Transient Electromagnetics (TEM)

The Role of Geophysics in Groundwater Modeling

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

Why Geophysics?

Geophysics is most powerful when used in combination with conventional measurements.

BH - 2 BH - 3 BH - 1

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

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

Information Extracted from TEM Data

High Resistivity – Red Sand and gravels (aquifer where saturated)

Low Resistivity – Blue Clay and silts (confining unit)

Case Study 1 3D Geological Modelling of Buried-valley System

Høyer et. al., 2015

Conceptual model of the study area

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

Case Study 2 Mapping of the Base of Aquifer in Areas of W. Nebraska

HTEM

Abraham et. al., 2011

Case Study 2 Base of Aquifer

Base of aquifer “picks” from AEM resistivity

Case Study 2 Base of Aquifer in Areas of W. Nebraska

Cooperative Hydrology Study (COHYST)

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

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

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

Valley Generation

Relative age of different valley generations. Oldest valleys are shown with darkest colors. The valleys have been divided into 8 different generations.

Valley Interpretation

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)

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.

Applications