Traditional localization techniques, such as radar and GPS, rely on RF waves, which require line-of-sight for effective operation. In contrast, applications based on low frequency magnetic fields benefit from high penetration ability to crown canopy, soil, and many other types of media. In previous work we introduced a method for 2-D localization using beacons of low frequency magnetic field. Here, we propose to expand the method for 3-D localization. A mathematical analysis of the beacons' magnetic fields results in closed-form formulas which enable simple localization calculations. The method has been tested using numerous computer simulations showing accurate localization results in noisy environment and in various beacon configurations. A field prototype of the system has been developed and tested in field conditions, validating simulation results. The obtained experimental results show that the mean localization error is smaller than 0.25 m, and the maximal localization error is less than 0.77 m. The simple implementation, together with the fair localization accuracy, make the proposed method attractive for many applications requiring field penetration ability, such as indoor robot navigation, underground cavity mapping, and many more.

Localization in 3-D Using Beacons of Low Frequency Magnetic Field

The paper gives a summary of geophysical aspects of man-made electromagnetic noise in the Earth as follows:


- EM distortion effects of man-made constructions below and over the Earth's surface defined as ‘passive-noise’,


- field observation of EM disturbances due to ‘active’ man-made sources,


- EM source mechanism of some important active sources from a geophysical point of view,


- efforts in order to improve the signal-to noise ratio by instrumental, methodological and dataprocessing ways,


- application of man-made EM noise for geophysical prospecting.

Geophysical aspects of man-made electromagnetic noise in the earth—A review

The advantage of magnetic resonance sounding (MRS) as compared to other classical geophysical methods is in its water selective approach and reduced ambiguity in determination of subsurface free water content and hydraulic properties of the media due to the nuclear magnetic resonance (NMR) principle applied. Two case examples are used to explain how hydrogeological parameters are obtained from an MRS survey. The first case example in Delft (the Netherlands) is a multiaquifer system characterized by large signal to noise ratio (S/N = 73), with a 24 m thick, shallow sand aquifer, confined by a 15 m thick clay layer. For the shallow aquifer, a very good match between MRS and borehole data was obtained with regard to effective porosity n(e) approximately 28% and specific drainage S(d) approximately 20%. The MRS interpretation at the level deeper than 39 m was disturbed by signal attenuation in the low resistivity (approximately 10 omega(m)) media. The second case of Serowe (Botswana) shows a fractured sandstone aquifer where hydrogeological parameters are well defined at depth > 74 m below ground surface despite quite a low S/N = 0.9 ratio, thanks to the negligible signal attenuation in the resistive environment. Finally, capabilities and limitations of the MRS technology are reviewed and discussed. MRS can contribute to subsurface hydrostratigraphy description, hydrogeological system parameterization, and improvement of well siting. The main limitations are survey dependence upon the value of the S/N ratio, signal attenuation in electrically conductive environments, nonuniformity of magnetic field, and some instrumental limitations. At locations sufficiently resistive to disregard the signal attenuation problems, the MRS S/N ratio determines how successfully MRS data can be acquired. Both signal and noise vary spatially; therefore, world scale maps providing guidelines on spatial variability of signal and noise are presented and their importance with respect to the MRS survey results is discussed. The noise varies also temporally; therefore, its diurnal and seasonal variability impact upon the MRS survey is covered as well.

Magnetic Resonance Sounding: New Method for Ground Water Assessment

Summary Mineral exploration using the Z-TEM or Z-Axis Tipper Electromagnetics, an airborne AFMAG system, has been guided by numerical modeling of the target types Numerical modeling is used to plan the survey in terms of survey line spacing, survey height, and expected signatures of the targets Knowing the Z-TEM response of a deposit of the type that is being explored for, aids in the interpretation of the results Numerical modeling has demonstrated that the Z-TEM system is ideally suited for large, deep deposits of low to high resistivity contrasts such as porphyry copper and SEDEX deposits

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Numerical Modeling of Z-TEM (airborne AFMAG) Responses to Guide Exploration Strategies

Electromagnetic Induction Methods for Environmental Problems

Experience with the AFMAG method has demonstrated that an electromagnetic exploration system using the Earth’s natural audiofrequency magnetic fields as an energy source is capable of mapping subsurface electrical structure in the upper kilometer of the Earth’s crust. We resolved the limitations of this method by adapting the tensor analysis and remote reference noise bias removal techniques from the geomagnetic induction and magnetotelluric methods to computation of the tippers. After a thorough spectral study of the natural magnetic fields, we designed lightweight magnetic field sensors capable of measuring the magnetic field throughout the year. We also built a digital acquisition and processing system with the ability to provide audiofrequency tipper results in the field. This new instrumentation was used in a study of the Mariposa, California site previously mapped with AFMAG. This study once again demonstrates the usefulness of natural magnetic field data in mapping an electrically conductive body. ...

H. F. Morrison

Papers

Geophysical exploration with audiofrequency natural magnetic fields