Magnetic Gradiometry Research Articles

Optimal total-field magnetometer configuration for near-surface applications

Within the geophysical community there has been considerable interest in developing instrumentation for measurement of magnetic gradients and in developing processing strategies to effectively interpret the collected magnetic gradient data (see, for example, TLE's Magnetic Gradiometry special section in 2006). There have been significant advances in sensor technologies in recent years, and true magnetic tensor gradiometry using SQUIDS (Superconducting Quantum Interference DeviceS) may soon be in reach, at least in an experimental form.

Archaeological prospecting at the Double Ditch State Historic Site, North Dakota, USA

AbstractA four‐year programme of remote sensing at the Double Ditch State Historic Site, a fortified earthlodge village in the Great Plains of North Dakota, USA, demonstrates the importance of combined prospecting methods for understanding the multidimensional nature of complex settlements. Aerial survey from a powered parachute acquired high‐resolution digital colour and thermal infrared imagery. The former distinguishes houses, borrow pits and ditches from middens and fill areas by changes in vegetation; the latter does the same through temperature variations that also highlight substantial village deposits and century‐old excavations. Magnetic gradiometry reveals two previously unknown fortification systems that vastly increase the settlement's area and projected population to perhaps 2000 individuals. Vast numbers of food storage pits that supported that population are also indicated. Vertical soundings and area surveys of magnetic susceptibility aid in the understanding of storage pit magnetism and anthropogenic soil mounding and removal. A high‐resolution digital model of topography documents surface expressions caused by ditches, houses, borrow pits and mounds; when combined with overlays of geophysical data understanding of remote sensing responses is improved and relationships between large mounds with village defenses are made clear. Ground‐penetrating radar gives insights into mounded midden interior forms and yields details about house interior components, while electrical resistance results improve definition of middens, other depositional areas, houses and borrow pits. This multisensor remote sensing programme reduced excavation costs by allowing a wide variety of archaeological features of interest to be targeted accurately. Excavations confirmed anomaly identifications and established a chronology that documents late fifteenth century origins to an ultimate abandonment about AD 1782. Copyright © 2008 John Wiley & Sons, Ltd.

Magnetic field gradients from the ST‐5 constellation: Improving magnetic and thermal models of the lithosphere

We report the development of a new technique (magnetic gradiometry) for satellite‐based remote sensing of the lithosphere. The measurements reported here represent the first systematic measurements of lithospheric magnetic field gradients, and were collected from a spinning spacecraft. The three‐satellite ST‐5 mission collected vector magnetic field observations at 300–800+ km altitudes over mid and high‐northern latitudes in 2006. Away from the auroral oval, and over the continents, the gradients of the low altitude (<400 km) total anomaly field are dominated by lithospheric magnetic fields. Using a seismic starting model, and magnetic field observations from ST‐5 and other recent satellite missions, we demonstrate how these techniques can be used to improve our knowledge of the processes involved in the thickened crust of the Colorado Plateau and the Sierra Madre Occidental.

A New Algorithm for the Seismo-Electrokinetic effect generated by different seismic waves and it’s application to shallow depths.

A new approach to modeling the seismoelectric (seismo-electrokinetic) effect is presented. It is based on the concept that at a specific time each location on the wave front that proceeds through the medium imparts equal pressure on the solid matrix and pore space. Consequently two fundamental equations contribute to the basis for the calculation of the electrokinetic effect (EKE): the first describes the relative displacement of particles in the sediment at the arrivals of the seismic wave and second, the transient streaming potential, induced in a pore, when the pressure pulse is applied to a cylindrical pore. To calculate the EKE on the surface the following are required: calculate the pressure gradient in a single pore caused by a seismic wave, the transfer from EKE arising in a single pore to the porous media, integrate the EKE signals all over the sediment space or layer and access the electrokinetic coefficients. A new algorithm has been developed. Being able to model field data mathematically shows the viability of this approach and allows the estimation of permeability of layers. To obtain a realistic geological and groundwater model test site, several geophysical methods were applied to a small area (shallow refraction seismic, DC resistivity, SP, FDEM, magnetic gradiometry). Comparing the EKE result modelled with the new algorithm with the above methods shows that the seismo-electrokinetic method can be successfully used for the exploration for ground water at shallow depths.

