Further by downloading this data the user acknowledges that they agree with the NERC data policy (), and the following conditions:

1. To cite the data in any publication as follows:

https://doi.org/10.5270/esa-8ffoo3e - PolarGap: "Filling the GOCE polar gap in Antarctica and ASIRAS flight around South Pole"

2. The user recognizes the limitations of data. Use of the data is at the users' own risk, and there is no warranty as to the quality or accuracy of any data, or the fitness of the data for your intended use. The data are not necessarily fully quality assured and cannot be expected to be free from measurement uncertainty, systematic biases, or errors of interpretation or analysis, and may include inaccuracies in error margins quoted with the data.

Methodology:

The raw aeromagnetic data was initially corrected for DC offsets due to aircraft procedures, such as pumping fuel from so called 'tip tanks'. Most of these corrections were in turns, but logistical considerations meant such corrections on lines were unavoidable. Due to the unusual radial survey design normal compensation for aircraft dynamics was not possible. However, visual comparison between recorded fluxgate and total field data on the aircraft suggested most dynamic noise had a frequency of <15 seconds (~900m). The tip corrected data was therefore filtered with a 15 second low pass filter to remove dynamic noise prior to decimation from 10Hz to 1Hz to enhance the efficiency of data processing. The total field magnetic data was corrected for the core field using the 2015 IGRF, implemented in the Geosoft software package.

The base-station correction for diurnal variations was derived from multiple base-station observations. Base station data at the PolarGAP field camps (Thiel Mountains and FD83) was collected using total-field sensors, with additional fluxgate magnetometer data from the Polar Experiment Network for Geospace Upper atmosphere Investigations (PENGUIn) at South Pole and other remote field sites. All base station data was converted into total field values, and each station corrected to have a zero mean across the survey period. The theoretical base station value at the aircraft was calculated using an inverse distance weighting scheme to merge base station observations (Shenwary et al 2011). This incorporates the base station corrections from all available base stations weighted by their distance to the survey aircraft. This technique provided a smooth transition between different base stations, and ensures the most 'local' base station dominates the correction. As short wavelength diurnal variations observed at a base station are often not fully representative of those occurring at large distances from the base station, the base station corrections were filtered with either a 60 or 30 minute low-pass filter. The base station data is also provided for reference.

After base station correction preliminary levelling of the 30 minute base station corrected magnetic data was carried out. This included minimising internal crossovers and discrepancies with other surveys in the region. In general the preferred levelling method removed a 0, or 1st order trend revealed by errors at a number of intersections. Where residual line to line discrepancies remained, more complex spline levelling was applied. Further improvement of the dataset by more detailed levelling may be possible.

The aeromagnetic data set includes the following channels:

Line: Sequential survey flight number from P03 to P39: decimals denote line breaks.

Lon: Longitude DDD.DDDDDDDD

Lat: Latitude DD.DDDDDDDD

Elev: Elevation on the WGS1984 ellipsoid

Date: Date YYYY/MM/DD

Time: Time (UTC) HH:MM:SS.SS

MagR: Raw total field magnetic data (nT)

MagC: Pseudo-compensated total field magnetic data (nT)

TCorr: Tip correction (nT)

RefField: IGRF correction (nT)

MagRTC: Mag after IGRF: compensation and tip correction (nT)

Bcorr_60min: IDW base station correction with 60 minute mean filter (nT)

Bcorr_30min: IDW base station correction with 30 minute mean filter (nT)

MagB60RTC: Final mag after 60 min base station correction (nT)

MagB30RTC: Final mag after 30 min base station correction (nT)

MagL_B30RTC: Magnetic anomaly after preliminary levelling (nT)

The magnetic base station data set includes the following channels:

Line: PolarGAP flight number

TimeUTC: Time UTC (HH:MM:SS.SS)

UTCDate: Date (YYYY/MM/DD)

Longitude_decimal_degrees: Survey aircraft longitude (DDD.DDDDDDDDD)

Latitude_decimal_degrees: Survey aircraft Latitude (DD.DDDDDDDDD)

x: Polar stereographic projected aircraft x position (true scale -71) (m)

y: Polar stereographic projected airc...(23)

Data collection:

Data was collected using the BAS aerogeophysicaly equipped twin otter VP-FBL. Magnetic data was collected using wing tip mounted Scintrex CS-3 High Sensitivity Cs magnetometers. Only data from the magnetometer with the lowest noise characteristics is reported here. Data was recorded at 10 Hz on a PicoEnvirotec logging system. Additional base station information from total-field magnitomiters at field camps and fluxgate magnitomiters from the Polar Experiment Network for Geospace Upper atmosphere Investigations (PENGUIn) network were used.

Data quality:

After the basic processing presented here the standard deviation of the crossover errors was 52 nT, after preliminary levelling the crossover error was 25 nT.

Data resolution:

The radial design of the survey makes defining a specific resolution problematic. Line spacing at the survey edges was 90 km, reducing towards South Pole. The provided data is sampled at 1Hz, along track which equates to a ~60m along track sample spacing. The true usable resolution of this dataset is proportional to the depth to source, which averages to 2900 m across the survey.