The feasibility of electromagnetic gradiometer measurements

Abstract

The quantities measured in transient electromagnetic (TEM) surveys are usually either magnetic field components or their time derivatives. Alternatively it might be advantageous to measure the spatial derivatives of these quantities. Such gradiometer measurements are expected to have lower noise levels due to the negative interference of ambient noise recorded by the two receiver coils. Error propagation models are used to compare quantitatively the noise sensitivities of conventional and gradiometer TEM data. To achieve this, eigenvalue decomposition is applied on synthetic data to derive the parameter uncertainties of layered‐earth models. The results indicate that near‐surface gradient measurements give a superior definition of the shallow conductivity structure, provided noise levels are 20–40 times smaller than those recorded by conventional EM instruments. For a fixed‐wing towed‐bird gradiometer system to be feasible, a noise reduction factor of at least 50–100 is required. One field test showed that noise reduction factors in excess of 60 are achievable with gradiometer measurements. However, other collected data indicate that the effectiveness of noise reduction can be hampered by the spatial variability of noise such as that encountered in built‐up areas. Synthetic data calculated for a vertical plate model confirm the limited depth of detection of vertical gradient data but also indicate some spatial derivatives which offer better lateral resolution than conventional EM data. This high sensitivity to the near‐surface conductivity structure suggests the application of EM gradiometers in areas such as environmental and archaeological mapping.

Improved karst exploration by VLF‐EM‐gradient survey: comparison with other geophysical methods

“…Spitzer et al () illustrated an example where magnetic field gradient measurements in a borehole were applied to magnetotellurics in order to avoid measuring the generally perturbed electric field. Sattel and Macnae () showed, both theoretically and in a case study, the advantages of gradient measurements applied to transient electromagnetic surveys, i.e. increase in resolution and reduction of noise.…”

Metalliferous mining geophysics — State of the art after a decade in the new millennium

“…This work is done using a frequencydomain system, but a similar procedure could be used for timedomain systems. Sattel and Macnae (2001) investigated the measurement of gradient data. They argued that the greater spatial resolution of the gradient measurement could resolve closely spaced features and, depending on the noise levels, might be able to resolve shallow features.…”

Transient process and optimal design of receiver coil for small‐loop transient electromagnetics

“…The transient electromagnetic (TEM) technique has been widely used for geological exploration in underground solid waste and pollutants survey (Tibana et al . ), archaeology (Sattel and Macnae ), tunnel prediction (Xue et al . ), soil evaluation (Cooper and Swift ), urban construction (Metwaly et al .…”

“…The transient electromagnetic (TEM) technique has been widely used for geological exploration in underground solid waste and pollutants survey (Tibana et al 2004), archaeology (Sattel and Macnae 2001), tunnel prediction (Xue et al 2007), soil evaluation (Cooper and Swift 1994), urban construction (Metwaly et al 2010) and groundwater investigation (Sandberg 1993;Mohamed et al 2002). In space constrained areas such as undulating mountains and coal mine fields, small-loop instruments are preferred for TEM exploration since large loops are difficult to deploy.…”