SkyTEM–a New High-resolution Helicopter Transient Electromagnetic System
TL;DR: SkyTEM as mentioned in this paper is a time-domain, helicopter electromagnetic system designed for hydrogeophysical and environmental investigation, which is a rapid alternative to ground-based, transient electromagnetic measurements, the resolution capabilities are comparable to that of a conventional 40 × 40 m 2 system.
Abstract: SkyTEM is a time-domain, helicopter electromagnetic system designed for hydrogeophysical and environmental investigation. Developed as a rapid alternative to ground-based, transient electromagnetic measurements, the resolution capabilities are comparable to that of a conventional 40 × 40 m 2 system. Independent of the helicopter, the entire system is carried as an external sling load. In the present system, the transmitter, mounted on a lightweight wooden lattice frame, is a four-turn 12.5 × 12.5 m 2 square loop, divided into segments for transmitting a low moment with one turn and a high moment with all four turns. The low moment uses about 30 A with a turn-off time of about 4 µs; the high moment draws approximately 50 A, and has a turn-off time of about 80 µs. The shielded, overdamped, multi-turn receiver loop is rigidly mounted on the side of the transmitter loop. This is essentially a central-loop configuration with a 1.5 m vertical offset. In vertical hover mode the SkyTEM responses were within 2% of those from a conventional ground-based system. Instrument bias level is not a concern as high-altitude tests showed that the background noise level is higher than the instrument bias level. By inverting a sounding from a test site to a standard model and then applying the SkyTEM system parameters to compute the forward response, conventional measurements were within 5% of SkyTEM responses for flight heights of 7.25, 10, and 20 m. Standard field operations include establishment of a repeat base station in the survey area where data are acquired approximately every 1.5 hours, when the helicopter is refuelled, to monitor system stability. Data acquired in a production survey were successful in detecting and delineating a buried-valley structure important in hydrogeophysical investigations.
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TL;DR: The paper identifies instruments, provides examples of their use, and describes how synergy between measurement and modelling could be achieved, and provides a vision for the use of electrical and magnetic geophysical instrumentation in watershed scale hydrology.
Abstract: We want to develop a dialogue between geophysicists and hydrologists interested in synergistically advancing process based watershed research. We identify recent advances in geophysical instrumentation, and provide a vision for the use of electrical and magnetic geophysical instrumentation in watershed scale hydrology. The focus of the paper is to identify instrumentation that could significantly advance this vision for geophysics and hydrology during the next 3–5 years. We acknowledge that this is one of a number of possible ways forward and seek only to offer a relatively narrow and achievable vision. The vision focuses on the measurement of geological structure and identification of flow paths using electrical and magnetic methods. The paper identifies instruments, provides examples of their use, and describes how synergy between measurement and modelling could be achieved. Of specific interest are the airborne systems that can cover large areas and are appropriate for watershed studies. Although airborne geophysics has been around for some time, only in the last few years have systems designed exclusively for hydrological applications begun to emerge. These systems, such as airborne electromagnetic (EM) and transient electromagnetic (TEM), could revolutionize hydrogeological interpretations. Our vision centers on developing nested and cross scale electrical and magnetic measurements that can be used to construct a three-dimensional (3D) electrical or magnetic model of the subsurface in watersheds. The methodological framework assumes a ‘top down’ approach using airborne methods to identify the large scale, dominant architecture of the subsurface. We recognize that the integration of geophysical measurement methods, and data, into watershed process characterization and modelling can only be achieved through dialogue. Especially, through the development of partnerships between geophysicists and hydrologists, partnerships that explore how the application of geophysics can answer critical hydrological science questions, and conversely provide an understanding of the limitations of geophysical measurements and interpretation. Copyright © 2008 John Wiley & Sons, Ltd.
364 citations
Cites methods from "SkyTEM–a New High-resolution Helico..."
...SkyTEM was designed specifically for hydrogeophysical and environmental investigations (Sørensen and Auken, 2004)....
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TL;DR: In this paper, a new methodology, spatially constrained inversion (SCI), is proposed to produce quasi-3D conductivity modeling of electromagnetic (EM) data using a 1D forward solution.
Abstract: We present a new methodology, spatially constrained inversion (SCI), that produces quasi-3D conductivity modeling of electromagnetic (EM) data using a 1D forward solution. Spatial constraints are set between the model parameters of nearest neighboring soundings. Data sets, models, and spatial constraints are inverted as one system. The constraints are built using Delaunay triangulation, which ensures automatic adaptation to data density variations. Model parameter information migrates horizontally through spatial constraints, increasing the resolution of layers that would be poorly resolved locally. SCI produces laterally smooth results with sharp layer boundaries that respect the 3D geological variations of sedimentary settings. SCI also suppresses the elongated artifacts commonly seen in interpretation results of profile-oriented data sets. In this study, SCI is applied to airborne time-domain EM data, but it can also be implemented with other ground-based or airborne data types.
318 citations
TL;DR: In this paper, the authors present a modular forward response algorithm that is independent of data type, making it easy to add support for new types of data types and instrument geometries.