Two‐dimensional inverse filtering for the rectification of the magnetic gradiometry signal

ABSTRACTThe magnetic difference, the quantity measured by magnetic gradiometers, is considered to be the convolution between a function that controls the anomaly pattern and another that controls the strength signal. These are called shape and amplitude functions, respectively. They are distinct and analytically determined; thus, after the assessment of a suitable model, its shape function can be inverted to serve as a filter. If this filter is next convolved with the measured field, a series of amplitude functions is recovered, provided that the subsurface structures can be simulated by a combination of a number of models similar to the one whose anomaly was inverted. The recovered series is essentially the subsurface distribution of the amplitude function.Alternatively, the scheme can be viewed as a transformation of the original field of magnetic differences. The signals, transformed in this context, comprise rectified monopolar versions of the originals, positioned directly above the centre of the targets. Furthermore, their amplitude is a measure of the magnetization of the targets. Thus, the new anomalies can be viewed as a kind of magnetization or susceptibility mapping. Inversion filters are computed in the Wiener mode by inverting the shape function of the anomalies caused by vertical sided finite prisms. This particular model is appropriate for a wide variety of targets that are commonly met in archaeological prospecting. Converting the magnetic signal from the dual lobe pattern to single monopolar anomalies cause them to resemble the usual outcome of resistivity exploration. In this context, this work aims to form a scheme that makes the magnetic gradiometry outcome directly comparable to the outcome of resistivity mapping. Furthermore, it can be applied without contradicting or excluding any other processing operation.The method is implemented in a FORTRAN program that is reasonably user‐ friendly. The efficiency of the scheme is tested both on synthetic and real data sets.

An Archaeological Study of the Battle of the Boyne at Oldbridge, Co Meath

This paper outlines the results of a pilot archaeological study carried out on the Oldbridge Estate, Co. Meath, Ireland, in advance of its development for the presentation of the events of the Battle of the Boyne which was fought by the forces of King William III and James II on 1st July 1690. Archival research including the examination of museum collections, cartographic sources, contemporary artistic depictions of the battle and recent aerial photographic coverages was followed up by the use of a range of archaeological techniques including systematic surface collection survey, soil phosphate analysis, magnetic gradiometry, metal detecting and limited test excavation. Additionally, marine geophysics equipment was used to profile the character of the River Boyne, especially in the area of the main crossings. The study revealed important new evidence including the establishment of the location of Oldbridge village, a pivotal feature in the conduct of the battle, the confirmation of the location of the first engagement on the day of the battle and the first systematically recovered artefacts from the battlefield.

Multi‐temporal geophysical survey of a Roman bath complex in Montegrotto Terme (Padova, northern Italy)

AbstractBetween 2000 and 2005, several geophysical surveys were carried out by the staff of the Department of Geosciences, University of Padova, on the site of the Terme Neroniane in Montegrotto, near Padova, covering a whole complex of Roman spa buildings. This work represented a good opportunity to compare the effectiveness of various techniques (ground‐penetrating radar, magnetic gradiometry and electrical resistivity tomography) and to seek the reasons for differing results. Depth, contrast between composition of the geological background and building materials, good state of structural preservation, and the lack of settlement following the Roman age supported experimental results. Old and more recent excavations, conducted in parallel with the University of Padova surveys, enabled continual comparisons between geophysical models and evidence from the site. Copyright © 2007 John Wiley & Sons, Ltd.

