Andreas Chwala

Magnetic full-tensor SQUID gradiometer system for geophysical applications

In 1997, the Institute for Physical High-Technology (IPHT) in Jena, Germany, started a program with the aim of developing an airborne full-tensor magnetic gradiometer superconducting quantum interference device (SQUID) using liquid-helium-based thin-film technology. Since that time, the sensor, system electronics, data acquisition system, and cryogenics have been developed and extensively tested. This article reports on successful tests of the intrinsic planar LTS (low-temperature superconductor) SQUID gradiometers which were conducted by towing the system from a helicopter as well as for a stinger-mounted configuration in a Cessna 208 aircraft.

A LTS-SQUID System for Archaeological Prospection and Its Practical Test in Peru

We present a new geomagnetic field measurement system for the detection of archaeological signatures in the soil. The system provides a unique fast mapping of large areas with high magnetic field gradient resolution as well as lateral precision. The data acquired by the device are geographically referenced and suitable for embedding in a geographic information system (GIS).

Archaeometric prospection with high-T/sub c/ SQUID gradiometers

Mapping of the Earths magnetic field or its gradient is a widely used method in archaeological prospection. The use of SQUIDs promises to be advantageous for archaeometry, since they combine a high field resolution with a large bandwidth. Compared to conventional Cs vapor sensors SQUIDs can be used for much faster magnetic mapping, allowing, for the first time, the investigation of huge archaeological features in a reasonable time period. We have investigated several SQUID systems for their usability in archaeometry by measuring a Neolithic double ring ditch enclosure. We have used two electronic High Temperature Superconductor SQUID (HTS SQUID) gradiometers with base lengths of about 60 cm and a Low Temperature Superconductor SQUID (LTS SQUID) gradiometer with a base length of 4 cm. Their intrinsic magnetic field resolution was 6 pT/m//spl radic/Hz for the HTS SQUID gradiometers and about 0.1 pT/m//spl radic/Hz for the LTS SQUID gradiometer. In contrast to Cs vapor gradiometers, which measure the gradient of the total magnetic field, SQUID gradiometers measure one component of the gradient tensor. Since measurements have to be performed whilst movement in the background of the Earths magnetic field, balancing is the limiting factor for the magnetic gradient field resolution of vector gradiometers.

SQUID gradiometers for archaeometry

The mapping of the Earths magnetic field or field gradient is a proven method in surface exploration and archaeometry. Caesium vapour magnetometers show the best magnetic field resolution of commercial devices, but their sampling frequency is limited to 10 Hz. Using SQUIDs it is possible to achieve the same or even better magnetic field resolution with a sampling frequency as high as 100 Hz or more. This allows significantly shorter acquisition times, which is essential for the mapping of large objects. In this paper we check the performance of our developed systems on a neolithic double-ring ditch enclosure near Weimar, Germany. We compare mappings of this area using an electronic caesium gradiometer, an electronic HTS SQUID gradiometer and an integrated planar LTS SQUID gradiometer. With all three systems the magnetic pattern of the ditch is visible; however, the electronic HTS gradiometer shows disturbances of the same order of magnitude as the gradient signal of the ditch, due to an insufficient common mode rejection whilst being moved. The planar LTS SQUID gradiometer shows superior performance. Its mapping shows a much better contrast and features that are not visible in the mapping of the caesium gradiometer.

HTS dc SQUID systems for geophysical prospection

We describe the use of high T/sub c/ flip-chip dc SQUID magnetometers in systems for several geophysical prospection methods. For use in the Transient Electro Magnetics (TEM) method, a one-channel SQUID magnetometer system (vertical field component) was adapted to rough conditions in the field. We report on measurements in South Africa over a geophysically interesting target in real production mode. Transients up to 200 ms could be recorded, which is about a factor of 100 more than with conventional coils. For investigation of the magnetic field gradient a two-channel system, built as a first-order electronic gradiometer with a baseline up to 1 m, was designed and manufactured. It can be used, e.g., for surface exploration or for archeometry. The system can be moved in the Earths magnetic field. We achieve a common mode rejection of 100. The reasons for the imbalance and the possibility of further improvement of the system are discussed.

