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Snowmobile mode surveys by ClearView Geophysics Inc.
- 2. Cesium Magnetics • GEM Systems Base Station (2) • Leapfrogged along grid • Base and rover synced to GPS time • Scintrex NavMag; 1 or 2 sensors • Aluminum sleigh • Rover sensor typically ~1m above surface • Readings at 10x per second • GPS NMEA ported into Navmag console for navigation and positioning • GPS antenna and mag sensor(s) on sleigh • Batteries, console and GPS equipment on snowmobile • Ideally use wide-track snowmobiles (e.g., Bearcat) • Data layback and latency corrected using in-house software <CVNAV.exe> • Also can resample by position
- 4. IMAGEM • Height above ground: 1/2 to 1m • Configuration: Rx and Tx on separate plastic sleighs • Horizontal co-planar geometry • Length: 3.7m • Width: 1.14m Transmitter • Type: Horizontal square equivalent (4.41 m2) • Current: nominally 700 A • # of turns: 1 • Moment: 3100 Am2 Receiver • Type: Horizontal Loop • Radius: Variable • Recording channels: 1-50 (on-time) • 51-200 (off-time) Signal • Waveform Alternating positive and negative half sine wave • On-time ch1 8us to ch50 686us • Off-time ch51 699us to ch200 4.53ms • Windows can be modified in a parameter file • Sounding frequency 30 Hz
- 6. IMAGEM and Cesium Mag
- 7. Ohm-Mapper • Discontinued by Geometrics but still available for rent/sale • 2.5m/5m/10m between Rx’s • Offset Tx for multiple n’s • 1 or 2 seconds per reading
- 8. Example data line with 5 receivers. *.stn is the raw data file viewed in MagMap test.dat is RES2D file exported from MagMap *.bmp is a quick inversion in demo mode... ftp://geom.geometrics.com/pub/mag/Softw are/magmap_latest.exe
- 9. PROMIS • 20m/50m/100m/200m separation cables • ~3 seconds per full sweep 110 Hz through 56 kHz
- 10. Geonics EM34-3 XL • XL version has increased Tx power and larger coil • S/N improved by 10x at 40-m separation • S/N improved by 4x at 10-m and 20-m separation • Can use tilt-meters but for vertical dipoles mode in relatively flat terrain with long sleighs deemed not necessary • Penetration ~0.75x and 1.5x horizontal and vertical dipole separation respectivley
- 11. Geonics EM31 & EM61 • EM31 has penetration of ~6-m vertical dipoles; EM61 ~2-3 m penetration • EM31 Connected to DAP logger that records EM31 I/P & O/P plus GPS positioning at 10x per second • However, each EM31 reading takes ~1 second to acquire which can be corrected • EM61 has single frequency 9.8 kHz • Other options such as GEM-2 with multiple frequencies, shorter configuration • See Geonics paper on why EM31 single freq. (TN-30)