This paper presents a new geophysical method of reservoir characterization and monitoring using the measurements of the controlled source time domain electromagnetic data by highly-sensitive SQUID (Superconducting Quantum Interference Device) magnetic sensors (SQUID-TEM). We have conducted a feasibility study of the SQUID-TEM survey for reservoir monitoring during CO2 sequestration in the Middle East test site. The SQUID-TEM measurements have been made before and after seawater/CO2 fluid injection into the subsurface reservoir. The goal was to produce 3D resistivity models around the injection borehole before and after the seawater/CO2 fluid injection and determine the injected fluid’s location after the injection. The results of the inversion of the post-injection survey data clearly showed the location of the conductive zone associated with the injected seawater/CO2 in the reservoir.

We have succeeded in fabricating a high-temperature superconducting (HTS) thin-film gradiometer with small imbalance by applying an HTS multilayer process, including crossovers, superconductive contacts, and ramp-edge Josephson junctions. The gradiometer consists of a series pickup coil and a gradiometric superconducting quantum interference device (SQUID) structure. Since the intrinsic imbalance of the series pickup coil is found to be small, a highly balanced gradiometer can be obtained by selecting the gradiometric SQUID inductor with imbalance that cancels the imbalance of the pickup coil out. The fabricated gradiometer showed the imbalance better than 10 –4 by applying a homogeneous magnetic field using a large circular coil in an outdoor experiment.

More than 50 years ago superconducting quantum interference devices (SQUIDs) were invented. Since then many applications opened up. Already in a 1980 workshop (Weinstock and Overton 1981 SQUID Applications to Geophysics (Society of Exploration Geophysicists)) the application of SQUIDs in geosciences was reviewed. The fabrication and cooling technologies, electronics and other SQUID system components underwent significant improvement within the past years. Thus, SQUIDs are today better suited, more sensitive and effective as well as robust and reliable in operation for geophysical measurements. Many successful application examples, demonstrations and discoveries of mineral resources have been made using them in laboratory devices for investigation of magnetic properties, magnetic exploration, transient electromagnetics and for superconducting gravimeters as well as gravity gradiometers. Therefore, this article intends to review the past, present, and some future aspects of SQUIDs in geo-scientific applications such as e.g. mineral exploration. Since this field is still very active and quite a number of developments are ongoing, this review cannot be comprehensive.

New barrier materials have been tried to be used for fabrication of ramp-edge-type Josephson junctions (JJs) and SQUIDs in which La0.1Er0.95Ba1.95Cu3Oy and SmBa2Cu3Oy are used as counter- and base-electrodes, respectively. The derivatives having the YBCO-type structure with a variety of lattice size, Al0.5Sr2Y0.5Ca0.5Cu2.4Zn0.1Oy, Mo0.5BaSrPr0.5Yb0.5Cu2.4Zn0.1Oy, Yb0.9La0.2Ba1.9Cu3Oy, Pr1.4Ba1.6Cu2.6Ga0.4Oy, La1.5Ba1.5Cu3Oy and TaBa2LaCu2Oy were examined. The structure and composition around the barrier region were investigated using an analytical transmission electron microscope for a sample prepared using the Yb0.9La0.2Ba1.9Cu3Oy barrier material. It was suggested that an oxygen-deficient perovskite phase exhibiting metallic conduction was crystallized in the barrier region. When TaBa2LaCu2Oy was used, large IcRn's around 200 μV and SQUID modulation voltages larger than 40 μV were observed. The attained large Rn in the JJs prepared using TaBa2LaCu2Oy might be attributed to suppression of metallic conduction due to appreciable Ta substitution for Cu in the barrier region.

We developed a hybrid cooling system consisting of a liquid-nitrogen Dewar and a cryocooler with the aim of cooling HTS-SQUIDs fast at operation sites, extending cooling period time, and avoiding noise increase caused by the cryocooler. Liquid nitrogen is evaporated mainly by thermal inflow into the Dewar. Thus, we tried reducing the thermal inflow into a glass Dewar by cooling its inner surface using a small Stirling cryocooler with a cooling capacity of 16 W at 77 K, and examined the cooling period and the operation procedure. We successfully kept 0.6 liter of liquid nitrogen for one week with 0.2 liter reduction. It was also indicated that long time and low noise operation of HTS-SQUIDs would be possible in the hybrid cooling system by temporarily stopping the cryocooler during measurements.

