Inversion of Airborne Electromagnetic Data For Overburden Mapping and Groundwater Exploration

Paterson, Norman R.; Reford, Stephen

doi:10.4095/122345

Cited by 16 publications

(13 citation statements)

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“…However, they don't allow detection of resistivity variations with depth and suffer in areas where the resistivity model departs significantly from that of a half-space. In order to distinguish vertical resistivity variations and obtain a formal measure of model validity one can use a multi-layer 1-D inversion (Beard, 2000;Constable et al, 1987;Fitterman and Deszcz-Pan, 1998;Paterson and Redford, 1986;Sengpiel and Siemon, 1998). These inversions are non-unique because they typically solve a heavily underdetermined problem.…”

Section: Frequency-domain Aem Inversion Methodsmentioning

confidence: 99%

Laterally Constrained Inversion of Fixed-Wing Frequency-Domain AEM Data

Tartaras¹,

Beamish²

2006

Near Surface 2006 - 12th EAGE European Meeting of Environmental and Engineering Geophysics

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SUMMARYNew highresolution airborne geophysical surveys of the UK, undertaken with the system developed under the Joint Airbornegeoscience Capability programme, established between the Geological Survey of Finland and the British Geological Survey, will provide large 4frequency airborne electromagnetic data sets. These data sets will be used to characterise the conductivity distribution of the subsurface for environmental and exploration purposes. To invert these large data sets in a fast and robust manner we have developed "LC1DINV", a laterally constrained onedimensional inversion algorithm. This algorithm inverts simultaneously for all observation points along a profile and regularises the inverse problem by requiring that differences between model parameters at adjacent points be small. We use the conjugate gradient method for minimising the data misfit subject to the lateral constraints and a priori model terms. We have inverted 4frequency data obtained over Suurpelto, a test area in southern Finland, characterised by conductive clays overlying a highly resistive granitic shield. The results show that LC1DINV can successfully locate the depth extent and variations of the clays. Comparison of these results with those obtained with two other types of inversion shows that LC1DINV produces welldefined layer boundaries and laterally smooth crosssections.

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Section: Frequency-domain Aem Inversion Methodsmentioning

confidence: 99%

Laterally Constrained Inversion of Fixed-Wing Frequency-Domain AEM Data

Tartaras¹,

Beamish²

2006

Near Surface 2006 - 12th EAGE European Meeting of Environmental and Engineering Geophysics

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“…One of the most common methods is the ridge-regression procedure (Inman, 1975) with stabilisation incorporated using a Marquardt (1970) approach. Paterson and Redford (1986) provide a description and examples of the problems associated with the 1D multilayer inversion of HEM data. One of the key points is that dual frequency airborne data provide an underdetermined case of model estimation i.e.…”

Section: D Inversionmentioning

confidence: 99%

“…non 1D) data adds to the reliability of an interpretation. 1D inversion of AEM data is becoming more widespread (Paterson and Redford, 1986;Sengpiel and Siemon, 1998) and the misfit parameter is useful with regard to model validity. Theoretical examples of 1D inversion applied to a two-layer earth are presented.…”

