scholarly journals Joint inversion of EM and magnetic data for near‐surface studies

Geophysics ◽  
2002 ◽  
Vol 67 (6) ◽  
pp. 1729-1739 ◽  
Author(s):  
Christophe Benech ◽  
Alain Tabbagh ◽  
Guy Desvignes
Keyword(s):  
Magnetic Susceptibility ◽  
Case Studies ◽  
Joint Inversion ◽  
Synthetic Data ◽  
Magnetic Data ◽  
Near Surface ◽  
Data Inversion ◽  
Electromagnetic Measurements ◽  
Geophysical Surveys ◽  
Depth Analysis

Magnetic and electromagnetic measurements are influenced by magnetic susceptibility and, thus, are widely used in geophysical surveys for archeology or pedology. To date, the data inversion is performed separately. A filtering process incorporating both types of data is presented here. After testing the algorithm with synthetic data, the algorithm is used in several case studies in archeological prospecting. This approach presents two advantages: establishing the presence of remanent magnetizations (viscous or thermoremanent), and achieving more refined depth analysis of the anomaly.

Inversion of aeromagnetic data using digital terrain models

Geophysics ◽  
1993 ◽  
Vol 58 (5) ◽  
pp. 645-652 ◽  
Author(s):  
Derek J. Woodward
Keyword(s):  
Joint Inversion ◽  
Digital Terrain Model ◽  
Vertical Direction ◽  
Inverse Function ◽  
Magnetic Data ◽  
Stability Field ◽  
Topographic Effects ◽  
Near Surface ◽  
Digital Terrain ◽  
White Island

Although draped magnetic surveys contain more information about the magnetization of the rocks near the surface of the earth than surveys at constant elevation, allowance for the effects of the terrain is critical for their correct interpretation. A new method for calculating the magnetic effect of the topography from a digital terrain model by integrating analytically in the vertical direction and then numerically in the horizontal plane is presented. This method lends itself to the calculation of anomalies when the magnetization of the rocks varies with position and thus is well suited to the inversion of draped aeromagnetic surveys to obtain the apparent magnetization of the surficial rocks. This inversion is achieved by repeated use of an approximate inverse function in the form of a two‐dimensional (2-D) filter that is applied to gridded data. An example, using draped magnetic data collected over White Island, an active volcanic island of high relief, shows that although the anomaly pattern is dominated by topographic effects, the distribution of near‐surface magnetic bodies can be determined by a joint inversion of the data and the topography. One of the highly magnetized areas of White Island is interestingly in the vicinity of the active crater, with another near the inner wall of the caldera where there are numerous fumaroles. It may be expected that the higher temperatures in these areas would reduce the magnetization. However, it appears that an explanation for the higher magnetization can be found in the stability field of the mineral magnetite.

scholarly journals Combining Resistivity and Aeromagnetic Geophysical Surveys for Groundwater Exploration in the Maghnia Plain of Algeria

2017 ◽  
Vol 2017 ◽  
pp. 1-14 ◽  
Author(s):  
Djamel Boubaya
Keyword(s):  
Groundwater Potential ◽  
Groundwater Exploration ◽  
Magnetic Data ◽  
Geoelectric Section ◽  
Resistive Layer ◽  
Near Surface ◽  
Groundwater Aquifer ◽  
Geophysical Surveys ◽  
Schlumberger Sounding ◽  
Topsoil Layer

The Maghnia plain in western Algeria is filled by Plio-Quaternary and Miocene sediments that rest unconformably on a basement of Jurassic rocks. Electrical sounding (VES), magnetic data, well information, and hydrogeological data have been used to explore for groundwater potential in the Maghnia plain. The interpretation of Schlumberger sounding data was first calibrated with the lithology of available nearby wells. Four geoelectrical layers were identified within the study area. They are a thin near surface topsoil layer with variable resistivities, a moderate resistive aquifer (15–30 ohm-m), a resistive aquifer (40–70 ohm-m), and a conductive clay layer (1–10 ohm-m). Near Sidi Mbarek, the geoelectric section is reduced to three layers: a topsoil layer, a conductive layer corresponding to the Miocene marls, and a deep resistive layer that correlates with the Oxfordian sandstones. The interpretation of VES data and the enhancement techniques of magnetic data enabled the identification of a number of unmapped faults that occur near recharge zones close to adjacent mountains. This study enabled us to study the extension of the known Plio-Quaternary aquifer of the Maghnia plain and to explore the possible existence of a second deep groundwater aquifer in Oxfordian sandstones.

