Altitude Correction - Magnetic Data

Use the Altitude Correction option (Geosoft.uxo.gxnet.dll(Geosoft.GX.UXO.UxoCorrectLineElevationMag;Run)*) from the UXO-Marine Mag > Data Corrections menu to adjust magnetic data to a constant altitude above bathymetry.

The tool uses the 1-D Drape dialog (DRAPE1D GX) to drape the data of the specified magnetic field channels to constant altitude above the seabed. This will help to remove the effects of variation in seabed clearance from line-to-line and/or at traverse-tie line intersections. It should be applied prior to any tie line levelling and microlevelling, so that the errors due to height variations are removed beforehand. For more information on 1-D Drape, see the Application Notes below.

Prior to running the Altitude Correction option, review your altitude data, and if required, filter to remove spikes. Spike in altitude data may cause issues due to the larger range.

Correct Magnetic Data to Constant Altitude dialog options

Sensor
Mag channel	Select the magnetic input channel. This is a mandatory field. Script Parameters: UXO_CORRECT_LINE_ELEVATION_MAG.CHANNEL_TO_CORRECT
Altitude channel (m)	Select the altitude channel for the mag sensor, in metres above the sea floor. This is a mandatory field. The altitude channel will be converted to elevation for DRAPE1D GX by adding it to the topography calculated in the next parameter. This will give the elevation of the mag sensor relative to sea level. Script Parameter: UXO_CORRECT_LINE_ELEVATION_MAG.ALTITUDE_CHANNEL
Depth channel (m)	Select the depth channel, in metres below the water (sea surface). If no depth channel is selected, it is the same as if you had a depth channel with a value of zero. The depth will be converted to topography for the DRAPE1D GX, by multiplying the channel by (-1) to make it negative. This is the elevation of the sea bottom relative to the sea level. Script Parameter: UXO_CORRECT_LINE_ELEVATION_MAG.DEPTH_CHANNEL
Output
Corrected Mag channel	Specify the name of the magnetic output channel. This is a mandatory field. Script Parameter: UXO_CORRECT_LINE_ELEVATION_MAG.OUTPUT_CHANNEL_NAME
New observation altitude (m)	Specify the new observation altitude (height of the drape), in metres above sea bottom. This is a mandatory field. The new observation height value is the desired constant altitude above sea bottom. Script Parameter: UXO_CORRECT_LINE_ELEVATION_MAG.NEW_OBSERVATION_ALTITUDE
Number of continuation levels.	Specify the number of continuation levels. This is a mandatory field. Default is 10, but the user may vary this parameter. The more levels used, the greater the accuracy will be, but the computation time will also be longer. The maximum number of levels is 99. Script Parameter: UXO_CORRECT_LINE_ELEVATION_MAG.NUMBER_OF_CONTINUATION_LEVELS
Low pass factor	Specify the Butterworth low-pass factor. This is a mandatory field. Default is 1.0. The DRAPE1D.GX automatically applies low-pass filters to the magnetic data when the New Observation Height value is less than the sensor altitude, i.e., when downward continuations are required. Given the downward continuation distance h, the Butterworth low-pass filter wavelength is computed as 2*pi*h*f in ground units, where f is the user-defined scaling factor. The scaling factor f has a default value of 1.0, and an allowable range of 0.1 to 10.0. This added scaling factor enables the user to adjust the low-pass filter strength according to the magnetic data high-frequency content, resulting from shallow sources and/or noise. Users should be reminded that too strong a low-pass filter would smooth the draped magnetic profiles when the new observation height is below the original sensor height (i.e. where downward continuation is in effect). Where the data are upward continued, the low-pass filter is not applied. Script Parameter: UXO_CORRECT_LINE_ELEVATION_MAG.LOW_PASS_WAVELENGTH
FFT Sampling Parameters
Distance increment (m)	Specify the re-sampling distance increment. If not specified, the nominal data spacing will be used. The distance increment should not be significantly smaller than the average original data sampling interval, otherwise unwanted high frequency noise may be introduced into the output data. If a small distance increment is to be used, the Linear interpolation method should be selected. Script Parameter: UXO_CORRECT_LINE_ELEVATION_MAG.DISTANCE_INCREMENT
Interpolation method	Select the interpolation method: Linear Akima Minimum curvature Default is Linear. Script Parameter: UXO_CORRECT_LINE_ELEVATION_MAG.INTERPOLATION_METHOD

Application Notes

The 1-D Drape dialog (DRAPE1D GX) works by performing height continuations on magnetic profile data to transform them from the original magnetic field on an arbitrary observation surface to the magnetic field on a new surface of specified height. Its most useful application is to remove the effects of variation in bottom clearance from line-to-line and/or at traverse-tie line intersections.

In order to transform from an arbitrary observation surface to one of constant height, 1-D Drape must perform a varying height continuation. There is no simple formula for calculating this exactly, but since the magnetic field is a continuous function of the altitude, it is possible to approximate a varying height continuation by interpolating between values from a set of profiles continued up or down by various constant heights.

The formula used is only correct for planar observation surfaces, but it is approximately correct for approximately planar surfaces, i.e. as long as the rate of change of altitude with position is much less than 1. This approximation is valid for most airborne and sea bottom survey data.

1-D Drape calculates the minimum and maximum distances between the old and the new observation heights, and then calculates a set of upward and/or downward continued profile's at a series of constant heights ranging between the minimum and maximum required continuation distances. For each point along the profile, the difference between the original height value and the new constant observation height is computed to obtain the continuation distance. 1-D Drape then interpolates between the magnetic values at the different continuation levels to give a magnetic field value for the new height.

The height continuation filter described above is stable for upward continuation but can be unstable for downward continuation in the presence of noise. To overcome this problem, 1-D Drape applies a low-pass filter in downward continuation as described in the parameter descriptions below.

1-D Drape assumes 1-D magnetic sources (i.e. sources strike perpendicular to the flightline direction and extend to infinity). This approximation produces little error for continuation heights that are small relative to the anomaly wavelengths, but can break down for large continuation heights in the presence of 2-D and 3-D sources. For the latter case, DRAPE2D.GX (similar process applied to grids) is recommended.

New Channels

When you run 1-D Drape a new Drape_Alt channel is created and assigned the new observation altitude values. These values are the height of the new drape surface above the sea floor.

When the Depth channel is selected, the Sea_Depth and Drape_Depth channels are created and added to the database.:

The Sea_Depth channel is the sensor depth channel + (plus) the Altitude channel.

The Drape_Depth channel is the Sea_Depth channel - (minus) the new observation altitude. This is the distance between the surface and the drape surface.

Drape_Alt + Drape_Depth = Sea_Depth.

Reference