To perform an early stage detection of the deterioration of steel parts used in infrastructures, a nondestructive testing (NDT) method is required, which is capable of evaluating hidden parts such as a steel deck covered with an asphalt pavement. To achieve this, a magnetic field detector should be sufficiently large to detect magnetic fields over a wide range to account for high liftoff. In addition, the detector must be capable of functioning at high frequencies for a high-speed inspection of surface cracks, and at low frequencies to compensate for the skin effect during the inspection of inner or rear side cracks. In this paper, we developed a magnetic field detector for eddy current testing in a wide frequency range by combining a high-temperature superconducting (HTS) coil made using DI-BSCCO tapes with a tunnel magnetoresistive (TMR) sensor. The TMR sensor was attached on the inside of the loop of the HTS coil to maximize the shielding characteristic. We applied the developed system to NDT, which can detect a signal from an artificial crack with a lift-off of 75 mm, which is greater than the thickness of the asphalt pavement. In addition, inner cracks that are 2.7 mm beneath the surface can be detected with a 10 Hz applied magnetic field.

We propose an HTS-SQUID vector magnetometer with a new configuration reducing the crosstalk between a pickup coil of one SQUID and feedback coils of other SQUIDs. The new configuration is designed to mount three SQUIDs at symmetric positions where the crosstalk should be ideally zero while keeping minimal height of a probe head. In an actual HTS-SQUID magnetometer, however, either pickup coil or feedback coil is inevitably dislocated from the ideal symmetric positions. Numerical simulation of crosstalk revealed that the new configuration has smaller crosstalk than that of a conventional cubic configuration even if the positioning error from the ideal symmetric positions is as small as 1 mm. Three types of SQUID vector magnetometer were fabricated and their crosstalk was measured. The averaged crosstalk of a SQUID vector magnetometer using the new configuration was actually improved by about one order of magnitude as compared with the conventional cubic configuration.

We have developed a three-channel high-temperature superconductor (HTS) superconducting quantum interference device (SQUID) eddy current testing (ECT) system for detection of a fatigue crack which occurs in a steel deck plate used in expressway bridges. To avoid physical interference between adjacent channels, a dipole coil with compact design is employed. A field test using the three channel ECT system was performed on an expressway bridge having an orthotropic steel deck structure under traffic lane restrictions. Although a signal corresponding to existence of a fatigue crack was not detected due to a low occurrence rate of fatigue crack, we could demonstrate a stable and long-time operation in an urban area during the daytime and acquisition of correct data corresponding to some structural features of the expressway bridge.

The high upper critical field and low anisotropy of the 122-type iron-based superconductor BaFe2As2 makes it promising for use in superconducting high field magnets. However, its critical current density (Jc) in high magnetic fields needs to be further improved. Here we show that for the film prepared by pulsed laser deposition method by controlling the deposition parameters (higher substrate temperature and lower growth rate), the crystallinity of BaFe2(As0.66P0.33)2 (Ba122:P) matrix is improved while maintaining a high density of incoherent BaZrO3 (BZO) nanoparticles (NPs) which together lead to significantly increased self field Jc. Our Ba122:P nanocomposite films also exhibit increased in-field Jc, reduced angular anisotropy of Jc and reduced detrimental effects of thermal fluctuations (creep rate) over a wide range of temperatures and magnetic field strength. The BZO NP doped Ba122:P films show high in-field Jc over 2.1 MA cm−2 even at 4 K and 9 T (μ0H∣∣c), which is significantly higher than that of standard Ba122:P films and conventional alloy superconducting wires. To understand the contribution of the various pinning centers, we applied a simple model, which was developed for cuprates, to Ba122:P film with all the parameters used derived by fitting to a limited set of experimental data (no free parameters) such that temperature, angle and field properties at other experimental conditions are then calculated. This simple model fits very well to the experimental results in these two very different material systems. We discuss the effectiveness of natural defect and BZO NPs on the ratio of Jc to the depairing current density. The superconducting properties for 122-type iron-based superconductors obtained through this work are considered promising for high-field applications.

