Base Station Correction

Use the Base Station Correction option (geogxnet.dll(Geosoft.GX.UAVSurvey.BaseStationCorrection;Run)*) from the UAV Merge Sorties > Pre-processing menu to perform the base station correction on raw surveyed magnetic data.

Base Station Correction dialog options

Survey database
Date channel	If this field is not provided, then the date is ignored and only the time stamp is cross-referenced. If a date channel is present in the database, then the combination of date and time is cross-referenced. Script Parameter: PLAN_UAV_SURVEY.DATE_CHANNEL
Time channel	Select the time channel in the GDB file to cross-reference with the base station data file. It is imperative to ensure that the mag base file and the database do overlap in time. Mag base correction will not be applied to readings with time stamps that are not covered in the base station file. Script Parameter: PLAN_UAV_SURVEY.TIME_CHANNEL
Input data channel	Select the mag channel to correct. Script Parameter: PLAN_UAV_SURVEY.INPUT_CHANNEL
Output corrected channel	Specify the corrected mag channel. Script Parameter: PLAN_UAV_SURVEY.OUTPUT_CHANNEL
Base station
Base station file	Select the base station file. This is an ASCII dump of base station magnetic field data coinciding in time with the surveyed data. The file must have a header containing the label for each field followed by the data. It must have the same number of fields on each record. The data must be tabulated with at the very least the time stamp and the base station reading. The time stamp in the file is used for cross-referencing the GDB data. Script Parameter: PLAN_UAV_SURVEY.BASE_STATION_CSV
Select label line	The first few lines of the data are displayed in this text box, as an aid to enable the user: • Ensure it is the correct file • Verify the date/time coverage to be coincident with the surveyed data • Identify if all necessary fields are present • Identify the date format Script Parameter: PLAN_UAV_SURVEY.BASE_STATION_LABEL_LINE
Date field	If the date field in the previous dialog has been left blank, this entry will not appear. Otherwise this field is cross-referenced with the database date channel. Script Parameter: PLAN_UAV_SURVEY.DATE_FIELD
Date format	Select the date format in the base station file. Script Parameter: PLAN_UAV_SURVEY.DATE_FORMAT
Time field	Select the time field in the base station file. This field is cross-referenced with the time channel in the database. Script Parameter: PLAN_UAV_SURVEY.TIME_FIELD
Data field	Select the field containing the base station data. Script Parameter: PLAN_UAV_SURVEY.DATA_FIELD

Application Notes

The magnetometers used in geophysical surveys measure minute variations in the earth’s magnetic field. The Earth’s magnetic field intensity varies naturally as the Earth rotates daily through the ionosphere (solar wind) of the sun. This effect can be exacerbated by events such as solar flares or geomagnetic storms. Magnetic storms (from sun spots) can cause jumps in the magnetic field by as much as 100 nT in a few minutes. This can greatly affect field magnetic survey data and may give erroneous results. This GX is used to correct magnetic survey data for such diurnal variations in the earth’s magnetic field.

A typical magnetic survey would involve placing a "base station" in a "magnetically clean" area which would mean it would be geologically quiet, away from cultural effects, etc. It is imperative that the site selected for the base station be absolutely free of disturbance while the instrument is collecting. Any unnatural variations in the data will compromise the accuracy of corrected values taken from the field magnetometer. The base station should be proximal to the field area (within 10 miles) and the same location should be used every day that magnetic data are collected. In the case of large sites, it is permissible to have multiple base station locations based on convenience for the current survey area. It is recommended that the base station sensor be placed at least 4 feet above the ground surface to minimize cultural and terrain induced interferences in the base station measurements.

The base station would be set to run in continuous mode throughout the duration of the survey. To ensure adequate sampling, a minimum rate of collection of 1 reading per minute is required by CEHNC for base station magnetometers, as specified in DID OE-005-05.01.

The clocks for the field and base magnetometers must be synchronized before data collection commences. The time stamps in both sets of data allow the field survey data to be normalized for diurnal drift. Use of a base station will also identify periods of magnetic storms, in which case, field data should be discarded.

Most modern magnetometers can be linked together and therefore the base station correction can be made immediately by using software that has been built into the instruments. However, older magnetometers do not have this capability and therefore this GX may be used in order to make the correction.

Base station data should be plotted in a profile and evaluated for the presence of drop-outs, spikes, magnetic storms, and/or any other features that will degrade the quality of the data. Data should be examined for short-term high amplitude anomalies that could be a result of someone approaching the base station in a vehicle, or otherwise bringing ferrous metal near the sensor. Applying a base station "correction" which contains short-term high amplitude noise may generate false positives in the collected field data.

Once the data has been deemed acceptable, a diurnal drift correction may be applied to the field magnetometer data.

By the nature of its smooth variation, the base station data is collected at a much lower frequency than the surveyed data. This data must be splined to match the data rate of the survey data. The base station data is splined using the non-linear filter. This filter has the advantage of not introducing wavelengths that are not already present in the data.

Although data can be filtered by various means to minimize or eliminate diurnal drift, the use of a base station magnetometer is preferable.