Integrated Remote Sensing and Excavation at Double Ditch State Historic Site, North Dakota

A four-year program of remote sensing at the Double Ditch State Historic Site (32BL8) demonstrates the utility of combined prospecting methods for understanding complex settlements when combined with traditional excavation methods. Magnetic gradiometry revealed countless subterranean storage pits, hearths, and two previously unknown fortification systems that vastly increase the settlement's area and projected population to perhaps 2,000 individuals. Electrical resistance surveys helped define middens, other depositional areas, houses, and earth-borrowing pits. Ground-penetrating radar yielded details about ditch, house, and mounded midden interior forms. Aerial survey from a powered parachute acquired high-resolution digital color and thermal infrared imagery. The former distinguished houses, borrow pits, and ditches from middens and fill areas by changes in vegetation; the latter did the same through temperature variations that also highlighted historic excavations. High-resolution topographic survey allowed documentation of topographic expressions caused by ditches, houses, borrow pits, and mounds. The remote sensing program reduced excavation costs by targeting features. Excavations confirmed anomaly identifications and established a chronology that documents late-fifteenth-century origins to an ultimate contraction in the eighteenth century, with abandonment after a smallpox outbreak about A.D. 1785. Evidence suggests that large mounds formed integral components of the village"s defenses. Excavations also reveal extensive earth moving for mound building, earthlodge coverings, and other reasons still unclear. This practice caused the truncation or obliteration of many earlier archaeological features and forces realization that long-occupied settlements were fluid through time with continual reworking of deposits, and complex depositional, use, and formation histories.

Impact of magnetic survey design on the imaging of small archeological objects: Practicability in gradiometer surveying

There are various strategies for magnetic surveying. Most are designed to capture a wide range of sources from near surface to deep subsurface structure. Magnetics, particularly magnetic gradiometry, is often used for discovering small objects at very shallow depths that have environmental or archaeological importance.

Magnetic multipoles: forward modelling the gradient tensor, and hybrid global/linearised inversion

With magnetic gradiometry emerging as a new tool for geophysical exploration, the mathematical modelling of gradient fields is necessary for interpretation of field measurements. Magnetic multipoles may be useful for the modelling of complex geological sources, and so formulae for their field responses are presented here in Cartesian form. The formulae required for the forward modelling of the magnetic gradient tensor can be expressed as a set of linear equations. This fact is exploited to create a hybrid global and local inversion technique whereby simulated multipoles must satisfy the gradient tensor measurements for more than a single measurement. The inversion process yields subsurface images showing the variation of multipole moment components, as each component is calculated to satisfy measured components of the magnetic gradient tensor. The images exhibit high probability (or small variance) if a multipole is found. The inversion routine is quick, and can be applied automatically to data as it is being collected, or applied to an already complete data set.

New archaeological discoveries through magnetic gradiometry: The early Celtic settlement on Mont Lassois, France

The burial complex of the “Lady of Vix” was discovered and excavated in the 1950s at the foot of Mont Lassois (Figure 1), a mountain situated close to the town of Chatillion-sur-Seine in the Bourgogne region of France. The assemblage of the burial goods was rather extraordinary, including such items as an artfully crafted golden necklace with winged horses and a voluminous wine-mixing vessel, probably made in a Greek workshop, capable of holding 1100 liters. According to archaeological research, this member of the aristocracy must have lived during the period between 550 and 500 BCE. Several large-scale geophysical research projects were undertaken in the vicinity of the burial complex during the last few years to learn the history of the settlements from that period. This led not only to the discovery of the expected settlement structures, but also to the discoveries of the remains of several large burial mounds that had been erected along the River Seine.

Introduction to this special section: Magnetic gradiometry

The earth's magnetic field at any point on or near the earth's surface is the vector sum of the contributions from two main sources; the primary field due to the dynamo in the earth's liquid core and the crustal field from the magnetic mineral content of local rocks. This vector has both an orientation and an amplitude. Move slightly in any direction and this orientation and amplitude will vary. Introduce a highly magnetic rock formation into an otherwise homogenous host, and the local magnetic vector will change. In a three-dimensional world, there are nine (3×3) spatial gradients forming a tensor which defines the anomalous field.

