A retrievable sensor installation technique for acquiring high frequency signals

The use of in-seam waves for void detection in mines requires the capability of capturing high frequency signals over large distances. For instance, the Airy phase of Love waves which are used for void detection in coal mines ranges from several hundred to over one thousand Hertz and the expected travel distance of these signals is at least 90 m (equivalent to a detection distance of 45 m) for the technique to be considered practical. In order to obtain high quality and broadband signals, sensors are conventionally grouted at the bottom of boreholes so that the attenuation due to the fractured surface is minimized and the coupling effect is improved. However, to be economically feasible, the expensive and high sensitive sensors must be retrievable so that they can be used repeatedly at the same or other locations. Because of these concerns, a retrievable sensor installation technique was developed. This paper provides a detailed review of the technique as well as a brief discussion of its applications. The technique is simple and reliable for both installation and retrieval operations and can be used for boreholes oriented in any directions. The technique has been demonstrated in over 200 sensor installation/retrieval operations under various borehole conditions, including bituminous coal, anthracite coal, shale, sandstone and trona. With this technique, we were able to detect the high frequency signals required for our projects. For instance, the signals used at a trona mine for void detection have a typical frequency of 5 kHz with the travel distance of 150–200 m. The results of these operations have shown that sensors installed in the prescribed manner exhibit predictable, consistent, and repeatable performance. The technique also provides an economical and reliable means for many other field seismic monitoring applications where high quality and broadband signals are essential, such as microseismic monitoring and geotomography studies.