Microlevel

Use the Microlevelling > Microlevel menu option (MICLEV GX) to remove any low-amplitude component of flight line noise still remaining in airborne survey data after tie line levelling.

PGW Microlleveling 3.0 - Microlevel dialog options

X Channel	Select the X channel from your open database. Script Parameters: MICLEV.XCHAN
Y Channel	Select the Y channel from your open database. Script Parameters: MICLEV.YCHAN
Data channel to level	Select the name of the data channel to level. Script Parameters: MICLEV.OCHAN
Decorrugation noise channel	Select the name of the decorrugation noise channel. Script Parameters: MICLEV.NCHAN
Output microlevelled channel	Specify the name of the output microlevelled channel. Script Parameter: MICLEV.MCHAN
Amplitude limit value	Specify the noise amplitude limit value. Script Parameter: MICLEV.AMPLIM
Amplitude limit mode	Select the amplitude limit mode, as zero or clip. Script Parameter: MICLEV.AMPMODE
Naudy filter length (same units as X,Y)	Specify the length of the Naudy low-pass filter (for noise profiles), in the same units as X,Y. Script Parameter: MICLEV.FLTLEN
Naudy filter tolerance	Specify the Naudy filter tolerance. Script Parameter: MICLEV.FLTTOL

Application Notes

The Extract Noise and Microlevel options on the Microlevelling menu, implement a procedure called micro- levelling which removes any low-amplitude component of flight line noise still remaining in airborne survey data after tie line levelling. Micro- levelling 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 loads an existing grid, or grids the input data channel, and then applies a directional high-pass filter(*) perpendicular to the flight line direction to produce a decorrugation noise grid. This grid is then extracted as a new channel in the database. 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 the final microlevelled output channel.

The Extract Noise option calculates default amplitude limit and filter length values for use in Microlevel, but the skilled user may be able to set better values for these parameters based on an inspection of the noise grid. (The micro-levelling process is broken up into two separate GXs 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. The user can estimate the maximum amplitude of the flight line noise, and set the noise amplitude limit value accordingly. Similarly the user 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.

Short Survey Lines: due to the nature of the Naudy filter, Microlevel will apply a low-pass Naudy filter to the amplitude-limited channel only to those survey lines with lengths that are at least four times the Naudy filter width. For lines with a length between one and four times the filter width, the Naudy filter has no affect, so that the microlevel correction channel is equivalent to the amplitude-limited channel. For lines with a length less than the filter width, the filter skips the line. If short lines are present, it is recommended that these be processed separately using a shorter filter width, particularly if the amplitude limiting has truncated or zeroed the noise channel.

There is an option of using either of two kinds of amplitude limiting. 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 datasets in which the noise grid contains a lot of high amplitude geological signals (e.g. shallow basement areas). For datasets in which the noise grid contains mainly flight line noise (e.g. sedimentary basins), the clip mode works better.

There is also an option to adjust the tolerance of the Naudy low-pass profile filter. A Naudy low-pass filter can actually introduce random short-wavelength noise into the data. However the amplitude of this noise is constrained to be less than the tolerance value. The smaller the tolerance the longer it takes for the filter process to run. The default tolerance is equal to 0.0001. The user may set any desired value for the tolerance, either greater or less than the default, depending on what level of noise is considered to be acceptable. A small value is recommended as it will prevent the introduction of chatter by the Naudy filter.

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 the Load Corrections 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.

The decorrugation noise filter is a sixth-order high-pass Butterworth filter with a 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 travelling 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.)