We developed an ultrasonic guided wave testing system for pipes utilizing magnetostrictive method and hightemperature superconductor (HTS) superconducting quantum interference device (SQUID) gradiometer. Two pre-magnetized nickel thin plates were adhered on an aluminium pipe. One of them with a field coil was used as a magnetostriction-based T (0, 1) mode guided wave transmitter, while the other was used as a receiver. Allround inspections of the pipe without and with a circumference slit were conducted by measuring the T (0,1) mode guide waves around circumference of the pipe using the HTS-SQUID-based guided wave testing system. The optimum experimental setup about excitation frequency and input current to the transmitter was investigated using ez-SQUID electronics. By applying a sine burst current of 0.2 A at 50 kHz with single cycle to the transmitter and rotating the pipe stepwise at 30°, “B-scan” images of the guided wave propagating on the pipe without and with slit were measured. An axisymmetric distribution of a reflected wave from the slit was well detected. We analyzed distribution of incident waves and reflected waves from the end of the pipe based on magnetization in the nickel plates using an electromagnetic simulator and proposed a new transducer.

We are developing an ultra-low field (ULF) magnetic resonance imaging (MRI) system with a high-temperature superconductor superconducting quantum interference device (SQUID) for the purpose of food contaminant inspection. Our previous ULF SQUID-MRI system for food inspection used a resonant circuit to detect a magnetic field, which was tuned at the Larmor frequency. However, when the MR signal frequency decreases, the signal bandwidth becomes narrower. Since the line width of the nuclear magnetic resonance spectrum of food with a short relaxation time becomes broad, it is difficult to measure the MR signal. Therefore, to address this issue, we employed a non-resonant method to widen the bandwidth. In this paper, we describe a ULF SQUID-MRI system using a non-resonant Cu wound flux transformer to image food with a shorter relaxation time. As a result, T1-weighted images of samples containing water and oil could be acquired by varying the polarizing time.

We examined a portable cryostat utilizing liquid nitrogen with temperature control function for operation of high-temperature superconducting quantum interference devices (HTS-SQUIDs) to have a higher slew rate. To operate a non-destructive evaluation system with HTS-SQUID in city areas with serious environmental electromagnetic noise, we often need a higher slew rate. A glass Dewar is installed in an outer case made of aluminum, which is sealed hermetically. The temperature of liquid nitrogen is controlled by using a vacuum pump connected to a feedback circuit, which monitors the temperature of liquid nitrogen. The temperature of liquid nitrogen could be controlled in the range between 73 and 77.2 K with a 0.1 K step. For example, the temperature was kept at 75 K with precision of +/- 0.05 K for 9 hs and SQUID was operated stably for this period. The Delta V and the slew rate of a test SQUID were increased from 22 mu V and 3.2 mT/s at 77.2 K to 37 mu V and 5.9 mT/s at 73 K, respectively. The white noise was also improved from 48 fT/root Hz at 77.2 K to 32 FT/root Hz at 74 K. We believe that this technology is useful to extend application of HTS-SQUIDs in the fields.

We have been developing an HTS-SQUID eddy current testing system using an HTS-SQUID module and external pickup coil made of Bi-2223 tape to investigate the feasibility of nondestructive evaluation of a steel plate paved with asphalt. In this paper, a new excitation coil intended as a current dipole source is designed and evaluated. The current dipole coil can be expected to generate a unidirectional excitation field and exhibit small field decay by distance and, thus, seems suitable for a measurement under a long liftoff condition such as the case for crack detection through an asphalt pavement. It is also found that a strong single peak appears just above a slit hole as a simulated crack by applying the excitation field using a tilted current dipole coil. The developed measurement system was mounted on a hand cart, and successful detection of a 50-mm-long slit hole and its stable operation were demonstrated in a field operation test.