Geophysical surveys of the East Kirkton Limestone, Viséan, West Lothian, Scotland

1993 ◽  
Vol 84 (3-4) ◽  
pp. 197-202 ◽  
Author(s):  
Jonathan J. Doody ◽  
Rona A. R. McGill ◽  
David Darby ◽  
David K. Smythe
Keyword(s):  
Hot Spring ◽  
Magnetic Data ◽  
The West ◽  
Near Surface ◽  
The North ◽  
New Information ◽  
Geophysical Surveys ◽  
Volcanic Formation ◽  
Electrical Sounding ◽  
Surface Occurrence

ABSTRACTMagnetic and resistivity geophysical surveys conducted across the only known exposure of the East Kirkton Limestone have produced new information upon its extent. This is important to determine because of its unique faunal assemblage and possible hot spring deposition, suggesting a potential for precious metal mineralisation. Magnetic anomalies are attributed to basalts within the Bathgate Hills Volcanic Formation. Modelling of the magnetic data demonstrates a general dip to the west of about 25°, and the presence of significant local faulting. Modelling of vertical electrical sounding data shows the East Kirkton sequence (the limestone and associated beds) to be a low resistivity layer within the more highly resistive volcanic sequence. The East Kirkton sequence is seen to deepen to the west, and also to the north probably by faulting. Therefore the present exposure is the only near surface occurrence of the East Kirkton Limestone locally, but within the area of the survey no lateral limits to the formation are observed.

3-D inversions of magnetic gradiometer data in archeological prospecting: Possibilities and limitations

Geophysics ◽  
2000 ◽  
Vol 65 (3) ◽  
pp. 849-860 ◽  
Author(s):  
Jörg Herwanger ◽  
Hansruedi Maurer ◽  
Alan G. Green ◽  
Jürg Leckebusch
Keyword(s):  
Magnetic Susceptibility ◽  
Synthetic Data ◽  
Vertical Gradient ◽  
Magnetic Data ◽  
Data Sets ◽  
Northern Switzerland ◽  
Recorded Data ◽  
Susceptibility Contrast ◽  
Depth Extent ◽  
Archeological Site

A vertical‐gradient magnetic system based on optically pumped Cesium sensors has been used to map subtle magnetic anomalies across infilled pit houses and ditches at a medieval archeological site in northern Switzerland. For estimating the locations and dimensions of these features from the recorded data, we have designed and implemented an appropriate inversion scheme. Tests of this scheme on realistic synthetic data sets suggested that suitable minimum magnetic susceptibility contrasts and smoothing parameters for the inversion may be directly extracted from the data. Inversions with minimum magnetic susceptibility contrasts generated causative bodies with maximum plausible sizes. By using higher magnetic susceptibility contrasts, a complete suite of models that matched the data equally well was produced. To constrain better the magnetic susceptibility constrast within a selected area of the archeological site, shallow samples of topsoil and sediment were analyzed in the laboratory. An inversion based on the measured magnetic susceptibility contrast yielded reliable estimates of the locations, 3-D geometries, and sizes of two small pit houses. The depth extent of one pit house was subsequently verified by shallow drilling. We concluded that inversions of vertical‐gradient magnetic data constrained by magnetic susceptibility or shallow borehole information are rapid and inexpensive means of providing key knowledge on the depth distribution of inductively magnetized bodies.

Gaussian envelope for 3D geomagnetic data inversion

Geophysics ◽  
2003 ◽  
Vol 68 (3) ◽  
pp. 996-1007 ◽  
Author(s):  
Fabio Caratori Tontini ◽  
Osvaldo Faggioni ◽  
Nicolò Beverini ◽  
Cosmo Carmisciano
Keyword(s):  
Synthetic Data ◽  
Iteration Scheme ◽  
Source Distribution ◽  
Magnetic Data ◽  
Real Field ◽  
Inversion Method ◽  
Data Set ◽  
Data Inversion ◽  
Geomagnetic Data ◽  
Subjective Input

We describe an inversion method for 3D geomagnetic data based on approximation of the source distribution by means of positive constrained Gaussian functions. In this way, smoothness and positivity are automatically imposed on the source without any subjective input from the user apart from selecting the number of functions to use. The algorithm has been tested with synthetic data in order to resolve sources at very different depths, using data from one measurement plane only. The forward modeling is based on prismatic cell parameterization, but the algebraic nonuniqueness is reduced because a relationship among the cells, expressed by the Gaussian envelope, is assumed to describe the spatial variation of the source distribution. We assume that there is no remanent magnetization and that the magnetic data are produced by induced magnetization only, neglecting any demagnetization effects. The algorithm proceeds by minimization of a χ2 misfit function between real and predicted data using a nonlinear Levenberg‐Marquardt iteration scheme, easily implemented on a desktop PC, without any additional regularization. We demonstrate the robustness and utility of the method using synthetic data corrupted by pseudorandom generated noise and a real field data set.

