As of today, airborne time-domain EM surveying is the most effective method of ore exploration at the stage of a detailed survey (scale from 1:25000 to 1:5000).
Essence of the method
The time-domain EM survey method is based on measurements of decay of magnetic field of eddy currents in a conductive environment. These currents result from the primary EM field (On-time mode) and from it being turned off (Off-time mode).The primary EM field is produced by electrical pulses sent through a horizontal, multi-turn closed loop towed by the aircraft on a cable.
The responses are measured with receiver induction coils of different orientation.
- The system design provides minimal clearance of a system transmitter from the ground surface, which results in decreasing of the exitation zone and improving of survey spatial resolution.
- Control of relative position of transmitter and receiver allows for removal of the primary field of influence from EM measurements.
- Low base frequency and high dipole moment provide deep penetration of the induced signals
- Measure of all three components of the secondary field provides more accurate mapping of conductive objects.
- Interpretable B-field data extends the range of conductivities detectable with EM survey targets.
- High-accurate airborne magnetometer is a required part of the system.
- The GPS Navstar-Glonass satellite constellation is used for aircraft positioning.
- Depending on the project task and geological-geophysical situation, the EM system can be integrated with a gamma-ray spectrometer.
- AGP employs the HELITEM (Fugro Airborne Survey) or similar system.
Airborne time-domain EM survey project tasks:
- Exploration for sulphide copper-nickel deposits;Exploration for polymetallic lead-zinc ore;
- Exploration for kimberlite and classification of magnetic anomalies of pipe type;
- Research into inner structure of ore control tectonic zones and tracking of ore-localizing fractures
- laterally and in depth;
- Detailed research into upper part of geological sequence.
HELITEM EM system (Fugro Airborne Surveys, Canada)
- dipole moment – 600×103 – 1.2×106 Am2;
- tow cable length – 70 m;
- signal – bipolar half-sine pulse;
- base frequency – 25 Hz;
- pulse width – 4 μs;
- coils orientation – X, Y, Z;
- measurement channels – 30;
- measurable fi eld – B and dB/dt;
- Digital recording – all raw data channels.
Airborne magnetometer (Fugro Airborne Surveys, Canada)
Airborne magnetic measurements are performed by a cesium magnetometer with the following basic parameters:
- Resolution – 0.001 nT;
- Measurements rate – 20 Hz – 1000 Hz;
- Tolerance to magnetic field gradient– up to 20000 nT/m;
- Measurements range – 17000 nT – 100000 nT.
Magnetometer sensor – (CS-3 or similar) is installed in towed-bird.
Airborne gamma-ray spectrometer (Radiation Solutions, Canada)
- Number of channels – 512 or 1024;
- Detectors – NaI (Tl) crystal detectors with total volume of up to 48 L;
- Energy resolution on 0.662 MeV (Cs-137) line better than 10%;
- Gamma-radiation spectrum measurement range 0.38 – 5.0 MeV;
- Gamma-radiation spectrum measurement rate – 1 sec;
- Automatic stabilization of the spectrometer energy scale on Th line.
Data acquisition software and positioning
The system collects and synchronizes data from EM and magnetometer channels, and satellite navigation data from GPS-CLONASS systems. As well, it provides flight steering and survey line tracking with special indicator installed on pilot’s cockpit panel.
Using this program in combination with GPS it provides positioning accuracy meeting the requirements for a geophysical survey up to a scale of 1:5000.
The use of base magnetometer stations and base GPS stations is provided for differential correction of navigational solutions.