Borehole Magnetometry Test for Evaluating a Caisson’s Reinforced Depth: Experimental Results and Theoretical Modeling

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The borehole magnetometry (BM) test was performed to evaluate the foundation reinforcement depth at a site where a telecommunication tower is supported by a single 2.3-m diameter caisson foundation of known as-built design. An OPTV probe containing a three-axial fluxgate magnetometer was lowered into a vertical borehole 2.35 m distant from the center of the caisson and the profiles for the total magnetic field flux density and its vertical component were acquired and used to generate, by subtracting the IGRF background, the profiles for the total and vertical component, anomalous magnetic field. Four distinct graphical methods were used to evaluate the reinforced depth from the anomalous profiles and their first- and second-order derivatives. Two of the methods, including the one proposed in this study, based on locating the inflection points in the derivative profiles, evaluated the reinforced depth very close to the as-built depth (8.0 m). The reinforcement intensity of magnetization was then evaluated using the B_A profile and a method based on the bipolar model. Theoretical modeling of B_A and B_(z,A) and the derivative profiles was then performed, using a 3-D prismatic model. By comparing modeled and experimental results, the induced magnetization was found to be an unsuited modeling assumption, with remanent magnetization being a better representation of the magnetic field around the caisson´s steel reinforcement, in agreement with the theory, given the high Koenigsberger ratio for steel. Also, the modeling revealed the need for a more complex representation of the magnetic sources, with added prisms to represent the effects of a magnetically-noisy environment and above-ground structures, as well as the presence of inhomogeneity and polarization changes along the reinforcement length.

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... Jalinoos et al. (2006) associate the lowermost peak in the magnetic intensity field with the pile bottom location. Hemsi et al. (2021) analyze the decay behavior and associate the pile tip with the lowermost inflection point in the magnetic derivative field. ...
Technical information about deep foundations of built structures may in some cases be unavailable or incomplete due to the lack of construction documentation and electronic media in which the reports are stored. This problem is particularly relevant for telecommunication towers constructed two-to-three decades ago but still under operation and subject to new loading demands from expanding networks. Nondestructive evaluation with geophysical methods has been applied to determine the depth to the bottom of deep foundations, a key parameter to evaluate their bearing capacity and settlements. This is done by lowering a borehole magnetometry (BM) sensor down a drillhole installed close to the unknown structure, thereby measuring the magnetic field along the testing drillhole. Determining the bottom depth of the structure using BMdata configures an edge detection problem: a classical problem in magnetic data interpretation, but in this case with poor data coverage (only a profile close to the structure) to characterize a complex magnetization distribution. Common practices to locate pile bottom from characteristic peak or inflection points in the observed magnetic anomaly may work at some sites but usually fail to account for the complex anomaly patterns from built structures. We summarize a set of numerical and field-work applications showing that no single characteristic attribute of the observed field can undoubtedly be used to determine the foundation depth. We verify that most of the measured field along the testing drillhole is disturbed by fields from facilities at the ground surface and develop a data inversion procedure enforcing data fitting to measurements at the lower portions of the drillhole. The efficacy of this technique is illustrated with field tests in different geologic scenarios with results compared with as-built documentation and/or independent evaluation with other methods.
Conference Paper
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This paper presents the results of a seismic test of recent application in Brazil, called parallel seismic. This test is intended to determine the maximum depth of a foundation element. Its application has been tested on a large diameter caisson foundation and the results were very satisfactory.
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Three (3) geophysical borehole logging methods were used for the determination of unknown depth of steel sheet piles. The logging methods included magnetic susceptibility, electrical induction, and fluxgate magnetometer test methods. The project involved widening of an existing navigation channel in the Port of Long Beach for navigation safety purposes. The depth of the steel sheet piles were needed in order to model the system for seismic performance and develop feasible dike and/or bulkhead strengthening schemes to facilitate dredging the channel. Geophysical logging was performed from three PVC-cased boreholes which were drilled within 3 ft (0.9 m) of the sheet piles with the clearest results obtained from the fluxgate magnetometer. The sheet piles bottoms were interpreted as a decrease in magnitude of the magnetic field. On the conductivity and magnetic susceptibility curves, sheet piles bottoms were indicated as the inflection points of the decreasing curves.