3-D joint inversion of airborne gravity gradiometry and magnetic data using a probabilistic method

2020 ◽  
Vol 223 (1) ◽  
pp. 301-322
Author(s):  
Meixia Geng ◽  
J Kim Welford ◽  
Colin G Farquharson ◽  
Alexander L Peace ◽  
Xiangyun Hu
Keyword(s):  
Magnetic Susceptibility ◽  
Physical Properties ◽  
Covariance Matrix ◽  
Correlation Coefficient ◽  
Joint Inversion ◽  
Probabilistic Method ◽  
Probabilistic Approach ◽  
Magnetic Data ◽  
Airborne Gravity ◽  
Cross Covariance

SUMMARY A probabilistic approach is presented for jointly inverting gravity gradient and magnetic data for 3-D subsurface distributions of density and magnetic susceptibility. The coupling of the physical property models is incorporated in the inversion by using a cross-covariance matrix of density and magnetic susceptibility. This enables structural similarity such as the orientation and spatial extent of the sources and cross-variance between the two physical properties to be incorporated. A user-defined correlation coefficient can control the level of similarity between the two models. By applying a marginalizing algorithm in the joint inversion, the inversion domain is allowed to be partitioned into various zones, each of which can have its own covariance, cross-covariance matrix, as well as correlation coefficient, depending upon the feature and similarity of sources. Thus, sources with different shapes, sizes and relationships between the two physical properties can be simultaneously recovered. The validity of the method is verified using three synthetic examples, which demonstrate how incorrect parameters of the cross-covariance matrix affect the inverted results. Finally, the proposed method is successfully applied to full tensor gradiometry and magnetic data collected over the Budgell Harbour Stock (BHS) intrusion in north-central Newfoundland, Canada. Compared with models generated from independent inversions, better definition and localization of the main intrusion, as well as associated lamprophyre dykes at shallow depth, are achieved by using the joint inversion. The resolved physical properties for the intrusions show good agreement with field observations of lamprophyre dykes in proximity to the BHS.

Imaging of 3D electromagnetic data at low-induction numbers

Geophysics ◽  
2012 ◽  
Vol 77 (4) ◽  
pp. WB47-WB57 ◽  
Author(s):  
Marco A. Pérez-Flores ◽  
Ricardo G. Antonio-Carpio ◽  
E. Gómez-Treviño ◽  
Ian Ferguson ◽  
S. Méndez-Delgado
Keyword(s):  
Synthetic Data ◽  
3D Models ◽  
Two Dimensions ◽  
Imaging Data ◽  
Magnetic Dipoles ◽  
Near Surface ◽  
Conductivity Distribution ◽  
Electromagnetic Data ◽  
Depth Discrimination ◽  
Electromagnetic Measurements

We expressed electromagnetic measurements at low induction numbers as spatial averages of the subsurface electrical conductivity distribution and developed an algorithm for the recovery of the latter in terms of the former. The basis of our approach is an integral equation whose averaging kernel is independent of the conductivity distribution. That is, the recovery of conductivity from the measurements leads to a linear inverse problem. Previous work in one and two dimensions demonstrated that using a kernel independent of conductivity leads to reasonably good results in quantitative interpretations. This study extended the approach to 3D models and to data taken along several profiles over a given area. The algorithm handles vertical and horizontal magnetic dipoles with multiple separations for appropriate depth discrimination. The approximation also handles issues like negative conductivity measurements, which commonly appear when crossing near-surface conductors. This happens particularly when using vertical magnetic dipoles; whose averaging kernel has significant negative weights in the space between the dipoles, something that does not happen for the horizontal dipoles. In general, the more complex the kernel, the more complicated the signature of any given anomaly. This makes qualitative interpretations of pseudosections somewhat difficult when dealing with more than one conductive or resistive body. The algorithm was validated using synthetic data for imaging data from horizontal or vertical coils or from a combination of them. Imaging of field data from a mine tailings site recovered a shallow 3D conductive anomaly associated with the tailings.

scholarly journals First geophysical results in the Archeological sites of Θούρια (Péloponnèse, Hellas) and Sibari-Thurii (southern Italy)

2018 ◽  
Vol 40 (3) ◽  
pp. 1080
Author(s):  
B. Di Fiore ◽  
D. Chianese ◽  
A. Loperte ◽  
G. Conte ◽  
A. Dibenedetto ◽  
...  
Keyword(s):  
High Resolution ◽  
Information Quality ◽  
Southern Italy ◽  
Magnetic Measurements ◽  
Early Stage ◽  
Geophysical Methods ◽  
Magnetic Data ◽  
Near Surface ◽  
Data Inversion ◽  
Near Surface Geophysics

High resolution techniques for data acquisition and processing procedures are increasingly applied in near-surface geophysics for archaeology. In this paper we present the preliminary results of two geophysical measurements campaigns aimed to the investigation of buried remains in the archaeological sites of Θουρία (Péloponnèse, Hellas) and Sibari (Southern Italy). In the first field survey the geophysical approach involved the integrated application of the geoelectrical and magnetic methods and an innovative tomographic analysis for the inversion of both resistivity and magnetic data. In the second case, we carried out high resolution magnetic measurements, interpreted by means of the use of an appropriate filtering procedure. The applied data inversion allows us to provide reliable space patterns of the most probable specific target boundaries, improving the information quality of geophysical methods. The results obtained at this early stage of data processing confirm some archaeological hypothesis about the investigated areas and confirm that the use of integrated geophysical methods allows the archaeologists to reduce the time and the costs of their surveys.

