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.

We present a portable optically pumped magnetometer instrument for ultrasensitive 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 $M_z$ 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 the realization of both the sensor head and electronics. We show shielded and unshielded measurements within a static magnetic field amplitude of $50\mu \mathrm{T}$, demonstrating a noise level of the sensor system down to $140\mathrm{fT}/\sqrt{\mathrm{Hz}}$ 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 toward its fundamental noise limit of $12\mathrm{fT}/\sqrt{\mathrm{Hz}}$. We assess the parameters defining the sensor bandwidth by theoretical modeling based on the Bloch equations. Finally, we compare our sensors’ performance to a commercial superconducting quantum interference device system in a measurement environment typical for geomagnetic observatory practice and geomagnetic prospection.

We present an optically pumped magnetometer working in a new operational mode—the light-shift dispersed Mz (LSD-Mz) mode. It is realized combining various features; (1) high power off-resonant optical pumping; (2) Mz configuration, where pumping light and magnetic field of interest are oriented parallel to each other; (3) use of small alkali metal vapor cells of identical properties in integrated array structures, where two such cells are pumped by circularly polarized light of opposite helicity; and (4) subtraction of the Mz signals of these two cells. The LSD-Mz magnetometer’s performance depends on the inherent and very complex interplay of input parameters. In order to find the configuration of optimal magnetometer resolution, a sensitivity analysis of the input parameters by means of Latin Hypercube Sampling was carried out. The resulting datasets of the multi-dimensional parameter space exploration were assessed by a subsequent physically reasonable interpretation. Finally, the best shot-noise limited magnetic field resolution was determined within that parameter space. As the result, using two 50 mm3 integrated vapor cells a magnetic field resolution below 10 fT/√Hz at Earth’s magnetic field strength is possible.

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.