Geophysical investigation of the June 6, 1944 D‐Day invasion site at Pointe du Hoc, Normandy, France

A near‐surface geophysical survey at the D‐Day invasion site on top of the cliffs at Pointe du Hoc in Normandy, France, was carried out using ground‐penetrating radar, electromagnetic induction and magnetic gradiometry equipment. The subsurface targets of investigation are predominantly buried concrete and steel structures and earthworks associated with the German coastal fortifications at this strongpoint of Adolf Hitler’s Atlantic Wall. The targets are readily detectable, embedded within the vadose zone of a weakly magnetic, electrically resistive, loess soil cover. The radar and electromagnetic induction responses lend themselves to plan‐view imaging of the subsurface, while the magnetics data reveal the presence of buried magnetic bodies in a more subtle fashion. Several intriguing geophysical signatures were discovered, including what may be the buried remains of a railway turntable, ordnance fragments in the bomb craters, a buried steel‐reinforced concrete trench, and a linear chain of machine‐gun firing positions. Geophysical prospecting is shown to be a very powerful tool for historical battlefield site characterization.

Integrating multidimensional geophysical data

AbstractSurveys that utilize multiple geophysical methods offer greater insights about the subsurface because each one generally yields different information. Common approaches to integrating or ‘fusing’ multidimensional geophysical data are investigated utilizing computer graphics, geographical information system (GIS), mathematical and statistical solutions. These approaches are synthesized into graphical, discrete and continuous domains. It is shown that graphical approaches allow complex visualizations of the subsurface, but only images are generated and their dimensionality tends to be low. Discrete methods incorporate any number of geophysical dimensions, allow application of powerful Boolean operations, and produce unambiguous maps of anomaly presence or absence, but many of these methods rely on arbitrary thresholds that define only robust anomalies. Continuous data integrations offer capabilities beyond other methods because robust and subtle anomalies are simultaneously expressed, new quantitative information is generated, and interpretive data are derived in the form of regression weights, factor loadings, and the like, that reveal interrelationships and underlying dimensionality. All approaches are applied to a common data set obtained at Army City, Kansas, a World War I era commercial complex that serviced troops in nearby Camp Funston (now Fort Riley). Utilizing data from six geophysical surveys (magnetic gradiometry, electrical resistivity, ground‐penetrating radar, magnetic susceptibility, soil conductivity, aerial thermography), various data integrations reveal the structure of this nearly forgotten town. Copyright © 2005 John Wiley & Sons, Ltd.

Geophysical investigation of the site of the former monastic settlement, Clonard, County Meath, Ireland

AbstractClonard, in County Meath, Ireland was a major ecclesiastical centre from the early sixth century to the twelfth century and buildings associated with the monastery were in existence until the late eighteenth century. However, today no extant buildings or features associated with the monastery are known. The geophysical investigation undertaken in this project has uncovered many anomalies which may be related to the monastic settlement at Clonard. A significant number of linear anomalies have been detected east of St Finian's church using magnetic gradiometry and twin electrode resistance surveying. Many of these anomalies are probably field boundaries; however, a much greater diversity of geophysical responses is located south and west of St Finian's church. A subsquare enclosure of 60 m sides is adjacent to a 300 m long palaeochannel. A fine network of intersecting low resistance anomalies probably represent former artificial drainage channels. Three large areas associated with anomalous magnetic readings were located which might indicate sites of human activity. In addition, there are four distinct zones which have large concentrations of high resistance values suggesting the presence of walls or buildings. Resistivity and ground‐penetrating radar depth slices show that one of these zones has characteristics which indicate the presence of an east–west aligned building approximately 7 m wide in a north–south direction and about 15 m long in an east–west direction. Copyright © 2005 John Wiley & Sons, Ltd.

Geophysical investigations at the ancient royal site of Rathcroghan, County Roscommon, Ireland

