SURVEY EQUIPMENT

The instrumentation was installed in an Aerospatiale AS350B turbine helicopter (Registration N350LE) which was provided by Bulldog Helicopters Ltd. The helicopter flew at an average airspeed of 100 km/h with an EM bird height of approximately 30 m.

Electromagnetic System

Model: DIGHEM^V

Type: Towed bird, symmetric dipole configuration operated at a nominal survey altitude of 30 metres. Coil separation is 8 metres for all coil-pairs except for the 56,000 Hz coil-pair which has a 6.3 metre coil separation.

Coil orientations/frequencies:

coaxial / 900 Hz
coplanar / 900 Hz
coaxial / 5,500 Hz
coplanar / 7,200 Hz
coplanar / 56,000 Hz

Channels recorded: 5 inphase channels

5 quadrature channels
4 monitor channels

Sensitivity:

0.1 ppm at 900 Hz
0.2 ppm at 5,500 Hz
0.2 ppm at 7,200 Hz
0.5 ppm at 56,000 Hz

Sample rate: 10 per second

The electromagnetic system utilizes a multi-coil coaxial/coplanar technique to energize conductors in different directions. The coaxial coils are vertical with their axes in the flight direction. The coplanar coils are horizontal. The secondary fields are sensed simultaneously by means of receiver coils which are maximum coupled to their respective transmitter coils. The system yields an inphase and a quadrature channel from each transmitter-receiver coil-pair.

Radar Altimeter

Manufacturer: Honeywell/Sperry

Type: AA 220
Sensitivity: 1 ft

The radar altimeter measures the vertical distance between the helicopter and the ground. This information is used in the processing algorithm which determines conductor depth.

The altimeter is calibrated by checking the reading when the helicopter lifts-off and reaches the end of the 100 ft EM bird cable.

Analog Recorder

Manufacturer: RMSInstruments
Type: DGR33 dot-matrix graphics recorder
Resolution: 4x4 dots/mm
Speed: 1.5 mm/sec

The analog profiles are recorded on chart paper in the aircraft during the survey. Table 2-1 lists the geophysical data channels and the vertical scale of each profile.

Digital Data Acquisition System

Manufacturer: Picodas
Type: PDAS 1000

The PDAS 1000 has a built-in hard drive for digital data storage, and two internal magnetometer counters. The data are downloaded from the hard drive to a DC2120 cassette at the end of each flight.

The digital data are used to generate several computed parameters. Both measured and computed parameters are plotted as "multi-channel stacked profiles" during data processing. These parameters are shown in Table 2-2. In Table 2-2, the log resistivity scale of 0.06 decade/mm means that the resistivity changes by an order of magnitude in 16.6 mm. The resistivities at 0, 33 and 67 mm up from the bottom of the digital profile are respectively 1,100 and 10,000 ohm-m.

Tracking Camera

Type: Panasonic Video
Model: AG 2400/WVCD132

Fiducial numbers are recorded continuously and are displayed on the margin of each image. This procedure ensures accurate correlation of analog and digital data with respect to visible features on the ground.

Navigation System (RT-DGPS)

Model: Sercel NR106, Real-time differential positioning
Type: SPS(L1 band), 10-channel, C/A code, 1575.42 MHz.
Sensitivity: -132 dBm, 0.5 second update
Accuracy: < 5 metres in differential mode,
± 50 metres in S/A (non differential) mode

The Global Positioning System (GPS) is a line of sight, satellite navigation system which utilizes time-coded signals from at least four of the twenty-four NAVSTAR satellites. In the differential mode, two GPS receivers are used. The base station unit is used as a reference which transmits real-time corrections to the mobile unit in the aircraft, via a UHF radio datalink. The on-board system calculates the flight path of the helicopter while providing real-time guidance. The raw XYZ data are recorded for both receivers, thereby permitting post-survey processing for accuracies of approximately 2 metres.

Although the base station receiver is able to calculate its own latitude and longitude, a higher degree of accuracy can be obtained if the reference unit is established on a known benchmark or triangulation point. The GPS records data relative to the WGS84 ellipsoid, which is the basis of the revised North American Datum (NAD83).

Conversion software is used to transform the WGS84 coordinates to the system displayed on the base maps.

Field Workstation

Manufacturer: Dighem
Model: FWS: V2.41
Type: 80386 based P.C.

A portable PC-based field workstation is used at the survey base to verify data quality and completeness. Flight tapes are dumped to a hard drive to permit the creation of a database. This process allows the field operators to display both the positional (flight path) and geophysical data on a screen or printer.