Abstract: Wepresentanoverviewofamature,robustandgeneralalgorithmprovidingasingleframeworkfortheinversion of most electromagnetic and electrical data types and instrument geometries. The implementation mainly uses a 1D earth formulation for electromagnetics and magnetic resonance sounding (MRS) responses, while the geoelectric responses are both 1D and 2D and the sheet's response models a 3D conductive sheet in a conductive host with an overburden of varying thicknessandresistivity.Inallcases,thefocusisplacedondeliveringfullsystemforwardmodellingacrossallsupportedtypes of data. Our implementation is modular, meaning that the bulk of the algorithm is independent of data type, making it easy to add support for new types. Having implemented forward response routines and file I/O for a given data type provides accesstoarobustandgeneralinversionengine.Thisengineincludessupportformixeddatatypes,arbitrarymodelparameter constraints, integration of prior information and calculation of both model parameter sensitivity analysis and depth of investigation.Wepresentareviewofourimplementationandmethodologyandshowfourdifferentexamplesillustratingthe versatility of the algorithm. The first example is a laterally constrained joint inversion (LCI) of surface time domain induced polarisation (TDIP) data and borehole TDIP data. The second example shows a spatially constrained inversion (SCI) of airbornetransientelectromagnetic(AEM)data.ThethirdexampleisaninversionandsensitivityanalysisofMRSdata,where the electrical structure is constrained with AEM data. The fourth example is an inversion of AEM data, where the model is described by a 3D sheet in a layered conductive host.
257 citations
TL;DR: In this paper, the advantages and limitations of airborne electromagnetic (AEM) surveys compared to ground-based geophysical methods used in groundwater surveys are discussed, based on typical field examples.
Abstract: For about three decades, airborne electromagnetic (AEM) systems have been used for groundwater exploration purposes. Airborne systems are appropriate for large-scale and efficient groundwater surveying. Due to the dependency of the electrical conductivity on both the clay content of the host material and the mineralization of the water, electromagnetic systems are suitable for providing information about the aquifer structures and water quality, respectively.
More helicopter than fixed-wing systems are used in airborne groundwater surveys. Helicopter-borne frequency-domain electromagnetic (HEM) systems use a towed rigid-boom. Helicopter-borne time-domain (HTEM) systems, which use a large transmitter loop and a small receiver within or above the transmitter, are generally designed for mineral exploration purposes but recent developments have made some of these systems usable for groundwater purposes as well.
The quantity measured, the secondary magnetic field, depends on the subsurface conductivity distribution. Due to the skin-effect, the penetration depths of the AEM fields depend on the system characteristics used: high-frequency data/early-time channels describe the shallower parts of the conducting subsurface and the low-frequency data/late-time channels the deeper parts. Typical investigation depths range from some ten metres (conductive grounds) to several hundred metres (resistive grounds), where the HEM systems are appropriate for shallow to medium deep (about 1–100 m) and the HTEM systems for medium deep to deep (about 10–400 m) investigations.
Generally, the secondary field values are inverted into resistivities and depths using homogeneous or layered half-space models. As the footprint of AEM systems is rather small, one-dimensional interpretation
of AEM data is sufficient in most cases and single-site inversion procedures are widely used. Laterally constrained inversion of AEM data often improves the stability of the inversion models, particularly
for noisy data. Higher dimensional inversion is still not possible for standard-size surveys.
Based on typical field examples the advantages as well as the limitations of AEM surveys compared
to long-established ground-based geophysical methods used in groundwater surveys are discussed.
In a case history from a German island an airborne frequency-domain system is used to successfully
locate freshwater lenses on top of saltwater. An example from Denmark shows how a time-domain system is used to locate large-scale buried structures forming ideal groundwater aquifers.
239 citations
Cites methods from "SkyTEM–a New High-resolution Helico..."
...5) has been developed for groundwater investigations by the HGG group at the University of Aarhus, Denmark (Sørensen and Auken 2004)....
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TL;DR: Tunnel valleys are large, elongate and irregular depressions cut beneath the margin of former ice sheets as mentioned in this paper, and they play a substantial role for the entire hydraulic system beneath ice sheets and thus also for ice sheet behaviour.
219 citations
Cites background from "SkyTEM–a New High-resolution Helico..."
...They mostly appear as coherent, homogeneous and clayey tills, and can be interpreted as subglacially deposited (Jørgensen et al., 2003b; Jørgensen and Sandersen, 2004; Kronborg et al., 2003; Sørensen et al., 2004)....
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...…drillings located in the valleys support the proposed age relationships by showing the presence of old, probably preElsterian, glacial sediments within one of the valleys from the oldest generation, and probably Saalian and Weichselian deposits in the younger generations (Sørensen et al., 2004)....
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...Exploration drillings in buried valleys often show thick sequences of waterlaid deposits (e.g. Foldager, 2003; Jørgensen and Sandersen, 2004; Sørensen et al., 2004), but frequent shifts in lithofacies are also commonly found (Fig....
