Geophysical Helicopter Survey of the WUDA Coal Fire Area

During the period from August, 8th to September, 6th 2004 AGRS (China Aero Geophysical Survey and Remote Sensing Centre for Land and Resources) has flown an aerogeophysical survey with electromagnetic (EM) and magnetic systems in the region of WUDA. The survey area which had a size of approximately 7.5 km in E-W direction and 15 km in N-S direction included all known coal fires in the WUDA area.

View from helicopter into the survey area, looking South

Base of the survey was the airport of Wuhai about 25 km north of the survey area.

Airport of Wuhai - the base for the aerogeophysical survey

It was the aim of this survey to prove that the exact location of the coal fires can be identified by measuring the electric resistivity of the soil up to depth of about 100 m and the total magnetic field.

The whole area has been covered with parallel E-W flight lines which have a line spacing of 50 m. In addition, N-S cross-profiles have been flown with a spacing of 250 m.

A-Star helicopter at take-off

The following aerogeophysical systems had been installed in an A-Star 350 B2 helicopter:

• IMPULSE 2 electromagnetic system
  This system has been developed by Aeroquest, Canada. A towed sonda ("bird") with a length of 7.62 m, a diameter of 0.76 m and a weight of 250 kg contains two different coil systems each with three different frequencies between 870 Hz and 23.250 Hz. The electromagnetic fields transmitted by transmitter coils in the front of the bird produce eddy currents in the underground. The electric resistivity of the soil can be calculated from the electromagnetic fields of these currents, measured by very sensitive receiver coils at the back of the bird.
• The magnetometer sensor
  a CS-3 Cesium sensor from Scintrex, Canada, was installed at the rear end of the bird. The output signal of the sensor, the so called Larmor frequency, is proportional to the field strength of the total magnetic field and is measured by a frequency counter.

IMPULSE 2 electromagnetic system during take-off

The navigation system, developed by AGRS, has installed an Ashtech GPS/GLONASS receiver, which uses the American GPS and the Russian GLONASS satellites simultaneously. The horizontal position error of this receiver should be less than 10 m provided that enough Russian satellites are available.

Keeping in mind that this was the first EM helicopter survey for a Chinese company AGRS has done a really good job in preparing and carrying out the survey. Except the noise caused by a lot of power lines in the survey area and the “sferics” activities which were really strong at times the quality of the EM and magnetometer data was good. Preliminary maps of the apparent resistivity and the magnetic values have been produced jointly by BGR and AGRS already in the field at the end of the survey. The final data evaluation will start at the beginning of 2005.

In the area: power-lines and mining activities interfere the electromagnetic data