Tracking geomagnetic fluctuations to picotesla accuracy using two superconducting quantum interference device vector magnetometers

SQUIDs can be used to monitor the three vector components of the geomagnetic field to a high precision at very low frequencies, yet as they are susceptible to external interference, the accuracy to which they can track changes in the dc field over long periods has been unclear. We have carried out simultaneous measurements of the geomagnetic field recorded using two independent 3-axis SQUID magnetometers at the Laboratoire Souterrain à Bas Bruit (LSBB). We demonstrate a technique to take the difference between a linear transform of the three signals from one magnetometer, and a reference signal from the other, in order to account for any difference in alignment and calibration, and track local signals at a sub-nT level. We confirmed that both systems tracked the same signal with an RMS difference as low as 56pT over a period of 72 h. To our knowledge this is the first such demonstration of the long term accuracy of SQUID magnetometers for monitoring geomagnetic fields

Monitoring geomagnetic signals of groundwater movement using multiple underground SQUID magnetometers

International audienceGroundwater can influence the geomagnetic field measured underground in at least two key ways. The water levels in rock will determine its electrical conductivity, and thus change the magnitude of the telluric currents induced in the rock by changing magnetic fields generated in the ionosphere. This can be studied by using multiple magnetometers at different underground locations. Secondly the flow of water through rock will generate a small magnetic signal, of unknown magnitude, through the electrokinetic effect. SQUID magnetometry has the potential to allow passive studies of groundwater changes in complex systems such as karst. We have monitored geomagnetic signals using two SQUID magnetometers at the LSBB underground laboratory, and set an initial limit on the magnitude of the electrokinetic signal. We now plan to carry out a longer term measurement using three SQUID systems as well as fluxgate sensors to track changes in the gradient of the magnetic field across the underground complex

Simultaneous geomagnetic monitoring with multiple SQUIDs and fluxgate sensors across underground laboratories

For two periods of several weeks duration in spring and autumn 2014, we monitored fluctuations in the geomagnetic field using 3-axis SQUID magnetometers at three locations within the LSBB tunnel network. Measurements at the water flow point C, and the GAS gallery supplemented the measurements by the permanent [SQUID]2 system in the Capsule. The frequency spectra of the magnetic fluctuations varied considerably between the three locations. These measurements have provided a unique data set to investigate the potential of underground SQUIDs to monitor the gradient of geomagnetic signals and thus investigate magnetotelluric effects and possible seasonal effects due to changes in the water content of the surrounding rock. We also compared the measurements of a SQUID and fluxgate system, and report on fluxgate measurements at the Boulby Underground Laboratory

Hydrodynamic organisation of the flows in the unsaturated zone of the Fontaine de Vaucluse karst system. First results

Karst systems contain important groundwater resources. Due to their complexity, they are generally under exploited. Particularly, the hydrodynamic functioning of the unsaturated zone is badly understood even if its important role is admitted Today, the hydrogeologists are agreeing with the important function(s) of the unsaturated zone in karst systems, but today this role(s) is badly characterized. As they are very complex systems, karst aquifers are generally under exploited. It is necessary to progress in the understanding of the functioning of the karst systems and particulary in the functioning of the unsaturated zone in order to develop the corresponding management tools. The study of the unsaturated zone of a karst system needs some access to this part of the aquifer. Speleological access is not sufficient because the major part of the water flows in not humanly enterable drains. The Low-Noise Underground Laboratory of Rustrel- Pays d’Apt (LSBB) is an artificial gallery digged in the unsaturated zone of the Fontaine de Vaucluse karst aquifer catchment area. It intersects arbitrarily the fault networks in depth and then the potential areas of flows through the unsaturated zone. From 2002 to 2009, 61 points where water regularly flows have been identified. For each, flow rate has been periodically monitored. This first global study of the acquired data shows a good relation between flows and geological structures. An organization of flows with depth and geology is also underlined. With increasing depth, flows seem to concentrate from numerous faults networks to a little number of high discontinuities

Monitoring geomagnetic signals of groundwater movement using multiple underground SQUID magnetometers

Groundwater can influence the geomagnetic field measured underground in at least two key ways. The water levels in rock will determine its electrical conductivity, and thus change the magnitude of the telluric currents induced in the rock by changing magnetic fields generated in the ionosphere. This can be studied by using multiple magnetometers at different underground locations. Secondly the flow of water through rock will generate a small magnetic signal, of unknown magnitude, through the electrokinetic effect. SQUID magnetometry has the potential to allow passive studies of groundwater changes in complex systems such as karst. We have monitored geomagnetic signals using two SQUID magnetometers at the LSBB underground laboratory, and set an initial limit on the magnitude of the electrokinetic signal. We now plan to carry out a longer term measurement using three SQUID systems as well as fluxgate sensors to track changes in the gradient of the magnetic field across the underground complex

Implementation of an unshielded SQUID as a geomagnetic sensor

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Seismo-ionosphere detection by underground SQUID in low-noise environment in LSBB-Rustrel, France

International audienceThe permanent operation of a 3 axes magnetometer in the LSBB below 550 m of calcite rock is a unique system of magnetic observation: a rejection rate better than 3 fT/Hz over 40 Hz. The observation of magneto-ionosphere responses to wave emissions both at the epicentre and at their arrival at LSBB for earthquakes of magnitude larger than 3 is reported. A simple model predicts the starting time of these events. These results are compared with those provided by Doppler sounders for ionosphere responses to Rayleigh waves

About the world-wide magnetic-background noise in the millihertz frequency range

In the millihertz range, a single magnetometer can detect magnetic waves in the near-field regime. For such long wavelengths, it can measure the world-wide magnetic-background noise due to any charge displacement on Earth and within its environment. In this frequency band, the normal modes of the Earth's free oscillations exist and when excited, they shake the air column above them, up to the ionosphere where the moving charges emit a magnetic fluctuation, via Ampère's law. We show the magnetic-background noise spectrum obtained by an FFT analysis of 72 consecutive hours of magnetic-seismic calm. It is mostly due to vertical charge oscillations. Even in the absence of a quake larger than Mw = 5.2, spherical and toroidal modes are deected. Instrumental and analytical perspectives are discussed

Towards an international network of superconducting magnetometers for geomagnetic and Earth-Ionosphere studies.

International audienc

First demonstration of transcontinental SQUID magnetometry. A

International audienc