Sensitivity functions of frequency-domain magnetic dipole-dipole systems

Geophysics ◽  
2007 ◽  
Vol 72 (2) ◽  
pp. F45-F56 ◽  
Author(s):  
Rasmus Juhl Tølbøll ◽  
Niels Bøie Christensen
Keyword(s):  
Frequency Domain ◽  
Magnetic Dipole ◽  
Central Zone ◽  
Practical Investigations ◽  
Flight Path ◽  
Lateral Resolution ◽  
Sensitivity Distribution ◽  
Airborne Electromagnetic ◽  
Lateral Extent ◽  
Target Characteristics

The resolution capabilities of airborne electromagnetic (AEM) frequency-domain systems are traditionally analyzed in terms of the footprint, which provides a simple measure of the lateral extent of the earth volume involved in a given measurement. However, considerably more detailed insight into the system resolution capabilities can be obtained by studying the 3D sensitivity distribution as defined by the Fréchet derivatives. A qualitative analysis of the 3D sensitivity distributions for six typical magnetic dipole-dipole configurations demonstrates that the spatial resolution characteristics differ widely and that the optimal coil configuration for practical investigations depends on the expected target characteristics. For all six coil configurations, the 3D sensitivity distributions reveal significant sign changesdownwards and outwards from the center, stressing the necessity of reliable starting models for successful inversion of frequency-domain AEM data. Likewise, the central zone of sensitivity for the in-phase component is always larger than for the quadrature, indicating an inferior lateral resolution of the former. A new sensitivity footprint is defined, based on the at-surface behavior of the sensitivity distribution, simply as the area where the sensitivity is at least 10% of its maximum value. For the vertical coaxial (VCA) coil configuration, the size of the sensitivity footprint in the [Formula: see text]-direction (perpendicular to the flight path) is approximately a factor of two larger than in the [Formula: see text]-direction (along the flight path), while there is virtually no difference for the horizontal coplanar (HCP) coil configuration. The ratio of the HCP to VCA sensitivity footprint exceeds one in both [Formula: see text]- and [Formula: see text]-directions, suggesting that the VCA coil configuration has the best lateral resolution.

scholarly journals Quantifying model structural uncertainty using airborne electromagnetic data

2020 ◽  
Vol 224 (1) ◽  
pp. 590-607
Author(s):  
Burke J Minsley ◽  
Nathan Leon Foks ◽  
Paul A Bedrosian
Keyword(s):  
Frequency Domain ◽  
Time Domain ◽  
Parameter Uncertainty ◽  
Probability Distributions ◽  
Joint Probability ◽  
Model Complexity ◽  
Structural Uncertainty ◽  
Mcmc Algorithm ◽  
Geophysical Parameter ◽  
Airborne Electromagnetic

SUMMARY The ability to quantify structural uncertainty in geological models that incorporate geophysical data is affected by two primary sources of uncertainty: geophysical parameter uncertainty and uncertainty in the relationship between geophysical parameters and geological properties of interest. Here, we introduce an open-source, trans-dimensional Bayesian Markov chain Monte Carlo (McMC) algorithm GeoBIPy—Geophysical Bayesian Inference in Python—for robust uncertainty analysis of time-domain or frequency-domain airborne electromagnetic (AEM) data. The McMC algorithm provides a robust assessment of geophysical parameter uncertainty using a trans-dimensional approach that lets the AEM data inform the level of model complexity necessary by allowing the number of model layers itself to be an unknown parameter. Additional components of the Bayesian algorithm allow the user to solve for parameters such as data errors or corrections to the measured instrument height above ground. Probability distributions for a user-specified number of lithologic classes are developed through posterior clustering of McMC-derived resistivity models. Estimates of geological model structural uncertainty are thus obtained through the joint probability of geophysical parameter uncertainty and the uncertainty in the definition of each class. Examples of the implementation of this algorithm are presented for both time-domain and frequency-domain AEM data acquired in Nebraska, USA.

scholarly journals Airborne Electromagnetic and Radiometric Peat Thickness Mapping of a Bog in Northwest Germany (Ahlen-Falkenberger Moor)