The pre-Christian royal site of Rathcroghan (ancient Cruachain) is situated about 5 km to the northwest of the village of Tulsk in Co. Roscommon, Ireland. The archaeological remains of over 50 visible monuments, of various type and antiquity, are concentrated over an area of some 10 km2. Prominently located at its approximate centre is Rathcroghan Mound, averaging 88 m in diameter and 6 m in height. The ArchaeoGeophysical Imaging Project, funded by the Heritage Council, has completed a major programme of archaeological and geophysical surveys at Rathcroghan, concentrating primarily on Rathcroghan Mound. Detailed microtopographic surveys have enabled the production of computer generated maps and terrain models. These have aided the identification of a number of low-relief features that had remained unnoticed during reconnaissance field inspection. Magnetic susceptibility survey has delimited zones of activity on the top of the mound and has also identified an ancient field system surrounding it. Magnetic gradiometry survey, conducted at differing sample resolutions, complements and extends the results of the magnetic susceptibility survey. An enclosure, 370 m in diameter, is identified encircling Rathcroghan Mound and a number of its satellite monuments. Also identified are the subsurface remains of foundation trenches for a series of large-scale, circular structures or enclosures, approximately 20 m to 32 m in diameter, which once occupied the summit of the mound. Another, 26 m in diameter, lies immediately to its north-northeast. Electrical resistance survey on Rathcroghan Mound, using different array types, confirm the presence of a series of circular and arcuate features, many of which correspond to those noted in the magnetic surveys. The compelling results of the Rathcroghan survey have enabled more extensive and meaningful archaeological comparisons to be made between this and the other major, pre-Christian, royal centres of Tara (Temhair), Co. Meath, Navan Fort (Emain Macha), Co. Armagh and Knockaulin (Dún Ailinne), Co. Kildare. Copyright © 2005 John Wiley & Sons, Ltd.

Environmental geophysics investigations in urban areas

Dr John M. Reynolds, managing director and principal geophysicist at Reynolds Geo- Sciences, presents some case histories illustrating how the obstacles of noisy urban sites have been overcome to produce highly useful interpretable data sets, often far exceeding the expectations of the clients involved. Urban areas represent some of the most challenging environments in which to undertake geophysical investigations. Restricted space, the necessity for traffic management, high ambient noise (electrical and vibration), underground utilities, etc. all help to complicate the design and setting out of the geophysical profiles or grids. In many cases, such sites could not be investigated as the acquisition methods, the type of equipment and subsequent data processing software were not readily available. Over the last few years, significant changes in the capabilities of the equipment and software have permitted geophysical surveys to be undertaken in these hitherto inappropriate areas. The first case history at Field Road, Reading, Berkshire, was by far the largest, most complex and technically challenging. It shows how with very careful survey design, particular attention to data acquisition, and specialist data reduction and modelling, a labyrinth of shallow chalk mine galleries were mapped successfully using micro-gravity. The second involved a housing development in northwest London where the intrusive investigations had failed to satisfy the requirements of the Environment Agency, the environmental regulator in England and Wales (Scotland has a separate but similar regulatory body). Furthermore, the client and his engineering advisers were sceptical about the use of geophysics. Despite the technical difficulties of a complex partially developed site, the geophysical investigation demonstrated clearly the benefits of such a survey when integrated with intrusive investigations targeted on the basis of the geophysical survey. The client and his advisers became keen advocates of the use of geophysics once the results were made known. The third example is a complete contrast to the first two in that, despite the site being in a town, it comprised open space. The method used was the tried and tested and now very well established technique using a Geonics EM31 ground conductivity meter. (Detailed descriptions of geophysical methods discussed here have been presented by Reynolds (1997)). It demonstrates that even simple, long established techniques used very simply can produce some very clear and graphic results. The last case history involved the development of a former landfill site in south Wales. The 12.6 ha site was being cleared and earthworks to level the site were well in progress. At this late stage, it was suggested that steel drums might have been buried on the site but their location was unknown. A detailed geophysical investigation on the extremely busy construction site was undertaken using predominantly magnetic gradiometry and drum graves were located successfully and proven by excavation. The additional feature of all these case histories has been the involvement of what has subsequently become known as an ‘Engineering Geophysics Adviser’, an extremely experienced geophysicist (or group of geophysicists) independent of the geophysical contractor. This role has been advocated within the most recent guidelines for the use of engineering (and environmental) geophysics (McDowell et al., 2002).

