2.2. Create Steering Data
2.2.1. Import Steering Data
Steering data created in any other software package can be imported into OpendTect (with the dip-steering plugin), provided the format is compatible with the unit usage in OpendTect. Before steering data can be used as such, the dip and/or azimuth data needs to be available. Use File - Import - Seismics to import volumes, and define attributes to convert these into the correct units if necessary (see below). With this functionality, the data can be converted into a proper OpendTect steering cube.
Dip values should be non-negative and should be in usec/m, or in m/m if your survey is in depth.
Azimuth should be in degrees from -180 to 180. Positive azimuth is derived from the inline in the direction of increasing crossline numbers. Azimuth = 0 indicates that the dip is dipping in the direction of increasing cross-line numbers. Azimuth = 90 indicates that the dip is dipping in the direction of increasing in-line numbers.
It does not really make sense to execute this import on multiple machines. But, if the data volumes are stored on a remote machine, execution in single machine mode on this remote machine may avoid unnecessary network traffic.
2.2.2. Calculate Steering Data
2.2.2.1. Description
In OpendTect it is possible to apply attributes and filters that follow the local dip-azimuth. This is what we call "Steering". The local dip-azimuth information is stored in the SteeringCube.
2.2.2.2. Create steering cube window
Select the input data cube (usually a seismic volume) and optionally the sub-volume to process.
Four types of Steering algorithm are supported:
BG Fast Steering
Standard
Combined
Precise
BG Fast Steering is a very fast algorithm that is developed by BG and is based on analysis of the gradient of the amplitude data, both vertically and horizontally. Without filtering this SteeringCube looks very noisy. A median filter setting of 1 1 7 has shown to give very acceptable results.
Standard, Combined and Precise are all Fast Fourier Transform based algorithms. They need a cube size specification. Standard is the recommended algorithm, and the Precise algorithm is most accurate but requires (considerably) more CPU time. The Combined algorithm uses the standard method, but applies the precise method when the standard method does not provide a stable solution.
The Precise algorithm uses zero padding of the signal before Fourier transformation. For example, if the input window is 7x7x7 samples, the algorithm adds zeros to all sides to obtain a 21x21x21 cube. A Fourier transform assumes that all inputs are periodic, and will give a high response at 1/(ns*dt). In case ns (the number of samples) is 7 and dt (the sample rate) is 4 ms, this will be at 36 Hz, interfering with the main frequencies of the signal. By zero padding, we shift this peak to 12 Hz. At the same time, the amplitude of the undesired peak is much lower. The Precise algorithm also uses another interpolation algorithm to find the maximum (hence dip) in the Fourier domain. The interpolation algorithm is 'true' 3D, i.e. a 3D signalis fitted at the maximum position. In the standard algorithm the maximum is found by three successive 1-D interpolations, which is much faster, but less precise - albeit more 'global' and thus for many situations even more suited.
More background information, including a benchmark of the different algorithms and visual examples of the quality that different algorithms provide are presented in Section 2.4.
The Specify maximum dip is optional. It limits dip values derived from the input data. Another option to avoid extreme dip values is to filter the steering data with a Median filter. This removes the outliers from the steering data. The stepouts are defined in samples, regardless sampling rate.
Optionally, the processing specifications as defined in the window can be saved. Provide a file name in the appropriate box to store the processing specification. If this space is left empty, the processing specification is not saved. If, for any reason, the processing is aborted, the process can be re-started with this parameter file using the Re-start option under the Processing menu.
The Proceed button opens the single machine or multi-machine processing mode. For more information on single and multi-machine processing, open the help menu from the Batch Processing window.
2.2.3. Filter
2.2.3.1. Description
SteeringCubes can be enhanced by applying a median dip filter to the steering data. Already during creation of the SteeringCube the steering data can be filtered, but this filtering can also be done afterwards.
2.2.3.2. Filter steering cube window
Select the input SteeringCube and, optionally, the sub-volume to process. The median filter stepout settings are defined in samples, regardless sampling rate. Batch jobs can be processed on a single machine or on multiple machines. For more information on single and multi machine processing, open the help menu from the Batch Processing window.
Optionally, the processing specification as defined in this window can be saved. Provide a file name in the appropriate box to store the processing specification. If this space is left empty, the processing specification is not saved. If for any reason the processing is aborted, the process can be re-started with this parameter file with the Re-start option under the Processing menu.
