We present pre-polarization surface nuclear magnetic resonance (PP-SNMR) measurements performed with a Superconducting QUantum Interference Device (SQUID) magnetometer on water-filled pallet boxes. The SQUID directly detects the three components of the magnetic field (B-field) NMR response, while conventional SNMR experiments would detect its time derivative and most of the time only a single component. Each of the three vector components of the magnetic field NMR response consists of a component oscillating at Larmor frequency and of a non-oscillating component. We extend the general SNMR theory to model the measured signals. For the non-oscillating signal, another magnetic decay with a large amplitude is superimposed on the signal originating from the water-filled boxes, and we were unable to extract the desired signal. For the oscillating signal component, however, we report good agreement between the measured signal and the forward model in amplitude and phase. Measuring all three components of the B-field introduces a sensitivity to lateral inhomogeneities, which we demonstrate by repeating the experiment with one and two emptied boxes.

We present a portable optically pumped magnetometer instrument for ultra-sensitive measurements within the Earth's magnetic field. The central part of the system is a sensor head operating a MEMS-based Cs vapor cell in the light-shift dispersed Mz mode. It is connected to a compact, battery-driven electronics module by a flexible cable. We briefly review the working principles of the device and detail on the realization of both, sensor head and electronics. We show shielded and unshielded measurements within a static magnetic field amplitude of 50 uT demonstrating a noise level of the sensor system down to 100 fT/\sqrt{Hz} and a sensor bandwidth of several 100 Hz. In a detailed analysis of sensor noise we reveal the system to be limited by technical sources with straightforward strategies for further improvement towards its fundamental noise limit of 12 fT/\sqrt{Hz}. We compare our sensors' performance to a commercial SQUID system in a measurement environment typical for geomagnetic observatory practice and geomagnetic prospection.

We report on the development of magnetic background field-tolerant superconducting quantum interference filter (SQIF) based on low-capacitance sub-micrometer sized cross-type Josephson junctions either as current sensing amplifiers—even on chip—for advanced superconducting quantum interference device (SQUID) readout circuits or as magnetic field sensor in flux transformer configuration especially for geophysical measurement systems. Their very small parasitic magnetic sensitive areas enable them to operate in the Earth magnetic field and allow for magnetically unshielded cool-down. The careful consideration of magnetic sensitive areas inside each SQUID in the SQIF in all three dimensions result in developed SQIFs with 28 SQUIDs in series exhibiting large voltage swing and transfer function of more than 2 mV and 500VA−1 , respectively. We report on the electrical parameters and field stability as well as on the noise performance of the devices under investigation. SQIFs and devices with additional on-chip flux-transformers show input referred current noise levels of 5.9 and 1.1pAHz−1/2 , respectively. We furthermore demonstrate their potential as SQIF-based magnetometers using them in addition with thin-film pickup loops resulting in a white magnetic field noise of 1.6fTHz−1/2 . The implemented SQIFs are thus highly compatible with state-of-the-art single SQUID-devices offering beneficial features such as a unique working point, making them excellent suited for implementation of e.g. geophysical instruments.

For the first time a mobile underwater full tensor magnetic gradiometer (FTMG) system based on low-T-c superconducting quantum interference devices (SQUIDs) has been deployed in order to scan the sea floor for magnetized targets. The application is mainly focused on waste deposits and unexploded ordnance (UXO), but could also include shallow geological features as well as archaeological remains. The main methods for detection and localisation of underwater UXO and waste deposits are side sonar scanning and magnetic mapping. While modern sonar scanners can achieve a very high spatial resolution and long detection range, they still have problems detecting targets under cover-for magnetic sensors a layer of non-magnetic sand or ooze does usually not have an effect on the signal apart from an increased distance. In this paper we discuss the setup of the mobile underwater FTMG SQUID system, its challenges and main performance features. It also illustrates its detection and localization capabilities in tests on known magnetic targets.

Transient electromagnetics (TEM) is a well-established method for mineral, groundwater, and geothermal exploration. Superconducting quantum interference device (SQUID)-based magnetic-field receivers used for TEM have quantitative advantages and higher sensitivity compared with commonly used induction coils. Special applications are deep soundings with target depths [Formula: see text] and settings with conductive overburden. However, SQUIDs have rarely been applied for TEM measurements in environments with significant anthropogenic noise. We compared a low-temperature SQUID with a commercially available induction coil in an area affected by anthropogenic noise. We acquired four fixed-loop data sets with totally 61 receiver stations close to Bad Frankenhausen, Germany. The high sensitivity of the SQUID enables low noise levels, which lead to longer high-quality transient data compared with the induction coil. The effect of anthropogenic and natural noise sources is more critical for the coil than for the SQUID data. In the vicinity of the transmitter loop, systematic distortion of the coil signals occurs at early times, most probably caused by sferic interferences. We have developed 1D inversion results of both receivers that matched well in general. However, the SQUID-based models were more consistent and showed greater depths of investigation. This led to a superior resolution of deeper layers and even enabled a potential detection of thin conducting targets at up to a 500 m depth. Moreover, we find that the SQUID data inversion revealed multidimensional effects within the conductive overburden. In this regard, we applied forward modeling to analyze systematic differences between inversion results of SQUID and coil data. We determine that low-temperature SQUIDs have the potential to significantly improve the reliability of subsurface models in suburban environments. Nevertheless, we recommend combined application of both types of receivers.

