transient electromagnetics (tem) - illinois water … · why geophysics? cons ... transient...
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Transient Electromagnetics (TEM)
The Role of Geophysics in Groundwater Modeling
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Pros Minimally invasive Large areal coverage Relatively low cost Fill information gap between wells
Why Geophysics?
Cons Indirect – correlation required Limited resolution Expertise required for interpretation
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Why Geophysics?
Geophysics is most powerful when used in combination with conventional measurements.
BH - 2 BH - 3 BH - 1
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40 m (131 ft)
40 m
10 m (32 ft)
10 m
Transmitter loop
Receiver 2
Receiver 1
WalkTEM
External power source
1. Ground-based Profiling Tool - WalkTEM
Moderate penetration: ≤ 150 m (492 ft)
Moderate near surface resolution
Moderate lateral resolution
20 soundings per day – ~1 km2
Transmitter
Receiver 2
Receiver 1
WalkTEM
Transient Electromagnetics - TEM
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Deep penetration: ≤ 600 m (1968 ft)
High resolution
Large ground coverage at relatively short time
Dense data coverage
Flight line
map
2. Airborne Profiling Tool - Helicopter TEM (HTEM)
Transient Electromagnetics - TEM
Light-weight (≤1,800 lb) technology allows instruments to be towed from a helicopter. i.e. non-intrusive.
Map based on 518 boreholes Map based on 1,400 HTEM soundings
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Information Extracted from TEM Data
High Resistivity – Red Sand and gravels (aquifer where saturated)
Low Resistivity – Blue Clay and silts (confining unit)
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Case Study 1 3D Geological Modelling of Buried-valley System
Høyer et. al., 2015
Conceptual model of the study area
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Case Study 1 Buried-valley System
Resistivity slice 5 m asl
Depth to the top of Paleogene clay
Extent of interpreted buried valleys
2D resistivity profile with modelled valley surfaces
2D lithofacies model
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Case Study 2 Mapping of the Base of Aquifer in Areas of W. Nebraska
HTEM
Abraham et. al., 2011
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Case Study 2 Base of Aquifer
Base of aquifer “picks” from AEM resistivity
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Case Study 2 Base of Aquifer in Areas of W. Nebraska
Cooperative Hydrology Study (COHYST)
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Difference in aquifer thickness between the new base of aquifer and the COHYST base of aquifer (Luckey and Cannia, 2006)
Saturated thickness calculated from the COHYST water table using (A) the new base of aquifer, and (B) the COHYST base of aquifer (Luckey and Cannia, 2006)
Additional 458 GI (34%) of water in
storage was identified after AEM survey (Abraham et. al., 2011)
Abraham et. al., 2011
Case Study 2 Aquifer Thickness & Saturated Thickness
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Quaternary deposits above bedrock
3-D HTEM Results
Cretaceous bedrock topography beneath Quaternary deposits Cretaceous bedrock units beneath the Quaternary system and principal aquifer
Abraham et. al., 2014
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BH-1 BH-3 BH-2
Geophysical data is compatible and transferable as surfaces for groundwater models, research, or studies.
Geophysical methods have potential to improve hydrological framework while reducing
investigative costs. Geophysics provides expanded spatiotemporal information. i.e. Fill gaps between
boreholes.
Conclusion
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Valley Generation
Relative age of different valley generations. Oldest valleys are shown with darkest colors. The valleys have been divided into 8 different generations.
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Valley Interpretation
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Case Study 2 Saturated Thickness
Negative net change of 406 GI (32%) of water in storage was identified after AEM survey
(Abraham et. al., 2011)
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1. Water and Environmental Applications - Groundwater investigation, saltwater intrusion, waste-site characterization, plume delineation, etc. 2. Geological Mapping - Lithologic identification, mapping thicknesses of strata and topography of the bedrock surface, mapping structures and paleochannels etc. 3. Mining Applications - Mineral exploration, aggregate deposits mapping, overburden thickness mapping, etc. 4. Geotechnical Engineering - Slope stability studies, overburden thickness mapping, mine site infrastructure, identification of fractures and faults, etc.
Applications
5. Oil and Gas - Shallow gas reservoirs, characterize deeper buried pathways e.g. paleochannels, incised or glacial fluvial valleys, etc.
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Applications