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As the demand for high-resolution gravity gradient data increases and surveys are undertaken over larger areas, new challenges for data processing have emerged. In the case of full-tensor gradiometry, the processor is faced with multiple derivative measurements of the gravity field with useful signal content down to a few hundred meters' wavelength. Ideally, all measurement data should be processed together in a joint scheme to exploit the fact that all components derive from a common source. We have investigated two methods used in commercial practice to process airborne full-tensor gravity gradient data; the methods result in enhanced, noise-reduced estimates of the tensor. The first is based around Fourier operators that perform integration and differentiation in the spatial frequency domain. By transforming the tensor measurements to a common component, the data can be combined in a way that reduces noise. The second method is based on the equivalentsource technique, where all measurements are inverted into a single density distribution. This technique incorporates a model that accommodates low-order drift in the measurements, thereby making the inversion less susceptible to correlated time-domain noise. A leveling stage is therefore not required in processing. In our work, using data generated from a geologic model along with noise and survey patterns taken from a real survey, we have analyzed the difference between the processed data and the known signal to show that, when considering the G zz component, the modified equivalent-source processing method can reduce the noise level by a factor of 2.4. The technique has proven useful for processing data from airborne gradiometer surveys over mountainous terrain where the flight lines tend to be flown at vastly differing heights.

We report the detection of individual optical and ultraviolet photons using a different approach to photon detection based on a superconducting tunnel junction. A 20×20 μm2 junction, employing a 100 nm niobium film and operated at a temperature of ∼0.4 K, has been used to detect individual photons with inherently high quantum efficiency (>45%) over a broad wavelength range (between 200 and 500 nm), yielding high temporal (sub-ms) resolution, spatial resolution determined by the junction size, under conditions of minimal dark current, and in the absence of read noise. The quantum efficiency is limited by surface reflection, and could be improved by the deposition of antireflection coatings. The theoretical wavelength response range continues into the far UV and soft x-ray region, and is presently limited beyond 500 nm largely by the available signal processing electronics. The device intrinsically functions at very high incident photon rates—with count rates of order ∼10 kHz or higher being feasible and again currently limited primarily by the signal processing electronics—thus providing a correspondingly enhanced dynamic range by several orders of magnitude compared with previous panoramic photon counting detectors. The measured charge output from the device is highly linear with photon energy resulting in an optical photon detection system with intrinsic spectral resolution, related to the critical temperature of the junction material and, in the current device, providing a limiting spectral resolution of about 50 nm. It is realistic in the future to envisage that these devices could be packaged into arrays, with the resulting system characteristics offering advantages over detectors based on semiconductors.

The I-V characteristics of strongly coupled symmetric niobium-based superconducting tunnel junctions are found to display steplike structures at voltages less than the gap voltage 2D/e. A thorough investigation into the influences of magnetic-Geld and temperature variations on the structures has been performed. In addition, measurements have been made that allow the homogeneity of the junction barrier to be determined. The experimental results indicate that the structures arise due to either self-coupling, multiple Andreev-reflection processes or multiple-particle tunneling. The data have been analyzed in terms of each of these theories. The results of this analysis appear to indicate that multiple-particle tunneling is the mechanism most likely to be responsible for the subgap structures. If this is the case, this would indicate that three-particle tunneling has been observed in niobium-based junctions. Specific features in the structures are also observed; the current steps do not appear at exactly the voltages expected, and some steps are sharper than others. A modified version of the theory describing multiple-particle tunneling is also presented. It is found that this model is in good agreement with the experimental data. In addition, it is able to describe all features of the structures and indicates the presence of two different, resolved gaps in the superconducting region next to the junction barrier.