For ground-based electromagnetic methods in geophysics dc superconducting quantum interference devices based on high-temperature superconductors are well suited as magnetic field sensors. Therefore, we introduce in this paper an advanced fabrication technology for high-temperature superconducting dc SQUIDs based on step-edge Josephson junctions. The dc SQUIDs are prepared on structured magnesium-oxide substrates. A trilayer of YBa 2 Cu 3 O 7-x /SrTiO 3 /YBa 2 Cu 3 O 7-x is deposited by pulsed laser deposition and structured with ion beam etching. The galvanometer-type design consists of four different dc SQUIDs directly coupled to a pickup loop. They exhibit large I c R n products and a voltage swing of more than 30 μV. The magnetic field noise amounts to about 20 fT/Hz 1/2 and 35 fT/Hz 1/2 for the white noise region as well as 25 fT/Hz 1/2 and 100 fT/Hz 1/2 at 100 Hz for ac-bias and dc-bias SQUID electronics, respectively. The advanced fabrication process thus enables to produce HTS dc SQUIDs intended for the use in geophysical applications with high throughput.

Within the framework of the multidisciplinary research project called INFLUINS, INtegrated FLUid dynamics IN Sedimentary basins, we use a highly sensitive magnetic field receiver based on Superconducting Quantum Interference Devices (SQUIDs) for the transient electromagnetic method and compare its performance specifications with a commercially available induction coil. Four fixed loops TEM measurements with 10 20 receiver stations each have been conducted along a survey line perpendicular to the known geologic strike direction at the test site Esperstedter Ried in Northern Thuringia, Germany. The signals of the SQUID receiver provide significantly better quality and are less affected by natural and man-made noise sources than the ones of the induction coil, which is proved by data error and noise measurement analysis. As a result, the 1D inverse modeling results of the SQUID data show lower misfit ratios and are more reliable compared to the coil.

We report on the development of an ultralow-noise thin-film-based superconducting quantum interference device (SQUID) magnetometer. A niobium thin-film pickup coil is connected to the input coil of a SQUID current sensor. The low capacitance of the used submicrometer cross-type Josephson junctions enables superior noise performance of the device. Application scenarios, e. g., in geophysics and ultralow-field magnetic resonance imaging, are discussed.

Forty years after the first application of Superconducting Quantum Interference Devices (SQUIDs) [1], [2] for geophysical purposes, they have recently become a valued tool for mineral exploration. One of the most common applications is time domain (or transient) electromagnetics (TEM), an active method, where the inductive response from the ground to a changing current (mostly rectangular) in a loop on the surface is measured. After the current in the transmitter coil is switched, eddy currents are excited in the ground, which decay in a manner dependent on the conductivity of the underlying geologic structure. The resulting secondary magnetic field at the surface is measured during the off-time by a receiver coil (induced voltage) or by a magnetometer (e.g. SQUID or fluxgate). The recorded transient signal quality is improved by stacking positive and negative decays. Alternatively, the TEM results can be inverted and give the electric conductivity of the ground over depth. Since SQUIDs measure the magnetic field with high sensitivity and a constant frequency transfer function, they show a superior performance compared to conventional induction coils, especially in the presence of strong conductors. As the primary field, and especially its slew rate, are quite large, SQUID systems need to have a large slew rate and dynamic range. Any flux jump would make the use of standard stacking algorithms impossible. IPHT and Supracon are developing and producing SQUID systems based on low temperature superconductors (LTS, in our case niobium), which are now state-of-the-art. Due to the large demand, we are additionally supplying systems with high temperature superconductors (HTS, in our case YBCO). While the low temperature SQUID systems have a better performance (noise and slew rate), the high temperature SQUID systems are easier to handle in the field. The superior performance of SQUIDs compared to induction coils is the most important factor for the detection of good conductors at large depth or ore bodies underneath conductive overburden.

