In Situ Measurements in Fractured Till Using Sidewall Sensors

Abstract

Subsurface parameters, such as hydraulic head, often vary markedly with depth in fine-grained glacial sediments, but sensors placed in vertical boreholes are poorly suited to resolve these variations. One problem is that conventional methods only allow one, or perhaps a few, sensors to be placed in each borehole. To address such limitations we have developed a method for accessing the sidewall of a borehole. The method uses a device that pushes sensors or sediment samplers laterally into the sidewall to distances slightly less than the diameter of the borehole. The device can obtain a core sample 15 cm long and 4 cm in diameter, and then insert a permeable sleeve for extracting water samples. The same device has been used to insert several types of electrodes capable of measuring water content (using TDR waveguides), Eh (using platinum electrodes), or electrical resistivity (using a miniature Wenner-type array). At a site near Flakkebjerg, Denmark, we installed 22 water samplers and 19 resistivity electrodes in a single borehole to measure hydraulic head gradients in detail and to monitor the vertical migration of ionic tracers. This approach can be used to install horizontally oriented TDR waveguides at virtually any depth, thereby extending the TDR technique to the study of deep vadose zones. At a contaminated site in the USA, TDR wave guides were installed to a depth of 12 m in glacial till. Other applications include measurement of Eh at a site where in situ chemical oxidization was used, and the in situ sensors provided results that are similar to data obtained from soil cores.