Wallaby and Joey can be used with extra receiver loops on booms projecting out in front of towing vehicles, carried on foot or towed on mats. These configurations tend to provide semi-independent datasets for modelling verification, can provide deep signal without shallow signal superimposed, and can couple preferentially with predominantly vertical targets such as weathered fracture zones. We typically call the loop in front of a vehicle a ‘front loop’. Any deep survey with considerable budget should have a ‘front loop’ added.
AgTEM electronics can be used with a small 12m x 12m hand laid loop or 300 x 300mm loops for testing & calibration purposes.
We seek co-operation in developing high resolution deep investigation systems utilizing 3D modelling of AgTEM data with numerous static receivers among which the AgTEM Wallaby passes.
For root zone soil depth investigation transient EM can struggle due to timing of returned signal and clients are directed to frequency domain solutions provided by Geonics, DualEM and others. Groundwater Imaging can assist in providing towing, acquisition and data processing solutions using instruments from such suppliers. In the future we hope to provide a root zone EM instrument. See also our ERT alternatives that have some niche advantages.
AgTEM survey fills a niche market with slight overlap with airborne EM. The table below compares the two technologies which vary basically just in scale. We deliberately do not focus on AEM provision because by specializing on AgTEM we can provide a better niche product while others focus on the larger AEM opportunities.
|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||Having no air layer, and small footprint, near surface detail is maximised. Our system typically provides 1m depth resolution near the surface with turnoff of less than 3 uS possible and sampling at 500kHz.||Due to the need to penetrate and measure and compensate for a variable thickness air layer, some near surface resolution is lost. Such losses become extreme when part of the footprint crosses steep terrain covered with trees such as incised rivers and billabongs.|
|Auxiliary Data Options||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.|