Practicalities, pitfalls and new developments in airborne magnetic gradiometry

Scott Hogg, whose company Scott Hogg & Associates, based in Toronto, Canada offers services to the exploration and airborne geophysical industry, reviews the latest refinements in the revitalised use of airborne magnetic gradiometers for the mining industry. The oil and gas exploration industry spurred the development of the first airborne magnetic gradiometers. The motivation was to use Euler’s equation with measured vertical gradient to calculate depth to magnetic source. Aeroservice introduced both a helicopter system and a fixed wing towed bird system in the 1960s. Interest in the Euler theory for magnetic analysis was rekindled in the mining industry 20 years later and continues to be the foundation of many new interpretation methods. In Canada, the GSC developed a vertical magnetic gradiometer in the 1970s. A national mapping programme began in the early 1980s and in response Canadian airborne survey contractors created a variety of fixed-wing and helicopter systems. The mining sector’s interest in airborne gradient measurement was based primarily on the increased spatial resolution and detail: small anomalies on the flanks of large features could be clearly resolved. Calculated vertical gradient maps, produced by simple filtering of total field, were found to provide almost the same benefit as measured gradient, at less cost, and the commercial interest in measured vertical gradient faded. Geometrics introduced a horizontal gradiometer in 1983. This development was of particular significance since, at the same time, they incorporated a technique developed by Nabighian and Hansen to derive a pseudo total field residual from measured horizontal gradients. The demise of the Geometric survey division took horizontal gradients off the horizon for a while. A decade later, this same concept was used by Nelson of the NRC, and more recently implemented in a variety of forms by De Beers and others. Geodass in Botswana, now Fugro, introduced the first 3- axis gradiometer in the early 1990s. In conjunction with the gradient measurements it provided a variety of compilation and mapping services that made use of the information. In Canada, Terraquest was the first to provide horizontal gradient measurement and Goldak the first to provide a full 3-axis fixed-wing gradiometer. At present, almost all of the airborne contractors offer horizontal gradient systems, and several can now provide full 3-axis configurations for simultaneous vertical and horizontal gradient measurement. Considerable interest in magnetic gradient measurement has arisen over the past few years. Some of the benefits of gradient are well founded and some are overstated and many are poorly understood. Magnetic gradient measurements can be used to advantage in interpretation. Vertical gradient maps, analytic signal maps, and a host of Euler based methods all use gradient information. The gradient information for these purposes may be calculated or measured. This review addresses methods that rely specifically on measured, not calculated, gradient. At present there are three such primary applications. The first is the potential to avoid diurnal interference, the second is to correct total field for variations in aircraft altitude, and the third is to make significant improvements in the accuracy and resolution of magnetic maps.

D‐Day: geophysical investigation of a World War II German site in Normandy, France

AbstractAlthough military sites have long been the subject of geophysical investigation, those associated with recent military campaigns are noticeably rare. In fact, although such techniques are increasingly used to identify the unwanted legacy of modern warfare, such as unexploded ordnances or mass burials, they have not been used to investigate the archaeology of such conflict. The survey here attempts to identify the archaeology of one military site (Puits d'Herode) that was part of Hitler's Atlantic Wall and to identify the key elements associated with its D‐Day defences. Magnetic gradiometry, twin‐probe resistance and ground‐penetrating radar were used at this site and, as a result of the nature of the buried archaeology, the magnetic technique proved most valuable. Trenches and bunkers previously known from aerial photographs were located accurately and a track believed to relate to the post‐D‐Day advance of the Allied forces across Puits d'Herode was also identified. The use of geophysical data in the investigation of this site has provided a new avenue to examine previously difficult topics such as the quality of intelligence available at the time of the conflict and the level and accuracy of shelling; although these may seem unlikely targets for archaeological geophysics, they are important to military historians for whom eye witness accounts are becoming increasingly scarce. Copyright © 2004 John Wiley & Sons, Ltd.

Soil and susceptibility: aspects of thermally induced magnetism within the dynamic pedological system

AbstractMagnetic surveys now form part of the panoply of techniques used in archaeological investigations. Techniques such as magnetic gradiometry depend on bodies of soil developing enhanced magnetic susceptibilities. This enhancement, whether via heating or by ongoing pedological processes, is due to reduction and oxidation cycles within the soil. The salient point of this paper is that, the soil is a dynamic medium in which iron is subject to ongoing soil process. Several case studies are examined to illustrate these points. Copyright © 2002 John Wiley & Sons, Ltd.