Measurement of magnetic vector or tensor quantities, namely of field or field gradient, delivers more details of the underlying geological setting in geomagnetic prospection than a scalar measurement of a single component or of the scalar total magnetic intensity. Currently, highest measurement resolutions are achievable with superconducting quantum interference device (SQUID)-based systems.Due to technological limitations, it is necessary to suppress the parasitic magnetic field response from the SQUID gradiometer signals, which are a superposition of one tensor component and all three orthogonal magnetic field components. This in turn requires an accurate estimation of the local magnetic field. Such a measurement can itself be achieved via three additional orthogonal SQUID reference magnetometers. It is the calibration of such a SQUID reference vector magnetometer system that is the subject of this paper.A number of vector magnetometer calibration methods are described in the literature. We present two methods that we have implemented and compared, for their suitability of rapid data processing and integration into a full tensor magnetic gradiometry, SQUID-based, system.We conclude that the calibration routines must necessarily model fabrication misalignments, field offset and scale factors, and include comparison with a reference magnetic field. In order to enable fast processing on site, the software must be able to function as a stand-alone toolbox.

To measure the noise performance of highly sensitive SQUID magnetometer systems directly is nearly impossible due to superimposed external noise. In magnetically unshielded environments in particular one needs sophisticated methods in order to get an estimate of the intrinsic noise. We compare different approaches to estimate the noise of our latest SQUID magnetometer systems in the Earth's magnetic field and compare the results with measurements in magnetic (and superconductive) shielding.

The fast and sensitive SQUID (Superconducting Quantum Interference Device) system, which was developed at IPHT Jena, allows the geo-magnetic prospection of large land areas. The system's simultaneous high-resolution recording of all components of the Earth's magnetic field gradient tensor represents a high-quality data base for precise inversion calculations. Thus, we developed a software tool for the fast and direct inversion of full-tensor data from especially dipole-like sources. Our motivation is to localize buried magnetic objects and inhomogeneities in the underground only by measuring the gradient components at the surface. The application of the algorithm will be shown by two examples, first on a synthetic data set and second on a real data set measured at the IPHT test site with well-defined buried targets.

Summary IPHT Jena and Supracon AG develop and fabricate full tensor magnetic gradiometer (FTMG) systems, based on low-temperature-superconducting (LTS) planar-type SQUID (Superconducting Quantum Interference Device) gradiometers of first order. The systems are operated by Spectrem Air Ltd., jointly owned by Anglo American and De Beers. This paper describes details of the system setup, data processing and achieved performance parameters. The sensors work at 4.2 K in liquid helium. With a base length of only 3.5 cm a real gradient measurement is possible. With six gradiometer the full magnetic gradient tensor can be measured with redundancy, allowing for high system reliability and the application of enhanced noise reduction techniques. The data of the FTMG system will allow for a 3D inversion for the underlying magnetic sources. Examples of 2D maps of tensor components with high spatial resolution are shown.

Geophysical exploration is getting more and more difficult-many of the easily explorable ore-bodies have been discovered and are already being exploited. Finding new mines requires new technologies and tools. Transient electromagnetics (TEM) is widely used in mineral exploration, but conventional sensors (especially induction coils) cannot fulfil the needs anymore: deep targets, very conductive targets or targets under conductive overburden are more easily (or sometimes only) detected using SQUIDs. In this paper we will focus on low temperature SQUID magnetometers. As the systems are applied worldwide it is necessary to strengthen them for all conceivable application scenarios. Here, we report on the latest development of these systems which are now routinely used in South Africa, Australia, Finland and Canada. This paper highlights the main features of the system and describes one example from mineral exploration.

A new generation of High Temperature Superconductive QUantum Interference Detector (HTS SQUID) systems, with an improved ac bias scheme, achieves very low noise even at low frequencies. Based on that technology, robust and reliable systems for measuring TEM under sub‐arctic conditions have been developed. Case studies, measured with these systems, will be reported by Woods at 80th annual SEG meeting 2010.

Summary In recent developments a full tensor magnetic gradient system has been deployed in South Africa. The instrument is made by IPHT (Institute of Photonic Technology, Jena, Germany) and flown by helicopter. In developing a ‘custom’ solution for taking the raw signal through to a final located geophysically sound data base, one of the biggest challenges has proven to be the compensation of aircraft / bird rotational movements, so that the reported magnetic curvature gradients in the world co-ordinate system are as free of these rotational errors as possible. Issues that arise include drift correction of the Euler Angles from the INS (Inertial Navigation System), removal of flux jumps from the SQUID and recovering the three components of the ‘B’ field. Compensation issues are limiting the achievable resolution of the field gradients. A novel least-squares rotational adjustment of the tensor signal in a moving window is proposed as an extra compensation step to achieve higher spatial coherency of the curvature gradients.

Airborne geophysics is a commonly used tool for the geophysical prospecting of large areas with high spatial resolution. Especially the mapping of anomalies of the Earth’s magnetic field is well suited for airborne operation. Up to now the measurement of the total field and it’s gradients with caesium vapour magnetometers is the most widely used method in airborne magnetic surveys. But there are advantages of using the tensor gradient instead of the total field gradients, which will be shortly discussed.