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.

Advancing process‐based watershed hydrological research using near‐surface geophysics: A vision for, and review of, electrical and magnetic geophysical methods

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.

SkyTEM–a New High-resolution Helicopter Transient Electromagnetic System

Fluid pressure in the crust may be controlled by different mechanisms according to depth, temperature, and the mineralogy of the host rocks. Where rocks are fluid-saturated, fluid pressure may approach lithostatic or hydrostatic pressure depending on the ductility of the wall rocks and the connectivity of pores and fractures. However, if the host rocks contain minerals formed at temperatures higher than those currently prevailing, they will react with fluids to produce hydrated (or carbonated) retrograde minerals, and the fluid pressure will be limited by thermodynamic equilibrium between high-grade reactant minerals and retrograde products. The thermodynamically constrained parameter, water fugacity, may have a value of tens to hundreds of bars in the lower crust. In practice, this means that for typical igneous or high-grade metamorphic rocks now occurring in stable lower crust, notional fluid pressures are substantially (1 to 3 orders of magnitude) lower than lithostatic. No free, connected fluid phase can be present in deep stable crust, and alternative explanations must be sought for the relatively high electrical conductivity of such rocks. The proposal that high lower crustal conductivity is due to thin grain boundary films of graphite is also unlikely to be generally true because films of sufficient thickness would be readily visible on broken surfaces of hand specimens. An alternative explanation of the discrepancy between laboratory and field measurements of the conductivity of high-grade rocks is that laboratory measurements are not normally made under appropriate conditions of rock-buffered fluid pressure.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/97JB00508

The petrologic case for a dry lower crust

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.

An overview of a highly versatile forward and stable inverse algorithm for airborne, ground-based and borehole electromagnetic and electric data

The continental middle to lower crust (CLC) remains one of the most enigmatic parts of the Earth about which comparatively little is known. Remote-sensing geophysical and geochemical data illustrate our gaps in complete knowledge of the state and composition of the CLC. Generally, it is thought that the CLC, when compared to the upper crust, is somewhat more uniform in its properties. However, studies undertaken in the last decade have revealed that the CLC can be as heterogeneous as the upper crust, but that some regions of surprising homogeneity exist. In this chapter, I will describe attempts to image one particular physical parameter of the CLC, namely its electrical conductivity a. Electromagnetic (EM) sounding methods, as with most geophysical methods, are sensitive to the structure that exists today, and as such are dependent on the current state and composition of the CLC, rather than its state at formation or as a consequence of tectonic or metamorphic activity as are petrological studies on exhumed samples from the CLC. All EM methods give responses that are volumetric averages of the Earth's conductivities sensed by the diffusive EM fields, and as such are in the same class of geophysical techniques as seismic surface wave studies, potential field methods, and geothermal investigations. These are distinct from seismic reflection and refraction methods which deal with non-diffusive waves. However, with EM methods the governing field equations (§2) and wide range of the physical parameter being sensed (electrical conductivity, §1.1) ensure a far greater resolving power to anomalies than with the other diffusive geophysical techniques. EM and seismic methods are the only geophysical techniques for which probing of the deep crust is assured; with all others there are inherent screening effects and depth ambiguities. Imaging the electrical conductivity structure beneath the surface has a number of uses, classified mainly by the depth of investigation: (1) Identifying zones of mineralization, which are of economic importance. (2) Detecting fluids in the deep crust, such as those above downgoing slabs, e.g., the Juan de Fuca plate (Kurtz et al., 1986a, 1990; EMSLAB, 1989). (3) Resolving one physical property of crustal zones. As expressed by Dohr et al. (1989): " ... magnetotellurics works like a broad paint brush, colouring the layers bounded by the seismic reflections" .