Section: Introductionmentioning

confidence: 99%

An assessment of inversion methods for AEM data applied to environmental studies

Beamish

2002

Journal of Applied Geophysics

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Airborne electromagnetic surveys are currently being flown over populated areas and applied to detailed problems using high flight line densities. Interpretation information is supplied through a model of the subsurface resistivity distribution. Theoretical and survey data are used here to study the character and reliability of such models. Although the survey data were obtained using a fixed-wing system, the corresponding associations with helicopter, towed-bird systems are discussed. Both Fraser half-space and 1D inversion techniques are considered in relation to their ability to distinguish geological, cultural and environmental influences on the survey data. Fraser half-space modelling provides the dual interpretation parameters of apparent resistivity and apparent depth at each operational frequency. The apparent resistivity was found to be a remarkably stable parameter and appears robust to the presence of a variety of at-surface cultural features. Such features provide both incorrect altitude data and multidimensional influences. Their influences are observed most strongly in the joint estimate of apparent depth and this accounts for the stability of the apparent resistivity. Positive apparent depths, in the example data, result from underestimated altitude measurements. It is demonstrated that increasingly negative apparent depths are associated with increasing misfits between a 1D model and the data. Centroid depth calculations, which are a transform of the Fraser half-space parameters, provide an example of the detection of non-1D influences on data obtained above a populated area. 1D inversion of both theoretical and survey data is examined. The simplest use of the 1D inversion method is in providing an estimate of a half-space resistivity. This can be undertaken prior to multi-layer inversion as an initial assessment. Underestimated altitude measurements also enter the problem and, in keeping with the Fraser pseudo-layer concept, an at-surface highly resistive layer of variable thickness, can be usefully introduced as a constrained parameter. It is clearly difficult to ascribe levels of significance to a 'measure' of misfit contained in a negative apparent depth with the dimensions of metres. The reliability of 1D models is better assessed using a formal misfit parameter. With the misfit parameter in place, the example data suggest that the 1D inversion methods provide reliable apparent resistivity values with a higher resolution than the equivalent information from the Fraser half-space estimates.3

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“…It is also possible to perform iterative least-squares inversion of AEM data (Paterson and Redford, 1986;Sengpiel and Siemon, 1998) to obtain an equivalent half space model. These procedures also provide a misfit that aids discrimination of invalid data (i.e.…”

Section: D Modellingmentioning

confidence: 99%

3D modelling of near-surface, environmental effects on AEM data

Beamish

2004

Journal of Applied Geophysics

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This study considers the 3D modelling of compact, at-surface conductive bodies on frequency domain airborne electromagnetic (AEM) survey data. The context is the use of AEM data for environmental and land quality applications. The 3D structures encountered are typically conductive, of limited thickness (< 20 m) and form 'point' source locations carrying potential environmental risk. The scale of such bodies may generate single profile, 'bulls-eye' anomalies. In attempts to recover geological information, such anomalies may be considered to represent noise. In environmental AEM, the correct interpretation of such features is important. The study uses a combination of theoretical models and trial fixed wing survey data obtained in populated areas of the UK. Scale issues are discussed in terms of the volumetric footprints of the induced electric field generated by systems flown at both low and high elevation. One of the primary uses of AEM survey data lies in the assessment of conductivity maps. These are typically obtained using 1D conductivity models at individual measurement points. In order to investigate the limitations of this approach, 3D modelling of conductive structures with dimensions less than 350 x 350 m and thicknesses extending to 20 m has been carried out. A 1D half space inversion of the data obtained at each frequency is then used to assess the behaviour of the spatial information. The results demonstrate that half space conductivity values obtained over compact 3D targets generally provide only apparent conductivity results. For thin, atsurface bodies, conductivity values are biased to lower values than the true conductivity except at high frequency.The spatial perturbation to both coupling ratios and 1D conductivity models can be laterally extensive. The results from 3D modelling indicate that the use of horizontal derivatives applied to the conductivity models offers enhanced edge detection. The practical application of such derivatives to both regional and local scale survey data is presented.. The special case of a near-surface, metallic pipeline has been modelled. The problem constitutes an inductive limit (current gathering) response in which the perturbation is largely confined to the in-phase coupling ratios. The main perturbations, in data and conductivity models, are within about 40 m of each side of the pipeline. The maximum perturbation to the conductivity model is only a factor of 1.5 above background. Detailed survey data across a former compact landfill (about 100 x 100 m) are used to compare the model behaviour predicted by the 3D modelling with survey results. The survey, conducted at two separate altitudes, provides a demonstration of 3D effects on 1D survey models as a function of frequency and elevation. Although the nature of the landfill materials and their location are not known precisely, the mapping information appears realistic.

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Inversion of Airborne Electromagnetic Data For Overburden Mapping and Groundwater Exploration

Cited by 16 publications

References 0 publications

Laterally Constrained Inversion of Fixed-Wing Frequency-Domain AEM Data

Laterally Constrained Inversion of Fixed-Wing Frequency-Domain AEM Data

An assessment of inversion methods for AEM data applied to environmental studies

3D modelling of near-surface, environmental effects on AEM data

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