Alexander Brandelik

A new in-situ sensor for large-scale snow cover monitoring

Abstract A new in situ sensor for the simultaneous measurement of snow water equivalent, snow density and liquid-water content is presented in this paper. The system consists of radio frequency transmission lines of up to 25 m length cast in a flat PVC band, which can be set up either horizontally to monitor single snow-layer properties or sloping from a mast to the soil surface to determine vertical snowpack properties. The dielectric coefficient along the flat-band cable is measured with a time-domain reflectometer at high frequencies, and with a low-frequency impedance analyzer. The performance of the sensor system was tested during two winter seasons (2001–03) at the high-alpine test site Weissfluhjoch, Davos, Switzerland. The cable suspension and set-up of the sloping cable was shown to be critical with regard to stability and the formation of unwanted air gaps along the cable. Overall, the sensing system proved quite robust and produced results in agreement with manual snowpack observations.

Superconducting Accelerating Cavities for High Energy e+-e--Storage Rings

In electron storage rings the high demand of RF power contributes considerably to both construction and operating costs. A cost optimization shows, that a storage ring with superconducting cavities is smaller and cheaper than one with normal cavities. Exchanging normal into superconducting cavities in an existing storage ring can increase the end energy by about 40%. Open questions like performance, especially in presence of synchrotron radiation, effects of higher order modes and handling of high beam power need tests in an operating storage ring. The preparations and components of a test in DORIS are described.

Vicarious calibration for remotely sensed hydro power water resource

As part of the natural resource management (NRM) the exploitation and assessment of water resources from melted snow, the snow water equivalent (SWE) for hydro power generation can proceed only by remote sensing. The evaluation algorithm EQeau is already introduced for this assessment. This connects the remotely data to the SWE exploiting the seasonal thermal resistance change of the soil and snow cover. On the other hand this technique needs representative vicarious ground calibrations, especially that of the snow density. At this end the devices used for the ground truth measurements are too small in comparison with a remote sensing pixel size and are not suitable for continous monitoring. The new method and device senses a more than 50 times larger measuring volume than the usual ones, its linear extension can be compared by a pixel side. The sensor is an unshielded flat band cable which will be embedded by snow fall and remains and measures there during the entire winter season. With time domain reflectometry (TDR) and low frequency measurements on this long sensor one can determine the density, the most important input for the calibration of the remotely sensed data. The method contributes to a better prognoses for avalanche and flood warning as well, because it measures the snow liquid water content also. The method and instrument are installed in four consecutive winter seasons.

A superconducting accelerating cavity in PETRA

To investigate the feasibility of superconducting accelerating cavities in electron-positron storage rings a single cell niobium cavity was constructed and tested in PETRA at DESY, Hamburg. The paper describes the experimental setup and summarizes the results obtained: Accelerating field gradients of the order of 3MV/m and Q-values of 109 can be obtained in an operating storage ring and maintained over many weeks. Synchrotron radiation impinging on the cavity surface does not affect the performance. Higher order modes excited by the beam were coupled out, their amplitudes were measured and agree with theory. PETRA could be operated at 5 GeV and 2 mA with the superconducting cavity alone.

Operation of a Superconducting Accelerating Cavity in PETRA

To investigate the feasibility of superconducting cavities in electron-positron storage rings a single cell Niobium cavity was tested in PETRA. The paper describes the experimental setup and summarizes the results obtained. Accelerating field gradients of 3 MV/m and Qvalues of 10/sup 9/ are measured in an operating storage ring and maintained over many weeks. Synchrotron radiation impinging on the cavity surface did not affect the performance. Higher order modes excited by the beam were coupled out; their amplitudes agree with theory. PETRA was operated at 5 GeV and 2 mA with the superconducting cavity alone.

Measurements on a Superconducting Accelerating Cavity for DORIS

An experiment has been set up to test a superconducting cavity in the e+-e--storage ring DORIS (DESY, Hamburg). The cavity achieved accelerating field gradients Eacc of 3.2 - 4 MV/m and Q-values up to 6×109. The input coupling was tested up to 45 kW at total reflection, corresponding to 90 kW at matched conditions. Two higher mode couplers have shown 102 ¿ Qext ¿ 104 for all modes below the beam tube cutoff frequency and >108 for the fundamental mode. The completed cryogenic system works as planned.

Bestimmung von Feuchteprofilen mittels TDR während eines Einstauversuches an einem naturgemäßen Deichmodell (Determination of Moisture Profiles using TDR on a Full-Scale Dike Model during a Simulated Flood)

Viele Anwendungen in der Hydrologie, der Landwirtschaft und des Bauwesens benötigen eine Aussage über die räumliche Verteilung der Feuchte im Boden oder in Bauwerken. Bisher war die Zeitbereichsreflektometrie (Time Domain Reflectometry (TDR)) als gebräuchliches Verfahren zur Messung der Feuchtigkeit auf die Bestimmung einer gemittelten oder sehr grob auflösenden Feuchteverteilung entlang der verwendeten Messleitungen beschränkt. Ein neues Verfahren zur Wiedergabe der Feuchteverteilung entlang einer Messleitung wurde entwickelt, welches nun den gesamten Informationsgehalt des von beiden Enden der Messleitung gewonnenen TDR-Signals für die Auswertung verwendet. Es basiert auf der Telegraphengleichung für ungleichförmige Übertragungsleitungen und einem Optimierungsansatz für die gleichzeitige Rekonstruktion von zwei Leitungsparametern mit hoher Auflösung entlang der Leitung. Die Tauglichkeit des Rekonstruktionsverfahrens unter Verwendung von realen Messdaten wurde an einem naturmaßstäblichen Modelldeich während eines simulierten Hochwassers getestet.

