Extract Noise
Use the Extract Noise menu option (DECORR GX) to calculate the default amplitude limit and filter length values for use in Microlevelling.
PGW Microlleveling 3.0 - Extract Noise dialog options
Application Notes
The Extract Noise and Microlevel menu options implement a procedure called microlevelling which removes any low-amplitude component of flight line noise still remaining in airborne survey data after tie line levelling. Microlevelling calculates a correction channel and adds it to the profile database. This correction is subtracted from the original data to give a set of levelled profiles, from which a final levelled grid may then be generated. Microlevelling has the advantage over standard methods of decorrugation that it better distinguishes flight line noise from geological signal, and thus can remove the noise without causing a loss in resolution of the data.
To microlevel data, first run Extract Noise, then Microlevel.Extract Noise offers two options for the grid of the channel to be microlevelled. If a grid prepared from this channel already exists,it may be specified, and when prompted to overwrite, the user should answer no. If the user wishes to prepare a new grid of the channel to be microlevelled, the Minimum Curvature Gridding algorithm is applied. The advanced button provides access to the standard minimum curvature gridding parameters. Once the gridding is completed, Extract Noise applies a directional high-pass filter (see below) perpendicular to the flight line direction, in order to produce a decorrugation noise grid.
(The default grid cell size is 1/5 of the line spacing. The user may specify a different cell size if desired. A smaller cell size will give a more accurate result, but a larger cell size will make the dialog run faster and use less disk space.) The noise grid is then extracted as a new channel in the database (default name is "dcor_noise"). This channel contains the line level drift component of the data, but it also contains some residual high-frequency components of the geological signal.
Microlevel applies amplitude limiting and low-pass filtering to the noise channel in order to remove this residual geological signal and leave only the component of line level drift, which is then subtracted from the original data to produce a levelled output channel (defult name is "miclev").
Extract Noise calculates default amplitude limit and filter length values for use in Microlevelling, but the skilled user may be able to set better values for these parameters based on an inspection of the noise grid. (The microlevelling process is broken up into two separate dialogs in order to enable you to do this.) Flight line noise should appear in the decorrugation noise grid as long stripes in the flight line direction, whereas geological anomalies should appear as small spots and cross-cutting lineaments, generally with a higher amplitude than the flight line noise, but with a shorter wavelength in the flight line direction. You can estimate the maximum amplitude of the flight line noise, and set the noise amplitude limit value accordingly. Similarly you can estimate the minimum wavelength of the level drift along the flight lines, and set the low-pass Naudy filter width to half this wavelength. The defaults are to set the amplitude limit equal to the standard deviation of the noise grid, and to set the filter width equal to five times the flight line spacing.
There are two types of amplitude limiting modes that can be applied, clip and zero. In clip mode any value outside the limit is set equal to the limit value. In zero mode any value outside the limit is set equal to zero. The clip mode makes more sense intuitively, but it has been found in practice that the zero mode may reject geologic signal better, depending on the particular data set. As a rule the zero mode works better on data sets in which the noise grid contains a lot of high-amplitude geological signals (e.g. shallow basement areas). For data sets in which the noise grid contains mainly flight line noise (e.g. sedimentary basins), the clip mode works better.
Microlevelling applies a level correction to the traverse lines only. If it is desired to grid the tie lines together with the microlevelled traverse lines, then it may be necessary to also apply a level correction to the tie lines so that their values agree with the microlevelled traverse lines at the intersections. This may be done as follows:
- Copy the tie line values to the microlevelled channel.
- Use the Intersections menu option to find cross-difference values for the microlevelled data.
- Use Load Correction menu option to load these cross-difference values to the tie lines.
- Apply the Full Level menu option to the tie lines. The output will be a set of tie lines that matches the microlevelled traverse lines at all intersections.
- Copy the microlevelled traverse line values into the same channel as the corrected tie line values.
Decorrugation Filter
The decorrugation noise filter is a sixth-order high-pass Butterworth filter with a default cutoff wavelength of four times the flight line spacing, combined with a directional filter. The directional filter coefficient as a function of angle is F=(sin(a))^2, where "a" is the angle between the direction of propagation of a wave and the flight line direction, i.e. F=0 for a wave traveling along the flight lines, and F=1 for a wave travelling perpendicular to them. (Note this is the exact opposite of what is usually called a decorrugation filter, since the intention here is to pass the noise only, rather than reject it.)
The default cutoff wavelength (4 * line spacing) gives good results if the data is already fairly well levelled to start with. In cases where many lines are badly mis-levelled, it may be necessary to set a longer cutoff wavelength, at the risk of removing more geological signal.
See Also:
Got a question? Visit the Seequent forums or Seequent support
© 2023 Seequent, The Bentley Subsurface Company
Privacy | Terms of Use