Kerguelen Islands, Base of Port-aux-Français - (PAF)

Location

Instruments

Recording

INTERMAGNET

Key dates

History

The first magnetic observatory operated in the Kerguelen Islands was established on the site of Observatory Bay (49.420 S - 69.889 E) by the German Expedition " D.S.P ". K.Luyken was the observer and made regular absolute observations during years 1902-1903.

In 1957, during the International Geophysical Year (IGY),the Kerguelen observatory (IAGA code KGL) was opened near the Port-aux-Francais base. This base is located on the shallow plain of the Courbet Peninsula, eastward of Kerguelen Island. The magnetic observatory was setup on the western side of a small valley, 300 meters away from the laboratory quarters of the main base.
This observatory comprised two huts. One of these huts housed the " La Cour " magnetograph, and the other one the  absolute pier (49.352 S - 70.215 E). Magnetic observation routine involved continuous recording of the magnetic field variations (H, D, Z), based on the " La Cour " magnetograph and absolute measurements performed with a Chasselon theodolite (D), a Cambridge inclinometer (I), a B.M.Z. and a Q.H.M.

From the beginning, large magnetic anomalies were detected around absolute and variometer shelters, being caused by the basaltic flows which outcrops near the coast line and everywhere around Port-aux-Francais. Thus, it was impossible to avoid this perturbation. The observatory was operated in these conditions until 1966.
In 1967, two photoelectric feedback field magnetometers (H and D components) and an optical pumping magnetometer (Ceasium vapor supplied by Varian) were set up in a new thermostated shelter. A digital " punched tape " recorder was used to sample the H,D and F elements at 1-minute intervals. This system was operating until 1971 and absolute measurements were still made by means of traditional instruments (Chasselon theodolite, Q.H.M., B.M.Z., completed by an ELSEC proton precession magnetometer.
In  1972, a VFO31 triaxial fluxgate variometer was installed at KGL observatory. A low-power device for digital recording
on magnetic tape was used (1973-1987). Since 1980 an EOST D-I fluxgate has replaced the traditional absolute instruments. A proton magnetometer was used for the measurements of total field F. " La Cour " magnetograph was definitively removed in 1981, after more than twenty years of continuous working.
In 1988 a new absolute hut was established for logistic reasons approximatively 3 km east of former KGL observatory site. The variometer shelter moved there as well. The new observatory was named Port-aux-Francais (IAGA code PAF) and references were modified. In 1988 a  new digital data logger was developed on a PC basis.
In september 2006, the VFO magnetometer was replaced by a suspended FGE triaxial magnetometer manufactured by DMI, due to a failure in the Z bias field supply.

Since April 22nd 2008, new 1Hz Acquisitions Systems (M.A.R.Cell 1.0- Magnetic Acquisition and Recording Cell 1.0) were installed. In December 2008, a  new variometer house was installed in order to replace the previous, deteriorated, one.

Absolute Measurements

The absolute pier (49.353 S- 70.262 E) did not change since 1988. Two jumps exist for the 1980-1997 period :

• one in 1981, due to the change of absolute instruments (the EOPG D-I flux replaced the old standards and a new point of measurement was adopted for H,D,Z and F elements. Local gradient involved a modification of the observatory references.
• one in 1988, due to the change of observatory site (the new absolute hut is located approximately 3km to the east of the old KGL observatory site). Local magnetic anomalies explain the observed jump.

Absolute measurements are performed on average every two days using the Declination-Inclination D-I fluxgate (D-I MAG88). Total field determinations are performed every minute with a proton Overhauser magnetometer SM90R. The accuracy of the absolute determinations is estimated 1 nT for H and Z, the equivalent of 1 nT for D, and 0.2 nT for F.

Variometers

The fluxgate variometer (suspended FGE, DMI) and the proton Overhauser magnetometer (SM90R) recording total intensity F are placed in the variometer shelter which is heated at a constant temperature 18 degrees Celsius. The triaxial variometer has a nominal output of 2.5 mV/nT and a dynamic of +/- 2000 nT. The long-term stability of the triaxial variometer is better than 1nT/month.

Accuracy of Data

The adopted baseline values are calculated by spline smoothing. The uncertainty in the adopted baseline values, as well in the final 1 - minute values, is estimated 1 nT for all elements.

Reference of the data

Detailed description of the instrumentation and method of data reduction are given in the annual bulletin of the Bureau Central de Magnetisme Terrestre (B.C.M.T., Paris).