Dip Magnitude and Dip Azimuth
Attributes Description: The Dip Magnitude and Dip Azimuth attributes are estimates of the descent and direction of seismic reflectors.
Interpretation Use: The Dip Magnitude and Dip Azimuth attributes enable the mapping of subtle trace-to-trace variations on reflection character. Also, a composite display of dip magnitude and azimuth can help to emphasize more relevant geologic information (Chopra and Marfurt, 2007). Attribute results are better analyzed in plan view or draped over a horizon display.
Recommended color palette: For the Dip Magnitude attribute, a grayscale gradient color scheme is suggested. The color progression could begin with white (to highlight useful geological features) and finish with black (to denote shadow areas), or vice-versa. For the Dip Azimuth, a rainbow color scheme is suggested. A specific color is assigned to different azimuth ranges, so then the display can be used to infer the dip direction of deformed rocks. We suggest using the histogram of values to guide setting color value thresholds.
Computation: The Dip Magnitude and Dip Azimuth use seismic amplitude data (time or depth domain) as input. The attributes are computed from the estimates of dip angle along the inline and crossline directions in a 3D survey (refer to Filter Dip Components -> Inline Dip, Crossline Dip, and Confidence attributes description section). To avoid smearing of faults, fractures, and other discontinuities, the algorithm uses a multi-window moving around an analysis point (Luo et al., 2002). The inline and crossline dip estimates are done using the window with the maximum coherency and are related to the estimated true dip magnitude and dip azimuth by these simpel geometric relationships (Marfurt, 2006):
The dip magnitude is greater than or equal to apparent dip pairs θx and θy. The dip azimuth is measured from the inline 3D seismic survey direction. A coordinate rotation is applied when the inline acquisition is not aligned with the North. The apparent dip pairs θx and θy are related to the estimated true dip and azimuth by the simple geometric relationships (Marfurt et al., 1998)
Note that in the presence of noisy input data, the following structure-oriented filters can be also applied as an optional step: i) Lower-Upper-Middle (LUM), ii) Multistage Median-based Modified Trimmed-Mean (MSMTM), iii) Alpha-Trimmed Mean, and iv) Mean. The Alpha-Trimmed Mean and Mean filters work best in areas where the data has random incoherent noise but might misestimate the amplitude. Also, the Alpha-Trimmed Mean filter is relatively insensitive to spikes in the data. Refer to Filter Dip Components -> Inline Dip, Crossline Dip, and Confidence attributes description section for a description of how these filters are computed.
- AASPI Documentation, http://mcee.ou.edu/aaspi/documentation/Volumetric_Attributes-filter_dip_components.pdf
- Chopra, S. and K. J. Marfurt, 2007, Seismic attributes for prospect identification and reservoir characterization: SEG Geophysical development series, 11, 27 – 43.
- Marfurt, K., 2006, Robust estimates of 3D reflector dip and azimuth: Geophysics, 71, P29 – P40.
- Marfurt, K., R. L. Kirlin, S. L. Farmer, and M. S. Bahorich, 1998, 3-D seismic attributes using a semblance-based coherency algorithm: Geophysics, 63, 1150 – 1165.