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References
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ARCO1
TL;DR: The time or frequency at which the electromagnetic response of a buried inhomogeneity can first be measured is determined by its depth of burial and the average conductivity of the overlying section; it is relatively independent of the type of source or receiver and their separation.
Abstract: The time or frequency at which the electromagnetic (EM) response of a buried inhomogeneity can first be measured is determined by its depth of burial and the average conductivity of the overlying section; it is relatively independent of the type of source or receiver and their separation. The ability to make measurements at this time or frequency, however, depends on the sensitivity and accuracy of the instrumentation, the signal strength, and the ambient noise level. These factors affect different EM sounding systems in surprisingly different ways.For the magnetotelluric (MT) method, it is possible to detect a buried half-space under about 1.5 skin depths of overburden. The maximum depth of investigation is virtually unbounded because of high signal strengths at low frequencies. Transient electromagnetic (TEM) soundings, on the other hand, have a limited depth of penetration, but are less affected by static shift errors. For TEM, a buried inhomogeneity can be detected under about one diffusion depth of overburden. For conventional near-zone sounding in which induced voltage is measured (impulse response), the depth of investigation is proportional to the 1/5 power of the source moment and ground resistivity. By contrast, if the receiver is a magnetometer (step response system), the depth of investigation is proportional to the 1/3 power of source moment and is no longer a function of resistivity. Magnetic-field measurements may, therefore, be superior for exploration in conductive areas such as sedimentary basins. Far-zone, or long-offset, TEM soundings are traditionally used for deep exploration. The depth of investigation for a voltage receiver is proportional to the 1/4 power of source moment and resistivity and is inversely proportional to the source-receiver separation. Magnetic-field measurements are difficult to make at long offsets because instrumental accuracy limits the measurement of the very slow decay of the magnetic field.Frequency-domain controlled-source systems are ideally suited for sounding at the very shallow depths needed for engineering, archaeological, and groundwater applications because of the relative ease of extending the measurements to arbitrarily high frequencies, and also because geometric soundings can be made at low induction numbers.
406 citations
"SkyTEM–a New High-resolution Helico..." refers background in this paper
...For an in-depth discussion on the depth of penetration versus background noise levels, see Spies (1989)....
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TL;DR: The feasibility of using the transient electromagnetic sounding (TS or TDEM) method for groundwater exploration can be studied by means of numerical models as discussed by the authors, which is best suited for locating conductive targets, and has very good vertical resolution.
Abstract: The feasibility of using the transient electromagnetic sounding (TS or TDEM) method for groundwater exploration can be studied by means of numerical models. As examples of its applicability to groundwater exploration, we study four groundwater exploration problems: (1) mapping of alluvial fill and gravel zones over bedrock; (2) mapping of sand and gravel lenses in till; (3) detection of salt or brackish water interfaces in freshwater aquifers; and (4) determination of hydrostratigraphy. These groundwater problems require determination of the depth to bedrock; location of resistive, high‐porosity zones associated with fresh water; determination of formation resistivity to assess water quality; and determination of lithology and geometry, respectively. The TS method is best suited for locating conductive targets, and has very good vertical resolution. Unlike other sounding techniques where the receiver‐transmitter array must be expanded to sound more deeply, the depth of investigation for the TS method is a...
383 citations
"SkyTEM–a New High-resolution Helico..." refers background in this paper
...…worldwide, in a wide variety of 195Exploration Geophysics (2004) Vol 35, No. 3195 geological conditions from basalts to sedimentary environments (Fitterman and Stewart 1986; Mills et al., 1988; McNeill, 1990; Sandberg and Hall, 1990; Christensen and Sørensen, 1998; Courteaud et al., 1998;…...
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TL;DR: In this article, the authors show that for a valley structure in a low-resistive layer, the 1-dimensional assumption is sufficient to track the presence of rather steep slopes, however, the insensitivity of the TEM method to resistors makes it difficult to determine a slope with a 1-D inversion and only the overall structure is defined.
294 citations
"SkyTEM–a New High-resolution Helico..." refers background or methods in this paper
...Aquifers in this part of the country are often associated with buried valleys incised into the low-resistivity tertiary clays (Jørgensen et al.. 2003; Auken et al., 2003; Danielsen et al., 2003)....
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..., 2003), and hydrogeological structures (Danielsen et al., 2003; Jørgensen et al., 2003)....
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...Aquifers in this part of the country are often associated with buried valleys incised into the low-resistivity Tertiary clays (Jørgensen et al.. 2003; Auken et al., 2003; Danielsen et al., 2003)....
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...The HiTEM system (Auken and Sørensen (2003); Danielsen et al., 2003) uses the PROTEM receiver with a transmitter capable of putting out 75 A in a 30 × 30 m2 loop resulting in a magnetic moment of about 67 000 A.m2....
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...As analysed with onedimensional (1D) modelling, the central-loop configuration is preferable to the offset-loop configuration because it is insensitive to near-surface resistivity variations and small changes in the transmitter-receiver separation (Danielsen et al., 2003)....
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