Moisture monitoring with subsurface transmission lines

Waste disposals are covered with high density and highly saturated clay layers, which prevent rain penetration through waste into groundwater. These layers without monitoring are overdesigned to achieve long term performance. In case of a containment control, which means water content control, the construction can be less expensive and defects can be selectively repaired. We developed a new subsurface moisture sensor system, which locates a water content change of 2% with an accuracy of 4 m in a clay layer on a field of 2000 m2. It consists of a network of unshielded high-frequency transmission lines, installed during the disposal construction. The electromagnetic propagation along these lines is affected by the dielectric coefficient of the surrounding media which is strongly related to its water content. Usually, soil moisture sensors with Time-Domain- Reflectometry (TDR) are much shorter than 1 m. By dielectric measurements on several soil types and by an advanced model of the effective line parameters in soil, we could design transmission lines with lengths up to 20 m. Additional coupling zones between lines are used to economize connecting cables and to enhance the space resolution. TDR methods have been found less accurate than appropriate frequency domain methods performed with a network analyzer. The demonstrated moisture distribution maps of this test field show small local and seasonal variation since 1997.

Advanced ground truth for remote sensing of soil moisture

The increase in accuracy of soil moisture retrieval from radar data requires a complete and accurate ground truth for calibration and verification. In order to support microwave backscattering models, one has to provide reliable permittivity and volumetric water content profiles of the soil. The conventional methods do not meet these requirements. The gravimetric sampling does not even deliver the volumetric water content. It has to be completed with an inaccurate measurement of the soil bulk density. Furthermore, the frequent probing of large areas is a tedious and expensive task. The electric conductivity is also a poor indicator due to its strong dependency on the salt content. Neutron probes have less and less acceptance and suffer from the shortcomings in measuring thin surface layers. The remaining possibility is the dielectric moisture determination by rf or microwaves, preferably in the optimal frequency range between 100 MHz and 1000 MHz. The measurement at single frequencies is better than the use of broadband pulses in the time domain, because of the capability to convert permittivities measured at one frequency to permittivities at the radar frequencies. Based on the measurement of the permittivity we have developed a combined sensor system for the ground truth. Our previously published frost-calibrated in-situ device evaluates the permittivity profile, the volumetric water content profile and the physical properties of the soil in regard to the permittivity of the solid component and in regard to the bound water. It uses a frequency of 250 MHz and it is installed at stationary locations. A second portable sensor extends the measuring area and completes the system. With frequencies of 500 MHz and 1000 MHz it is intermediate to the in-situ sensor and also to the radar frequencies in L-, C- and X-band. The scanning rate is about 100 m2 in 10 minutes, which is suited for extensive field campaigns. This portable sensor is based on a Goubau-type open-ended surface waveguide, where a simple dielectric coated stick touches the soil surface. Its sensing area is sufficiently large to integrate over soil inhomogeneities. The diameter of the area and the penetration depth of the electromagnetic wave are adjusted by the frequency and the dimensions of the stick. The relation between permittivity and water content is determined by the in-situ sensor, resulting in an overall system accuracy of better than 6% for the volumetric water content.

Calibration of remotely sensed soil moisture

All remotely sensed soil moisture data have to be calibrated by reference measurements on ground. The gravimetric sampling (oven drying) is not suitable. It is time consuming, expensive, and destroys the site. Also the neutron probe has less and less acceptance. The remaining possibility is the dielectric moisture determination by rf or microwaves. The optimal measuring frequency for such a device is between 100 MHz and 1000 MHz. Our previously published frost-calibrated in-situ sensor uses 250 MHz. The difference to the used remote sensing frequencies, in L-C- and X-bands is too large. In addition the stationary in-situ device measures only its closest neighborhood. The remotely sensed pixel however is much larger. Therefore, we developed an intermediate sensor for a good compromise in these discrepancies. With frequencies of 540 MHz and 940 MHz it is intermediate to the in-situ sensor (250 MHz) and to the used L-C-X-bands. It is a Goubau-type open-ended surface waveguide, a simple dielectric coated stick, touching the soil surface. The reflected wave on the Goubau-line is a function of the moisture from a well-defined area under the line (0 0.4 m). So, it is intermediate concerning the measuring area, as well. We need approximately 10 minutes for scanning an area of 100 m2. This time is short enough. On the other side, this surface moisture sensor will be calibrated by a very accurate frost-calibrated in-situ sensor. An overall system accuracy of 6% can be achieved by this two step indirect ground truth.