2020 ◽  
Vol 12 (2) ◽  
pp. 203 ◽  
Author(s):  
Bernhard Siemon ◽  
Malte Ibs-von Seht ◽  
Stefan Frank
Keyword(s):  
Large Scale ◽  
Spatial Information ◽  
Borehole Data ◽  
Northwest Germany ◽  
Surface Data ◽  
Airborne Electromagnetic ◽  
Lateral Extent ◽  
Elevated Surface ◽  
Airborne Electromagnetic Data ◽  
Gradient Approach

Knowledge on peat volumes is essential to estimate carbon stocks accurately and to facilitate appropriate peatland management. This study used airborne electromagnetic and radiometric data to estimate the volume of a bog. Airborne methods provide an alternative to ground-based methods, which are labor intensive and unfeasible to capture large-scale (>10 km2) spatial information. An airborne geophysical survey conducted in 2004 covered large parts of the Ahlen-Falkenberger Moor, an Atlantic peat bog (39 km2) close to the German North Sea coast. The lateral extent of the bog was derived from low radiometric and elevated surface data. The vertical extent resulted from smooth resistivity models derived from 1D inversion of airborne electromagnetic data, in combination with a steepest gradient approach, which indicated the base of the less resistive peat. Relative peat thicknesses were also derived from decreasing radiation over peatlands. The scaling factor (µa = 0.28 m−1) required to transform the exposure rates (negative log-values) to thicknesses was calculated using the electromagnetic results as reference. The mean difference of combined airborne results and peat thicknesses of about 100 boreholes is very small (0.0 ± 1.1 m). Although locally some (5%) deviations (>2 m) from the borehole results do occur, the approach presented here enables fast peat volume mapping of large areas without an imperative necessity of borehole data.

Inversion of airborne geophysics over the DO-27/DO-18 kimberlites — Part 2: Electromagnetics

2017 ◽  
Vol 5 (3) ◽  
pp. T313-T325 ◽  
Author(s):  
Dominique Fournier ◽  
Seogi Kang ◽  
Michael S. McMillan ◽  
Douglas W. Oldenburg
Keyword(s):  
Frequency Domain ◽  
Time Domain ◽  
Three Dimensions ◽  
Data Sets ◽  
Conductivity Model ◽  
Time Domain Data ◽  
Conductivity Structure ◽  
Lake Bottom ◽  
Geologic Interpretation ◽  
Lateral Extent

We focus on the task of finding a 3D conductivity structure for the DO-18 and DO-27 kimberlites, historically known as the Tli Kwi Cho (TKC) kimberlite complex in the Northwest Territories, Canada. Two airborne electromagnetic (EM) surveys are analyzed: a frequency-domain DIGHEM and a time-domain VTEM survey. Airborne time-domain data at TKC are particularly challenging because of the negative values that exist even at the earliest time channels. Heretofore, such data have not been inverted in three dimensions. In our analysis, we start by inverting frequency-domain data and positive VTEM data with a laterally constrained 1D inversion. This is important for assessing the noise levels associated with the data and for estimating the general conductivity structure. The analysis is then extended to a 3D inversion with our most recent optimized and parallelized inversion codes. We first address the issue about whether the conductivity anomaly is due to a shallow flat-lying conductor (associated with the lake bottom) or a vertical conductive pipe; we conclude that it is the latter. Both data sets are then cooperatively inverted to obtain a consistent 3D conductivity model for TKC that can be used for geologic interpretation. The conductivity model is then jointly interpreted with the density and magnetic susceptibility models from a previous paper. The addition of conductivity enriches the interpretation made with the potential fields in characterizing several distinct petrophysical kimberlite units. The final conductivity model also helps better define the lateral extent and upper boundary of the kimberlite pipes. This conductivity model is a crucial component of the follow-up paper in which our colleagues invert the airborne EM data to recover the time-dependent chargeability that further advances our geologic interpretation.

scholarly journals Combining Hydraulic Head Analysis with Airborne Electromagnetics to Detect and Map Impermeable Aquifer Boundaries

Water ◽  
2018 ◽  
Vol 10 (8) ◽  
pp. 975 ◽  
Author(s):  
Jesse Korus
Keyword(s):  
Pumping Test ◽  
Hydraulic Head ◽  
Water Levels ◽  
Observation Well ◽  
Geophysical Model ◽  
Airborne Electromagnetic ◽  
Lateral Extent ◽  
Geological Units ◽  
Airborne Electromagnetics