/pdf/electrical-conductivity-of-the-continental-lower-crust-1gi1ipggnh.pdf

Electrical conductivity of the continental lower crust

An adaptation of the Macnae-Lamontagne method allows transform of airborne step-response electromagnetic (EM) data to a conductivity-depth image. The algorithm is based on a nonlinear transformation of the amplitude of the measured response at each delay time to an apparent mirror image depth. Using matrix algebra, the set of mirror image depth-delay time data pairs can then be used to derive a conductivity section. Data can be efficiently processed on a personal computer at rates faster or comparable to the rate required for collection. Stable conductivity fitting as a function of depth is obtained by damping the matrix inversion by specification of the first- and second-derivative smoothness weights of the fitted conductivity-depth sounding. Damping parameters may be either fixed or varied along the profile; their choice can be constrained by geologic control. Stability of the process is enhanced by accounting for the transmitter and receiver tilts.The mirror image depth-delay time data can also be used directly with simple regression to obtain the best-fitting thin-sheet and half-space models. With one novel assumption, the thin-sheet model can be converted to a thick-sheet overburden model without prespecification of either its conductivity or thickness. Depending on the geology, these simple models may prove quite useful.The conductivity imaging algorithm has been applied to a test data set collected with the SPECTREM system. The stability and speed of the imaging process were confirmed and have demonstrated airborne EM sounding to depths well over 400 m in an area with quite conductive sediments. Comparing the results with a better resolved image obtained from ground UTEM data shows that the airborne data can adequately define the geometry of the uppermost conductor encountered in the section. The geophysical results are consistent with geologic control and measurements of resistivity obtained from well logs.

Conductivity-depth imaging of airborne electromagnetic step-response data

DESPITE the importance of aqueous fluids in tectonic, metamorphic and magmatic activity in the Earth's crust, few techniques are available for directly observing their distribution at depth, even in stable continental crust. Such observations as have been made (electromagnetic deep sounding, which by inference1,2 images saline fluids, and super-deep drilling3) suggest that fluids are present in varying amounts throughout the crust, in direct conflict4 with the normal petrological inference of a very dry deep crust. Here we report the results of an unusually high-resolution electromagnetic sounding method to image the deep electrical-conductivity structure below a 30-km profile in Ontario which crosses Archaean lower-crustal material of high metamorphic grade exposed by a Proterozoic thrust fault, the Ivanhoe Lake cataclastic zone. The electrical-resistivity image obtained shows, embedded in very resistive crystalline crust, two nearly continuous, sub-horizontal zones of enhanced electrical conductivity at depths of 2–3 and 5–6 km. Below this, the commonly observed2,5-7 mid-crustal rise in conductivity is seen at depths of 15–20 km. The image does not correlate well with the structure inferred from surface geology, suggesting that the transport and distribution of fluids was not strongly controlled by lithological boundaries or the thrust fault itself.

Imaging of deep fluids in Archaean crust

We have developed a model that calculates the Electromagnetic (EM) response for multiple, planar conductors located in a resistive host. The program, called MultiLOOP, will handle any system waveform in time or frequency domain and will model fixed, moving and coincident loop receiver geometries. The assumption made to simplify the solution to allow for implementation on a microcomputer is that the current in the plates can be constrained to flow in ribbons, and that no currents may galvanically cross from one ribbon to another. Provided the tramnsmitter and receiver are reasonably distant from the conductors, the computed EM response agrees well with that computed by other modelling programs.

Multiple Conductor Modeling Using Program MultiLOOP

On October 22, 1997, Inco announced the discovery of a new deposit located at the south end of the Copper Cliff offset, a dike extending south from the Sudbury Igneous Complex (SIC) near the city of Sudbury, Ontario. The deposit was detected in 1992 as an off-hole borehole EM anomaly in a hole drilled from surface under Kelly Lake. A subsequent hole, 93670 targeted this off-hole conductor intersected 51 feet of ore grade mineralization; the first of the Kelly Lake deposit.

http://cronegeophysics.com/crone/Kelly%20Lake%20Sudbury%20Onratio,%20VMS,%20Nickel%20Copper.pdf

The Role of Borehole EM in the Discovery and Definition of the Kelly Lake Ni-Cu Deposit, Sudbury, Canada.

A simple but very effective technique can be formulated M remove much of the seccwlary response of cultural conductors. The method relies on es& mation of the secondary current flowing within a conductor at each smalpe time by indirect measurement of its local vertical magnetic field, and it does not require au a-priori knowledge of the subsurface conductivity snuchue. Since me induced current flowing within a cultural conductor can be estimated at any point by a short traverse across it, the method cnn account reasonably for variations in induced current along the cultural feature’s length caused by multiple grounding and variation in the primary electric field This technique has been very successfully applied to data collected in areas cut by numerous cultural features of complex geometry.

Stripping Cultural Conductor Responses From EM Data

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