Tools and Techniques: Active-Source Electromagnetic Methods

Fitterman, David V.

doi:10.1016/b978-0-444-53802-4.00193-7

Cited by 18 publications

(17 citation statements)

References 68 publications

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“…(3) The operating principles of the inductive time-domain electromagnetic (TEM) technique are described in Nabighian and Macnae (1991) and Fitterman (2015). The main distinction between TEM and rigid-boom EMI, which operates in the frequency-domain, is that the former involves subsurface energization by a transient step current rather than a time-harmonic current.…”

Section: Methodsmentioning

confidence: 99%

Multi-Layered High Permeability Conduits Connecting Onshore and Offshore Coastal Aquifers

et al. 2020

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Groundwater resources in coastal regions are facing enormous pressure caused by population growth and climate change. Few studies have investigated whether offshore freshened groundwater systems are connected with terrestrial aquifers recharged by meteoric water, or paleo-groundwater systems that are no longer associated with terrestrial aquifers. Distinguishing between the two has important implications for potential extraction to alleviate water stress for many coastal communities, yet very little is known about these connections, mainly because it is difficult to acquire continuous subsurface information across the coastal transition zone. This study presents a first attempt to bridge this gap by combining three complementary near-surface electromagnetic methods to image groundwater pathways within braided alluvial gravels along the Canterbury coast, South Island, New Zealand. We show that collocated electromagnetic induction, ground penetrating radar, and transient electromagnetic measurements, which are sensitive to electrical contrasts between fresh (low conductivity) and saline (high conductivity) groundwater, adequately characterize hydrogeologic variations beneath a mixed sand gravel beach in close proximity to the Ashburton River mouth. The combined measurements-providing information at three different depths of investigation and resolution-show several conductive zones that are correlated with spatial variations in subsurface hydrogeology. We interpret the conductive zones as high permeability conduits corresponding to lenses of well-sorted gravels and secondary channel fill deposits within the braided river deposit architecture. The geophysical surveys provide the basis for a discharge model that fits our observations, namely that there is evidence of a multilayered system focusing groundwater flow through stacked high permeability gravel layers analogous to a subterranean river network. Coincident geophysical surveys in a region further offshore indicate the presence of a large, newly discovered freshened groundwater system, suggesting that the offshore system in the Canterbury Bight is connected with the terrestrial aquifer system.

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Section: Methodsmentioning

confidence: 99%

Multi-Layered High Permeability Conduits Connecting Onshore and Offshore Coastal Aquifers

et al. 2020

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“…4d). In a homogeneous half-space, the RX-voltage decay curve follows a predictable shape (Fitterman, 2015). Departures from this shape indicate the presence of heterogeneity, such as layering or lateral variations in subsurface electrical conductivity.…”

Section: Time-domain Electromagneticsmentioning

confidence: 96%

Coupling hydrogeophysics with hydrodynamic modelling to infer subsurface hydraulic architecture of an alluvial floodplain

Martin

Everett

Knappett

2020

Near Surface Geophysics

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This paper underscores the importance of spatially dense geophysical data sets for making informed decisions in water management strategies. Such decisions may require understanding how site-specific subsurface architecture -especially hydraulic connectivity -impacts the response of a shallow aquifer to anthropogenic hydrologic disturbances (e.g. over-pumping of a shallow aquifer). At a 0.2-km2 alluvial floodplain site characterized by thick clay over fine sand to gravel and shale bedrock in the subtropical, sub-humid belt of the Gulf Coast of the United States, we image an asymmetrically shaped, compartmentalized, sand-dominated channel-belt using electrical resistivity tomography and 31 time-domain electromagnetic soundings probing to depths of ∼40 m and ∼90 m, respectively. Lithological interpretation and a hydrological model are developed based on the geophysical data and nearby sediment cores, where the resistivity of the groundwater is 9.1 Ωm. In a modelling scenario wherein the compartmentalized sand channel-belt starts out dry (i.e. an over-pumped shallow aquifer), we simulate 26 weeks of infiltration due to flooding of the surface. Preferential filling of the channel-belt occurs through its sides rather than from above, generating a new understanding of the hydraulic connectivity around and into asymmetrically shaped, sand-dominated channel-belts. This insight can inform decisions about the optimal placement of shallow water wells in a heterogeneous alluvial floodplain aquifer system and also highlights the dangers of over-pumping.

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“…1b). The operating principles of the inductive TEM technique are described in Nabighian and Macnae (1991) and Fitterman (2015). The survey parameters included four turns, a 10 × 10 m 2 square TX loop and a TX current output of 1 A.…”

Section: Near-surface Geophysicsmentioning

confidence: 99%

Groundwater erosion of coastal gullies along the Canterbury coast (New Zealand): a rapid and episodic process controlled by rainfall intensity and substrate variability

et al. 2021

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Abstract. Gully formation has been associated to groundwater seepage in unconsolidated sand- to gravel-sized sediments. Our understanding of gully evolution by groundwater seepage mostly relies on experiments and numerical simulations, and these rarely take into consideration contrasts in lithology and permeability. In addition, process-based observations and detailed instrumental analyses are rare. As a result, we have a poor understanding of the temporal scale of gully formation by groundwater seepage and the influence of geological heterogeneity on their formation. This is particularly the case for coastal gullies, where the role of groundwater in their formation and evolution has rarely been assessed. We address these knowledge gaps along the Canterbury coast of the South Island (New Zealand) by integrating field observations, luminescence dating, multi-temporal unoccupied aerial vehicle and satellite data, time domain electromagnetic data and slope stability modelling. We show that gully formation is a key process shaping the sandy gravel cliffs of the Canterbury coastline. It is an episodic process associated to groundwater flow that occurs once every 227 d on average, when rainfall intensities exceed 40 mm d−1. The majority of the gullies in a study area southeast (SE) of Ashburton have undergone erosion, predominantly by elongation, during the last 11 years, with the most recent episode occurring 3 years ago. Gullies longer than 200 m are relict features formed by higher groundwater flow and surface erosion > 2 ka ago. Gullies can form at rates of up to 30 m d−1 via two processes, namely the formation of alcoves and tunnels by groundwater seepage, followed by retrogressive slope failure due to undermining and a decrease in shear strength driven by excess pore pressure development. The location of gullies is determined by the occurrence of hydraulically conductive zones, such as relict braided river channels and possibly tunnels, and of sand lenses exposed across sandy gravel cliffs. We also show that the gully planform shape is generally geometrically similar at consecutive stages of evolution. These outcomes will facilitate the reconstruction and prediction of a prevalent erosive process and overlooked geohazard along the Canterbury coastline.

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Tools and Techniques: Active-Source Electromagnetic Methods

Cited by 18 publications

References 68 publications

Multi-Layered High Permeability Conduits Connecting Onshore and Offshore Coastal Aquifers

Multi-Layered High Permeability Conduits Connecting Onshore and Offshore Coastal Aquifers

Coupling hydrogeophysics with hydrodynamic modelling to infer subsurface hydraulic architecture of an alluvial floodplain

Groundwater erosion of coastal gullies along the Canterbury coast (New Zealand): a rapid and episodic process controlled by rainfall intensity and substrate variability

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