There are a significant number of bridges for which information regarding the foundation is missing or incomplete. It is extremely challenging to evaluate the performance of such unknown foundation bridges (UFB), particularly against scour or when their foundations are reused. For critical UFB, it is often necessary to estimate performance by developing an appropriate working model of subsurface foundation conditions. Typically, nondestructive testing (NDT) has served as the most viable alternative due to the costs and risk associated with excavation, coring, and probing. NDT can be quite difficult to perform and their results can contain significant uncertainty in highly urban settings. The primary objective of this study was therefore to compare performance of borehole NDT methods when evaluating the depth of two in-service unknown foundations (concrete-filled steel pipe piles and H-piles) in highly urban settings. The borehole magnetic, parallel seismic, borehole sonic, and borehole radar methods were implemented to determine the foundation bottom locations. Though uncertainty was present in all measurements, the borehole magnetometer and radar results proved the most conclusive. Parallel seismic testing did not yield any evidence of foundations due to issues with background noise and lack of direct access to the foundation. Likewise, borehole sonic testing was generally inconclusive due to issues with sensor directivity and attenuation. Borehole magnetometer estimated the depth to the foundation bottom as 8.6 m and 9.2 m at the two sites. Borehole radar estimates for the depth to foundation bottom compared favorably at 9.8 m and 8.0 m for the two sites. Given the borehole construction and depth to competent rock at the sites, these results for borehole magnetometer and radar were likely a minimum estimate for the location of the foundation bottoms. Such information can help evaluate long-term performance of this system as part of rehabilitation and reuse efforts.
Rock Magnetism, first published in 1997, is a comprehensive treatment of fine particle magnetism and the magnetic properties of rocks. Starting from atomic magnetism and magnetostatic principles, the authors explain why domains and micromagnetic structures form in ferromagnetic crystals and how these lead to magnetic memory in the form of thermal, chemical and other remanent magnetizations. The phenomenal stability of these magnetizations, providing a record of plate tectonic motions over millions of years, is explained by thermal activation theory. One chapter is devoted to practical tests of domain state and paleomagnetic stability; another deals with pseudo-single-domain magnetism. The final four chapters place magnetism in the context of igneous, sedimentary, metamorphic, and extraterrestrial rocks. This book will be of great value to graduate students and researchers in geophysics and geology, particularly in paleomagnetism and rock magnetism, as well as physicists and electrical engineers interested in fine-particle magnetism and magnetic recording.
As the quality of welded joints transfers to the load carrying capacity of structures, welded joints naturally become the object of diagnostic tests. Welded joints are mainly tested in order to find out the places with structural or mechanical non-homogeneities. It is caused by the fact that the existence of welding imperfections significantly decreases mechanical properties of the welded joint and especially its fatigue strength. The paper presents some results of diagnostic tests for welded joints of steel components carried out with the use of passive and active magnetic methods. Faults of the welded joint occurring within a tested object cause the phenomenon of magnetic anomalies existing over a tested surface and this phenomenon has been used to detect welding imperfections. A specially designed measurement matrix consisting of 768 one-axial miniature magnetic field sensors has been used to record the magnetic flux leakage. The paper includes a comparison of the effectiveness of diagnosing using the passive and active magnetic method. Moreover, it was shown that the magnetic diagnostics provides rapid inspection of quality of welded joints with no need for special preparation of joints for testing which is relevant for its practical applications and results in low costs of such tests.
Just a few meters below the Earth's surface lie features of great importance, from geological faults which can produce devastating earthquakes, to lost archaeological treasures! This refreshing, up-to-date book explores the foundations of interpretation theory and the latest developments in near-surface techniques, used to complement traditional geophysical methods for deep-exploration targets. Clear but rigorous, the book explains theory and practice in simple physical terms, supported by intermediate-level mathematics. Techniques covered include magnetics, resistivity, seismic reflection and refraction, surface waves, induced polarization, self-potential, electromagnetic induction, ground-penetrating radar, magnetic resonance, interferometry, seismoelectric and more. Sections on data analysis and inverse theory are provided and chapters are illustrated by case studies, giving students and professionals the tools to plan, conduct and analyze a near-surface geophysical survey. This is an important textbook for advanced-undergraduate and graduate students in geophysics and a valuable reference for practising geophysicists, geologists, hydrologists, archaeologists, and civil and geotechnical engineers.
The need for control data in interpretation of surface magnetometer surveys has led to the development of borehole instruments for measuring magnetic susceptibility and total magnetic field in situ. The susceptibility instrument is an alternating current induction device, and by separation of the quadrature components simultaneous recording of the magnetic susceptibility and the electrical conductivity is possible. The susceptibility log has many features that depart from ordinary electric logs. The instrument has a sensitivity of the order of 1 × 10 - 6 cgs units and sufficient contrast has been found in the sediments to yield a log of considerable lithologic character. This magnetic character suggests the use of the susceptibility tool in the field of special well logging, particularly for geologic correlation, and for tracer studies. The general assumption that the magnetic susceptibility of the sediments is sufficiently low compared to basic igneous rocks so that sedimentary rocks have little effect on surface magnetometer measurements has been verified. Since the magnetic susceptibility and electrical conductivity logs are made with an induction instrument, an electrolyte is not required in the hole and the logs are independent of the drilling fluid, except that the conductivity log is influenced by highly conductive muds. The total field log is made with a three element self‐orientating saturable core magnetometer that has been developed for borehole use. This log has not been used extensively. In addition to reflecting changes in polarization of the formations, it is influenced by formation susceptibility. Logs have been made of the total field going into and through igneous plugs. The paper presents examples of these logs along with a brief description of the instruments developed to produce them.
