CalMagNet ? an array of search coil magnetometers monitoring ultra low frequency activity in California

International audienceThe California Magnetometer Network (CalMagNet) consists of sixty-eight triaxial search-coil magnetometer systems measuring Ultra Low Frequency (ULF), 0.001?16 Hz, magnetic field fluctuations in California. CalMagNet provides data for comprehensive multi-point measurements of specific events in the Pc 1?Pc 5 range at mid-latitudes as well as a systematic, long-term study of ULF signals in active fault regions in California. Typical events include geomagnetic micropulsations and spectral resonant structures associated with the ionospheric Alfvén resonator. This paper provides a technical overview of the CalMagNet sensors and data processing systems. The network is composed of ten reference stations and fifty-eight local monitoring stations. The primary instruments at each site are three orthogonal induction coil magnetometers. A geophone monitors local site vibration. The systems are designed for future sensor expansion and include resources for monitoring four additional channels. Data is currently sampled at 32 samples per second with a 24-bit converter and time tagged with a GPS-based timing system. Several examples of representative magnetic fluctuations and signals as measured by the array are given

Analytical ultrasonics for evaluation of composite materials response. Part 2: Generation and detection

To evaluate the response of composite materials, it is imperative that the input excitation as well as the observed output be well characterized. This characterization ideally should be in terms of displacements as a function of time with high spatial resolution. Additionally, the ability to prescribe these features for the excitation is highly desirable. Various methods for generating and detecting ultrasound in advanced composite materials are examined. Characterization and tailoring of input excitation is considered for contact and noncontact, mechanical, and electromechanical devices. Type of response as well as temporal and spatial resolution of detection methods are discussed as well. Results of investigations at Virginia Tech in application of these techniques to characterizing the response of advanced composites are presented

Electromagnetic Imaging of the marine subsurface : a novel approach to assess sediment patterns and dynamics on clastic shelf systems

Electromagnetic (EM) imaging is a new approach to investigate marine near-surface sediments. The EM data provide information about electric conductivity and magnetic susceptibility of the sediments. Both are important physical parameters in exploration geophysics. Electric conductivity of marine sediments is a function of porosity, tortuosity and chemistry of the pore fluid. Magnetic susceptibility indicates the magnetic particle concentration and is hence related to the mineral composition of the sediment. In this thesis data processing, inversion and machine learning methods for a novel marine EM profiling system are developed, with the goal to explore the internal structure and spatial variability of sediment patterns in coastal and shelf regions. The investigated EM data were acquired on the NW Iberian shelf during the Meteor cruise M84/4b with the bottom towed electromagnetic profiler MARUM NERIDIS III. This non-conductive, non-magnetic fiberglass sled accommodates a controlled source electromagnetic system based on a frequency-domain concentric-loop EM induction sensor. In order to estimate quantitative seafloor sediment properties from the NERIDIS III EM data, the approach developed in this thesis follows three main steps: The first step is to calibrate the EM data such that instrument related bias is removed and the EM response is solely controlled by the frequency of the source signal, the system geometry, the electric conductivity and magnetic susceptibility of the seawater and the sediment. Calibration is necessary to make data from different measurements and surveys comparable and to enable solving of the ill-posed inverse problem for electric conductivity and magnetic susceptibility. This thesis shows that calibrating the primary EM field alone, by means of independently measured water conductivity and constant water susceptibility, is not sufficient. Therefore, a calibration methodology is developed which firstly calibrates the recorded EM data to compensate for bias in the primary EM field followed by a secondary EM field calibration by means of ground-truth data. The second step involves the inversion of the EM data, which can be subdivided into a half-space and 1-D inversion. The half-space inversion aims for the reconstruction of bulk sediment conductivity and susceptibility of the uppermost approximately 0.5 to 1 m. It is demonstrated that recovered half-space conductivity and susceptibility well reflect the main sediment patterns on the NW Iberian shelf and allow the reconstruction of sediment pathways. The 1-D inversion can be used to reconstruct the vertical conductivity structure of the subsurface. An algorithm is developed which employs the half-space susceptibility as a priori information and hence allows the utilisation of the in-phase component of the complex earth response increasing the depth of investigation. It is shown that vertical conductivity variations down to approximately 3 m below the seafloor can be reconstructed revealing the internal structure of the Galician Mud Belt. The third step covers the predictive modelling of grain-size from the electric conductivity and magnetic susceptibility of the sediment. Correlation analyses are carried out which reveal a strong relation between the electromagnetic and textural sediment properties. A radial basis function network is developed which predicts the entire grain-size distribution for each EM measurement location along shelf wide survey lines. The predicted grain-size distributions are used to identify the well-known sediment facies on the NW Iberian shelf and give new insights into their distribution and transitions

LC Sensor for biological tissue characterization

Over the past few decades, there has been growing interest and increased research on bio-implantable devices using RF telemetry links, enabling the continuous monitoring and recording of physiological data. However the dispersive properties of tissues make this a formidable task. In the present work, a novel technique for tissue characterization using an LC sensor is developed which allows for the extraction of the relative permittivity, and the conductivity of dispersive tissues. The resonant frequency of the sensor is monitored by measuring the input impedance of an external antenna, and correlated to the desired quantities. The impact of multi-layered dispersive tissues on the setup of the telemetry link is demonstrated where the role of the capacitor is analyzed. The sensor consists of a planar inductor, and an interdigital capacitor. Using an equivalent circuit model of the sensor that accounts for the properties of the encapsulating tissue, analytical expressions have been developed for the extraction of the tissue permittivity and conductivity. In addition, the effect of the thicknesses of the tissue layers on the sensor resonant frequency is studied. It is seen that the resonant frequency is strongly affected by the properties of the first layer, whereas additional layers prove to have little to no effect. A saturation thickness is defined that allows for the sensor to be implanted at a depth where it is only affected by the properties of the layer in which it is embedded. In order to analyze the telemetry system, a single loop antenna is evaluated in proximity to the biological tissue layer and the interaction of the electromagnetic field with the body is assessed in terms of specific absorption rate (SAR). It is studied through different multi-layered models composed of skin, fat and muscle, with typical values of tissue thickness. The introduction of multiple tissue layers as well as the misalignment effect is investigated from the SAR distribution. Finally, experimental validation has been performed with a telemetry link that consists of a loop antenna and a fabricated LC sensor immersed in single and multiple dispersive regions

Magnetic sensors and gradiometers for detection of objects

Disertační práce popisuje vývoj nových detekčních zařízení s anizotropními magnetorezistoryThis thesis describes development of innovative sensor systems based on anisotropi