Altitude Correction - Gradient Data

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

Correct Gradient Sensor Data to Constant Altitude dialog options

Sensor Configuration table	Select your sensor configuration table (*.csv) file. See the Gradient Sensor Offset Correction Application Notes for information on the format of the Sensor Configuration Table (CSV) file. Script Parameters: UXO_CORRECT_LINE_ELEVATION.SENSOR_CONFIGURATION_TABLE
Sensor label position	Select the sensor label position using the dropdown list. The default is Suffix. Script Parameters: UXO_CORRECT_LINE_ELEVATION.SENSOR_LABEL_POSITION
Sensor table	Select the four input channels (X Channel, Y Channel, Z Channel and Sensor Data Channel) for each sensor. Start typing in the "Sensor Label" text box to match and auto-select channel names by prefix/suffix (depending on the "Label position" dropdown setting), or you can select the input channels from the dropdown lists. Script Parameter: UXO_CORRECT_LINE_ELEVATION.Label_Z UXO_CORRECT_LINE_ELEVATION.Label_Depth UXO_CORRECT_LINE_ELEVATION.Label_Field
Output channel name	Specify the name of the output channel. This is a mandatory field. Script Parameter: UXO_CORRECT_LINE_ELEVATION.OUTPUT_CHANNEL_NAME_LEFT
New platform observation altitude (m)	Specify the new observation altitude (height of the drape) in metres above sea bottom. This is a mandatory field. Script Parameter: UXO_CORRECT_LINE_ELEVATION.NEW_OBSERVATION_ALTITUDE
Number of continuation levels.	Specify the number of continuation levels. This is a mandatory field. Default is 10, but you 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.NUMBER_OF_CONTINUATION_LEVELS
Low pass wavelength	Specify the Butterworth low-pass wavelength. This is a mandatory field. Default is 1.0, and an allowable range of 0.1 to 10.0. See Application Notes below. Script Parameter: UXO_CORRECT_LINE_ELEVATION.LOW_PASS_WAVELENGTH
FFT Sampling Parameters
Distance increment	Specify the re-sampling distance increment. If not specified, the nominal data spacing will be used, in metres. 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.DISTANCE_INCREMENT
Interpolation method	Select the interpolation method: Linear Akima Minimum curvature Default is Linear. Script Parameter: UXO_CORRECT_LINE_ELEVATION.INTERPOLATION_METHOD

Application Notes

The Survey Elevation, the elevation of the sensor relative to the sea level is calculated by multiplying the Depth the channel by (-1) to make it negative.

The Topography Elevation, the elevation of the sea bottom relative to the sea level, is calculated by adding the Altitude and Depth channels and multiplying by (-1) to make it negative.

This tool 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, you 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.

This tool 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. It 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. A low-pass filter is automatically applied 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 scaling factor enables you to adjust the low-pass filter strength according to the magnetic data high-frequency content, resulting from shallow sources and/or noise. Note a too strong of a low-pass filter (i.e. a high scale factor) would excessively smooth the draped magnetic profiles. Where the data are upward continued, the low-pass filter is not applied.

This corrections assumes 1-D magnetic sources (i.e. sources strike perpendicular to the flight-line 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.

Reference