3D inversion for multipulse airborne transient electromagnetic data

Geophysics ◽  
2016 ◽  
Vol 81 (6) ◽  
pp. E401-E408 ◽  
Author(s):  
Yunhe Liu ◽  
Changchun Yin
Keyword(s):  
Mineral Exploration ◽  
Synthetic Data ◽  
Underground Structures ◽  
Sensitivity Matrix ◽  
Near Surface ◽  
Power Pulse ◽  
Data Inversion ◽  
Transient Electromagnetic ◽  
1D Modeling ◽  
The Time Domain

Multipulse airborne transient electromagnetic (ATEM) systems transmit one high-power pulse and one low-power pulse containing more high-frequency EM signals. Such systems have better near-surface resolutions while maintaining the depth of exploration of other conventional systems. ATEM systems are especially suitable for geologic mapping and mineral exploration. The inversion of multipulse ATEM data has been mainly limited to 1D modeling, which is not suitable for complex underground structures. We have investigated an algorithm for 3D multipulse ATEM data inversion based on direct Gauss-Newton optimization with quite-fast convergence. The forward problems were solved in the frequency-domain based on the secondary scattered electrical field equation, and then the inverse Fourier transform and the convolution with transmitting waveform were applied to calculate the arbitrary waveform response and sensitivity matrix in the time domain. To optimize the number of computations and memory, we further used an EM “footprint” concept in our inversions to reduce the forward model size and sparse the sensitivity matrix. The inversion results of synthetic data showed that our 3D algorithm is very effective for inverting the multipulse data with results combining advantageous resolutions of different transmitting pulses. Finally, we applied our algorithm to invert real survey data obtained at McMurray, Alberta, Canada, to further test its effectiveness.

Near-surface geophysical surveys at the Duport gold deposit, Ontario, Canada: Relating airborne responses to small-scale geologic features

2016 ◽  
Vol 4 (3) ◽  
pp. SH39-SH60 ◽  
Author(s):  
Ian J. Ferguson ◽  
Jeffrey B. Young ◽  
Becky J. Cook ◽  
Ashley B. C. Krakowka ◽  
Cassandra Tycholiz
Keyword(s):  
Magnetic Susceptibility ◽  
Gold Deposit ◽  
Magnetic Anomaly ◽  
Geophysical Data ◽  
Data Availability ◽  
Electromagnetic Response ◽  
Small Scale ◽  
Near Surface ◽  
Geophysical Surveys ◽  
Enhanced Conductivity

Near-surface geophysical measurements using magnetometer, magnetic susceptibility, terrain conductivity, and time-domain electromagnetic instruments were made at the shear-hosted Duport gold deposit on Cameron Island in Shoal Lake, western Ontario, Canada, to help relate airborne total magnetic intensity (TMI) and helicopter electromagnetic survey data to small-scale geologic features. The magnetic airborne response provides a weak indication of a narrow anomaly within the Duport deformation zone, and the airborne electromagnetic response provides an indication of enhanced conductivity in the northwest of Cameron Island. In contrast, surface magnetic responses are dominated by the narrow 10,000–15,000 nT magnetic anomaly of a talc-chlorite-dolomite schistose basalt unit, a feature barely visible in the airborne TMI data. This geologic unit hosts the veins containing gold mineralization, so the surface TMI data provide a valuable response for delineating the corresponding rocks. Modeling of the TMI data indicates that the unit has a susceptibility of up to 0.7 SI and a corresponding magnetite content of up to 20%. The TMI data also reveal along-strike variations in the magnetic anomaly providing information on the component of ultramafic rocks in the protolith of the unit. The surface geophysical data allow the enhanced conductivity in the northwest part of Cameron Island to be attributed to several narrow sulfidized zones containing up to 10% pyrrhotite. Additional conductive and positively magnetized zones are associated with concentrations of 5%–10% of magnetite. The near-surface geophysics at the Duport deposit provided a rapid and inexpensive method for defining the magnetic and electrical properties of the geologic units at the site and for defining the exact location, width, and internal structure of features observed in the airborne geophysical data. Availability of magnetic susceptibility measurements from a single drill core enhanced the accuracy of the interpretations and the ability to relate the near-surface geophysical responses to geologic features.

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