Soils are differentiated vertically by coupled chemical, mechanical, and biological transport processes. Soil properties vary with depth, depending on the subsurface stresses, the extent of mixing, and the balance between mass removal in solution or suspension and mass accumulation near the surface. Channels left by decayed roots and burrowing animals allow organic and inorganic detritus and precipitates to move through the soil from above. Accumulation occurs at depths where small pores restrict further passage. Consecutive phases of translocation and root growth stir the soil; these processes constitute an invasive dilatational process that leads to positive cumulative strains. In contrast, below the depth of root penetration and mass additions, mineral dissolution by descending organic acids leads to internal collapse under overburden load. This softened and condensed precursor horizon is transformed into soil by biological activity, which stirs and expands the evolving residuum by invasion by roots and macropore networks that allows mixing of materials from above.
Superconducting quantum interference filters (SQIFs) have been created using two dimensional arrays of YBCO step-edge Josephson junctions connected together in series and parallel configurations via superconducting loops with a range of loop areas and loop inductances. A SQIF response, as evidenced by a single large anti-peak at zero applied flux, is reported at 77 K for step-edge junction arrays with the junction number N = 1 000 up to 20 000. The SQIF sensitivity (slope of peak) increased linearly with N up to a maximum of 1530 V T−1. Array parameters related to geometry and average junction characteristics are investigated in order to understand and improve the SQIF performance in high temperature superconducting arrays. Initial investigations also focus on the effect of the SQUID inductance factor on the SQIF sensitivity by varying both the mean critical current and the mean inductance of the loops in the array. The RF response to a 30 MHz signal is demonstrated.
Since December 1992, CSIRO and BHP have been field trialing rf HTS SQUID magnetometers for mineral prospecting applications. Ten field trials in widely varying environments (from -16°C to +4OoC ambient temperatures) in mostly remote locations saw the development of a system which can be operated in many configurations including ground based and airborne Transient ElectroMagnetics (TEM). The magnetometer system has been developed to a point where, at late times in TEM applications, the SQUID system has a higher signal-to-noise level than the competing traditional coil technology. In some trials, a SQUID magnetometer detected anomalies at later times than were observed with the coil system, indicating an enhanced ability to detect highly conductive targets. This paper reviews development of our 3-axis SQUID magnetometer. SQUID systems as B field sensors have advantages over coils which are dB/dt type sensors. We will discuss the importance of these advantages for mineral prospecting in regions with a conducting soil cover or overburden typical of the Australian landscape.
The quantum interference effects of one-dimensional (1D) parallel arrays of high-temperature superconducting (HTS) SQUIDs were investigated experimentally and theoretically via the voltagemagnetic field responses for 4-81 Josephson junctions. The sensitivity of the arrays generally decreased as the number of junctions (and SQUIDs) in parallel increased, contrary to the predictions of models in the low (zero) inductance limit. A full theoretical description was developed to describe 1D parallel HTS SQUID arrays with finite inductances in an applied magnetic field, by extending the model for a single DC SQUID to multiple loops in parallel and including the flux generated by currents circulating through all loops in the array. Calculations were extended from SQUID arrays with equal loop areas to arrays with a distribution of loop areas, otherwise known as superconducting quantum interference filters. The model uses parameters relevant to HTS arrays, including typical variations (up to 30%) in HTS Josephson junction parameters, such as critical current and normal resistance. The effect of the location of the current biasing leads was also explored through the calculations. This model shows good agreement with experimentally measured 1D arrays of different lengths and highlights the importance of the geometry of the current biasing leads to the arrays when optimizing the array response.
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