Impermeable aquifer boundaries affect the flow of groundwater, transport of contaminants, and the drawdown of water levels in response to pumping. Hydraulic methods can detect the presence of such boundaries, but these methods are not suited for mapping complex, 3D geological bodies. Airborne electromagnetic (AEM) methods produce 3D geophysical images of the subsurface at depths relevant to most groundwater investigations. Interpreting a geophysical model requires supporting information, and hydraulic heads offer the most direct means of assessing the hydrostratigraphic function of interpreted geological units. This paper presents three examples of combined hydraulic and AEM analysis of impermeable boundaries in glacial deposits of eastern Nebraska, USA. Impermeable boundaries were detected in a long-term hydrograph from an observation well, a short-duration pumping test, and a water table map. AEM methods, including frequency-domain and time-domain AEM, successfully imaged the impermeable boundaries, providing additional details about the lateral extent of the geological bodies. Hydraulic head analysis can be used to verify the hydrostratigraphic interpretation of AEM, aid in the correlation of boundaries through areas of noisy AEM data, and inform the design of AEM surveys at local to regional scales.

Relief effects correction on frequency-domain electromagnetic data

Geophysics ◽  
2019 ◽  
Vol 84 (1) ◽  
pp. E1-E11
Author(s):  
Rimary Valera Sifontes ◽  
Hédison Kiuity Sato
Keyword(s):  
Frequency Domain ◽  
Magnetic Dipole ◽  
Signal To Noise Ratio ◽  
Physical Parameters ◽  
Correction Procedure ◽  
Dipole Axis ◽  
Electromagnetic Data ◽  
Limited Effectiveness ◽  
Land Survey ◽  
Coil Size

During a frequency-domain electromagnetic (FDEM) land survey using transmitter-receiver distances of kilometer order, the receiver and transmitter may be at different altitudes. To increase the signal-to-noise ratio, the transmitting coil size must be increased to the order of a hundred meters and its geometry will be determined by the terrain roughness. Therefore, the equivalent magnetic dipole axis may be neither vertical nor normal to the mean plane representing the terrain surface. Considering the perpendicular loop-loop arrangement, these factors modify the expected secondary magnetic field in two ways: (1) A horizontal primary field arises at the receiving coil position as well as (2) the secondary fields induced by the abnormal currents in the subsurface caused by the tilting of the transmitter dipole axis. A correction procedure is proposed to remove these effects on field FDEM data and tested by using simulated FDEM data with two- or three-layered tilted models to represent the earth with a dipping surface and a nonvertically oriented transmitter magnetic dipole representing a large coil laid on rough terrain. The results demonstrate that the proposed correction procedure has a limited effectiveness, but it can be applied to the FDEM data collected on terrain surfaces having small dipping angles. It is observed that maximum values of the transmitter dipole or surficial plane tilt angle should be 2° to ensure error values in the apparent conductivity less than 10%. Even for the said value, in some combinations of geometric and physical parameters, the tilting and dipping angles can be increased to the order of 5°.

scholarly journals 2.5D Inversion Algorithm of Frequency-Domain Airborne Electromagnetics with Topography

2016 ◽  
Vol 2016 ◽  
pp. 1-10 ◽  
Author(s):  
Jianjun Xi ◽  
Wenben Li
Keyword(s):  
Frequency Domain ◽  
Large Scale ◽  
Jacobian Matrix ◽  
Sparse Matrix ◽  
Inversion Algorithm ◽  
Linear System Of Equations ◽  
Electromagnetic Data ◽  
Electric And Magnetic Fields ◽  
Airborne Electromagnetic ◽  
Airborne Electromagnetic Data

We presented a 2.5D inversion algorithm with topography for frequency-domain airborne electromagnetic data. The forward modeling is based on edge finite element method and uses the irregular hexahedron to adapt the topography. The electric and magnetic fields are split into primary (background) and secondary (scattered) field to eliminate the source singularity. For the multisources of frequency-domain airborne electromagnetic method, we use the large-scale sparse matrix parallel shared memory direct solver PARDISO to solve the linear system of equations efficiently. The inversion algorithm is based on Gauss-Newton method, which has the efficient convergence rate. The Jacobian matrix is calculated by “adjoint forward modelling” efficiently. The synthetic inversion examples indicated that our proposed method is correct and effective. Furthermore, ignoring the topography effect can lead to incorrect results and interpretations.

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