A study is made of magnetic anomalies due to prism-shaped bodies with arbitrary polarization. Expressions of the total field and its first and second derivatives are derived on the assumption of uniform magnetization through-out the body. Formulas for all possible cases in connection with a rectangular prism with vertical sides can be ob-tained either directly from this paper or by simple extension of the formulas given here. Using the exact expressions given in this paper, the total field and its derivatives are evaluated conveniently and rapidly with the aid of a digital computer. The effect of the dip angle and declination of the polarization vector on the size and shape of the magnetic anomaly is then studied for the case when the earth' s normal total field vector has a dip angle of 60" and declination of 0". With an increase in the dip angle of the polarization vector, the negative anomaly occurring on the north of the causative body diminishes in magnitude, whereas the positive and second derivative anomalies increase to maximum values and then decrease. With an increase in declination, this latter trend is repeated with the positive anomaly but the negative and second-derivative anomalies decrease systematically. Both the positive and second-derivative anomalies become more and more symmetrical with respect to the prismatic body with increase in either the inclination or declination of the polarization vector.
The petroleum discoveries in Australia have mostly been made after application of a conventional geophysical approach: airborne magnetics and gravity have focused on part of a basin, and then detail for determining drill sites has been provided by interpretation of extensive seismic surveys. It is now evident that high quality magnetics can provide new detail relevant to regional reviews, and can probably assist in targeting drillholes.
Developments which have led to an enormous increase in the use of magnetic surveys for mineral exploration during the past 30 years are reviewed here. Advances in instrumentation and digital compilation of data have come about largely due to the extensive use of the airborne magnetometer as a geologic mapping tool. Currently there is a growing interest in the use of the aeromagnetic gradiometer for exploration surveys in the Precambrian Shield areas. The advantages of gradiometry as a complement to total field surveys is becoming well recognized. Miniaturized electronics has reduced the cost and size of many magnetic survey instruments and ancillary equipments. Advanced compensation techniques have made it possible to make optimum use of the increased sensitivity of magnetometers for various high-resolution applications.Quantitative interpretation of magnetic data in terms of models of causative bodies has advanced largely due to the development of computerized multiparameter inversion methods. Many of these permit interpreter interaction through computer-graphic display system to impose sensible geologic constraints. Several survey contractors have the software and hardware facilities to perform various data enhancement techniques and also interactive modelling. Susceptibility mapping and magnetization mapping techniques are of great potential utility in mineral exploration. There is still a great need for studies into the relationship between rock magnetism and magnetic anomalies.
The parallel seismic (PS) method is used for determination of the unknown or undocumented depth of foundations, mostly piles. It was developed several decades ago and has not much changed since. PS is based on impulse generation on the pile head and registration of travel times in a borehole parallel to the foundation. The method is applicable to many foundation types, including single piles, pile walls, or sheet piles. While the accuracy is known to be high if the distance between foundation and borehole is small, the foundation length is overestimated with increasing distance. This paper presents the results of a systematic study on the influence of geometric and material parameters on the measurement results and explains the effects by studying the underlying wave phenomena based on numerical studies. It can be shown that several parameters (e.g. borehole inclination) have a strong influence. Foundation flaws and soil layers have also to be taken into account, while the possibilities in the other direction (derivation of soil parameters or detection of flaws from the results) are limited. Based on the simulation results, a new mathematical algorithm for data interpretation has been developed which takes into account the soil layers and the borehole inclination. This novel data interpretation scheme was used in combination with different data inversion methods. The new interpretation method was successfully validated using several sets of simulated data. Not only was it shown to be more accurate than all other available methods, but it also extended the maximum allowable pile–borehole distance to 2–3 m. Today, parallel seismics is the only method applicable on both metallic and non-metallic foundations which can be used without calibration. It has the largest range of all borehole methods. To enhance its efficiency it can be combined with downhole seismic measurements in the very same borehole to retrieve soil parameters
The paper deals with the remanent magnetism method (RM-method) to identify unsafe prestressed structures concerning sudden collapses of structural components due to fractures of the prestressing steel. It describes the non-destructive testing method that allows to localize fractures of single prestressing steel wires even in post-tensioned tendons. The method uses the magnetic field of externally magnetized tendons, measured at the concrete surface. The parameters that determine the characteristic fracture signal have been studied and are reported. The method has been successfully applied on full size units.Magnetische Ortung von Spanndrahtbrüchen in SpannbetonDie Remanenzmagnetismus-Methode wird genutzt, um Spannbetonbauteile, die aufgrund von möglichen Spannstahlbrüchen einsturzgefährdet sind, zu untersuchen. Brüche einzelner Spanndrähte eines von der Betonoberfläche aus magnetisierten Spannglieds in Spannbetonbauteilen mit nachträglichem Verbund erzeugen ein charakteristisches magnetisches Streufeld, das an der Betonoberfläche nachgewiesen wird. Die Parameter, die die Ausprägung des Bruchsignals bestimmen, wurden untersucht. Das Verfahren wurde bereits mehrmals erfolgreich an Bauwerken angewandt.
