OPERATIONAL CHARACTERISTICS OF SMALL-SIZED ANTENNA DEVICES OF THE MEDIUM FREQUENCY RANGE IN MINES

Представлены результаты исследования трех малогабаритных антенн (электрического,магнитного и магнитоэлектрического типов)СВ-диапазона с целью определения особенностей их эксплуатации в рудниках и шахтах,т.е. в диспергирующих средах. Рассмотрены импедансные характеристики и изменение частотных зависимостей коэффициента стоячей волны в ближней зоне антенн, обусловленное непостоянством электрических характеристик горных пород при перемещении антенн вдоль горной выработки. Проведена оценка стабильности работы портативных радиостанций при подводимой к малогабаритной антенне мощности 1 Вт с учетом времени,характерного для сеансов связи. Даны рекомендации по эксплуатации антенн в условиях рудников и шахт. The article investigates three main types of small antennas (electric, magnetic and magnetoelectric) of the medium frequency range in order to determine the features of their operation in mines, i.e. in dispersing media. The impedance characteristics and the change in the frequency dependences of the standing wave ratio are considered when the situation in the near zone changes. An assessment of the stability of the operation of portable radio stations at an input power of 1 W has been carried out, taking into account the time characteristic of communication sessions. Recommendations for the operation of antennas in mines are given.

Comparison on seismometer sensitivity following ISO 16063-11 standard

Accelerometer calibration by primary and secondary means is now very well covered both in low and medium frequency range. This is obviously not the case for seismometers which are widely used in the world. Up to now, seismometers only have internal checks thanks to an integrated coil. Calibration of these devices address different challenges like large mass and volumes, non-rotatable devices, specific mounting. This inter laboratory comparison will help to investigate these fields and to give feedback to develop a methodology to calibrate seismometers.

Medium-frequency electromagnetic device to measure electric conductivity and dielectric permittivity of soils

An electromagnetic (EM) tool working in the medium-frequency range allows the determination of the electric conductivity and dielectric permittivity of soils with a single measurement. It brings information about different state parameters of soils, especially their water and clay contents for a significant volume of investigation. To investigate these properties, a medium-frequency-range EM prototype, the CE120, was built using a perpendicular coil Slingram configuration with a working frequency of 1.56 MHz and a fixed coil spacing of 1.2 m. This configuration was chosen using modeling with the purpose of measuring electric resistivities up to a few thousands of ohm-meters and relative dielectric permittivities as low as two. These thresholds match the expected parameters values in the medium frequency range. Moreover, the CE120 characteristics allowed for an investigation depth between 2 and 2.5 m, depending on the nature of the soil. The prototype was tested on two different soils with water variations: sandy alluvia and clay-loam soil. The electric resistivities of the sandy alluvia can reach [Formula: see text], which is higher than the detection threshold of the CE120. Only the measured dielectric permittivity can be interpreted in terms of water contrasts. Four different zones corresponding to four different water contents were detected. For the clay-loam soil, the electric conductivity and dielectric permittivity measure the water content variations created by the wheat roots. At 1.56 MHz, the high-frequency equations linking the dielectric permittivity to the volumetric water content have a limited validity. Laboratory measurements for each different soil type are necessary to deduce empirical relations. In both cases, the results are coherent with the measured mass water content variations.