Quantum Detection Meets Archaeology – Magnetic Prospection with SQUIDs, Highly Sensitive and Fast

A new measurement system was built for magnetic prospection in archaeology. The new device extends the capability of fluxgate- and caesium magnetometer-based systems to large-area mapping as well as high magnetic and lateral resolution. The SQUID system passed its first toughness test during a survey in the Peruvian Palpa region in 2005. Within a couple of days an impressive magnetic database of several hectares was created. This georeferenced archaeological and geological information is used for specific excavations and contributes to the comprehension of the historical contexts of the Palpa region.

Low-noise Y-Ba-Cu-O flip-chip dc SQUID magnetometers

We have prepared low-noise flip-chip SQUID magnetometers by using dc SQUIDs with large flux-to-voltage transfer functions and superconducting flux transformers. The dc SQUIDs have a square washer geometry and are prepared on a SrTiO/sub 3/ bicrystal substrate with a misorientation angle of 30/spl deg/. The maximum peak-to-peak flux modulated voltage varies typically between 20 and 60 /spl mu/V for a SQUID inductance of 80 pH. The YBa/sub 2/Cu/sub 3/O/sub 7-x/ flux transformers are prepared on a SrTiO/sub 3/ single crystal polished on both sides. The preparation process of the flux transformers involves the deposition of YBCO and SrTiO/sub 3/ layers by pulsed laser ablation and pattern definition by ion beam etching. The flip-chip magnetometers are encapsulated and robust to temperature cycling. We operate the magnetometers in magnetically shielded and in unshielded environment by using a direct-coupled read-out electronics. The field sensitivity of the magnetometers is equal to 2.2 nT//spl Phi//sub 0/. The field resolution of the magnetometers is typically <40 fT/Hz/sup 1/2/ at 1 kHz and about 100 fT/Hz/sup 1/2/ at 1 Hz inside shielding. The suitability of these devices for geophysical applications is shown by Transient Electromagnetics measurements.

HTS SQUID gradiometer for application without shielding

An HTS SQUID gradiometer consisting of a series gradiometer pickup loop in flip-chip configuration with a small washer SQUID is presented. The series gradiometer configuration is advantageous for applications without shielding because of the avoidance of circulating shielding currents in the pickup loop. With a 20 mm × 10 mm substrate for the pickup loop a large effective area of 1.25 mm2 and a gradiometer baseline of 9.6 mm are achieved. The balance of the gradiometer impaired by the washer read-out SQUID can significantly be enhanced by the weighted subtraction of a reference SQUIDs signal. With this a balance of 5 × 104 is reached. The noise-limited magnetic field gradient resolution of the balanced SQUID gradiometer is 38 fT cm−1 Hz−1/2. This paper was presented at the 8th International Superconductive Electronics Conference, Osaka, Japan, 19–22 June 2001.

HTS dc SQUID behavior in external magnetic fields

Several types of HTS dc SQUID magnetometers - directly coupled magnetometers with either a conventionally solid pickup loop or with 17 narrow loops in parallel and flip-chip magnetometers-are tested concerning their behavior in external fields. The stability against flux penetration was measured by the determination of the magnetic hysteresis. This stability is increased by steep structure edges in the SQUID and in the pickup loop. The intrinsic noise of SQUIDs during operation in external fields was determined by simultaneous measurement of the spectra of two SQUIDs. A directly coupled magnetometer with narrow parallel loops showed much smaller intrinsic noise after movement in the Earths magnetic field than one with a solid pickup loop.

JESSY DEEP: Jena SQUID systems for deep earth exploration

Summary 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 80 th annual SEG meeting 2010.