This book bridges the gap between the classic texts on potential theory and modern books on applied geophysics. It begins with Newton's second law of motion and concludes with topics on state-of-the-art interpretations of gravity and magnetic data. The introductory chapters discuss potential theory with emphasis on those aspects particularly important to earth scientists, such as Laplace's equation, Newtonian potential, magnetostatic and electrostatic fields, conduction of heat, and spherical harmonic analysis. Difficult concepts are illustrated with easily visualized examples from steady-state heat flow. Later chapters apply these theoretical concepts specifically to the interpretation of gravity and magnetic anomalies, with emphasis on anomalies caused by crustal and orthospheric sources. The book includes numerous exercises and a variety of computer subroutines written in FORTRAN that provide insight into the underlying theory discussed in the text and provide a reference source with which readers can develop their own computer programs.
Vector field formulation based on the Poisson theorem allows an automatic determination of rock physical properties (magnetization to density ratio—MDR—and the magnetization inclination—MI) from combined processing of gravity and magnetic geophysical data. The basic assumptions (i.e., Poisson conditions) are: that gravity and magnetic fields share common sources, and that these sources have a uniform magnetization direction and MDR. In addition, the previously existing formulation was restricted to profile data, and assumed sufficiently elongated (2-D) sources. For sources that violate Poisson conditions or have a 3-D geometry, the apparent values of MDR and MI that are generated in this way have an unclear relationship to the actual properties in the subsurface. We present Fortran programs that estimate MDR and MI values for 3-D sources through processing of gridded gravity and magnetic data. Tests with simple geophysical models indicate that magnetization polarity can be successfully recovered by MDR–MI processing, even in cases where juxtaposed bodies cannot be clearly distinguished on the basis of anomaly data. These results may be useful in crustal studies, especially in mapping magnetization polarity from marine-based gravity and magnetic data.
Computer programs based on the exact calculations of the gravity and magnetic anomalies of polygonal prisms are faster in operation and more accurate than previous programs based on the numerical integration of polygonal laminas. The prism programs also are of more general application than existing computer programs that are based on the exact gravity and magnetic effects of rectangular prisms. There are no restrictions on the use of the exact formula for the gravitational attraction of a polygonal prism, but the formulas for the magnetic effect are restricted in that demagnetization is not considered, and a finite answer is not obtained in the unrealistic circumstance where an observation point coincides with an edge of the prism.Least-squares methods permit calculation of the gravity or magnetic effect of models without knowledge of the density or magnetization contrasts, respectively, by comparison of the observed anomalies with theoretical dimensionless values to determine contrasts as regression c
Magnetometer surveys above gas pipelines show stress induced magnetic anomalies at pipe bends. This suggests a potential technique for the noninvasive monitoring of stress in buried pipelines, etc. Laboratory measurements of the magnetic field changes due to the elastic bending of 110 mm diameter pipe are presented. The effects of orientation with respect to the earths field and of internal pressure are reported. Hysteretic effects are found to be important.
Grand Paris Express Metro Project - Line 15 - Geophysical Investigations of Two Existing Structures in Interaction with the Tunnel under the Seine River
  • C Jassionnesse
  • F Costa
  • C Mogénier
  • F Bévier
A Borehole Magnetic Logging Tool for Estimating Unknown Foundation Depths
  • C H Jo
  • Y H Cha
  • J H Choi
Utilization of the Magnetometric Method in the Urban Environment
  • V L Galli
A Three-Component Borehole Magnetometer Probe for Mineral Investigations and Geologic Research
  • J H Scott
  • G G Olson
Essentials of Paleomagnetism: Fifth Web Edition
  • L Tauxe
  • S K Banerjee
  • R F Butler
  • R Van Der
  • Voo
Development of Data Inversion Software to Determine the Depth of Foundation Using Borehole Magnetometry (BM) Test” (in Portuguese)
  • M C Santos