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Airborne EM Compared with Land Based and Waterborne Systems

EM imaging technology is offered by others operating expensive, elaborate airborne geophysical systems (airborne EM). On open farmland or rangeland we may have an advantage while on smaller, goal oriented projects we can often complete entire surveys for less than the mobilisation costs of airborne EM systems. Our waterborne survey solutions offer data robustness, detail, diversity and comprehensiveness not possible with airborne systems. Where a high degree of detail must be mapped, especially in the midst of metallic interference, our system may offer the only survey solution.

Consider the following comparison table:

Property Land Based Towed TEM Airborne EM
Footprint Compact – Almost completely isolated to equipment dimensions at ground level where cultural interference is problematic; wider at depth. Larger due to flying height; also, highly variable as flying height and orientation change continuously. Deleterious coupling with surface metallic features such as fences is maximised.
Course Across Ground Very flexible – can be adjusted on-the-run according to survey findings. Confined by vegetation, fences and other obstacles – this is typically not problematic for groundwater investigations. Ability to change course to test responses of fences and then keep an appropriate separation. Limited to straight or slightly curved lines due to aircraft speed. Survey progresses too fast for comprehensive on-the-fly assessment of data and course adjustment.
Cultural Interference Minimal coupling problem occurs until equipment passes right over or under metal objects. Effective survey has been conducted, in many instances, right beneath powerlines and within 1m of fences. The system cannot survey effectively over buried copper telecom cables and this often presents a problem when survey is restricted to road margins containing such cables. Coupling with surface metallic objects is maximised whenever such objects pass under the equipment footprint which may be several 10’s of metres wide at ground level.
Near Surface Detail DGPS or survey grade RTK DGPS topography, oriented located time lapse photography, magnetics and gamma ray spectrometry are available. Bore can be located and sampled, cultural features can be mapped, and geological mapping can occur en-route. DGPS topography, low-level video, magnetic and gamma ray spectrometry all are typically also available.
Survey Speed 5 to 10 km/hr. Slower speed permits much more stacking per km thus much greater signal to noise for equivalent power. Data can be sampled in much more detail with a moving average filter than for airborne systems. 15 to 150 km/hr. 120km/hr typical. Data acquisition is much faster and thus less detailed but due to higher running costs of airborne systems, not necessarily cheaper.
Mobilisation & Demobilisation Costs Only a lightweight trailer, 4wd and one operator need to mobilize. The survey trailer folds up and is mounted on a small dolly trailer in minutes for high speed road travel. An aircraft, pilot, aircraft maintenance personnel, bulky equipment, and a ground geophysical crew must mobilise.

Additional Matters Relating to Waterborne Acquisition

Waterborne Geo-electric Airborne EM
Response to Tree Canopy and River Incision Survey from the flat, consistent electrical conductivity water surface or the water bed permits acquisition of robust detail data in the substrate immediately beneath the water body bed. Errors in measuring and compensating for height above ground may be extreme over incised watercourses covered with tall dense riparian vegetation.
Depth Resolution Decimetre scale near-bed substrate resolution is possible with submerged streamers and sub-metre resolution is possible with floating streamers. Studies of canal siltation and hazards of saline inflow to rivers require such resolution. Resolution of a couple of metres near the water bed is estimated for conductive substrates. Steep incision of rivers and cultural features present along canals can render depth resolution ineffective.
Additional Parameters Professional sonar and RTK DGPS provide bathymetric data. Waterbody width can be added with laser rangefinders. Located, oriented, time-lapse photography. Mapping of cultural features. Continuous water property sampling (EC, pH, redox etc.). Substrate coring and permeability testing is feasible in a second pass. Located oriented video is typical.
Lateral coverage The waterborne system provides data only under and in very close proximity to the waterbody. An airborne system is suited to acquiring multiple parallel passes of data along river corridors. Such data strongly compliments quality data collected along watercourses themselves with waterborne equipment.