm, using its default calculation cube size of 3 and additional median filtering 113. Although depending on the geology, data quality, available computation time and purpose other choices can be made. A number of examples are presented:

For a dataset of good quality with only small and low variance in dips, the BG Fast Steering method performs well enough. With a median filtering with step-out 1 1 3 applied the result is already very acceptable.
For a poor quality dataset, one of the FFT algorithms should be used because the BG Fast Steering algorithm is sensitive to noise and will produce too much outliers. Minor vertical filtering might also help improve the results.
For detailed studies at target level, the precise FFT algorithm can be considered for a sub-volume within the area of interest.
When creating chrono-stratigraphy, the Event Steering is optimal because it looks for similar events (min, max) on neighboring traces in inline/crossline directions.

It is always possible to go back and spend much more time in producing a SteeringCube using the FFT precise algorithm.

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