A new borehole wire extensometer with high accuracy and stability for observation of local geodynamic processes

Abstract

Very stable and reliable instruments with high accuracy are required in field measurements for continuous monitoring local geodynamic processes, such as tectonic movements, ground motions in landslide prone areas, etc. A sensitive borehole wire extensometer with low energy consumption was developed in the Geodetic and Geophysical Research Institute of the Hungarian Academy of Sciences to observe very small vertical movements (in the order of a few millimeters) of the upper layer of the soil due to hydrological, meteorological and biological processes. The newly developed instrument eliminates the disadvantages of the borehole wire extensometers which are presently used. Its sensitivity and stability are much higher than these parameters of the previous instruments. The instrument is able to measure distance variations without instrumental drift in a range of 0-4 mm with a resolution of better than 1 µm. Since the effect of the yearly temperature variations can be easily removed from 2 the extensometric data record, the compensation for the short-periodic (daily) thermal effects on the instrument was of high priority during the design of the instrument. This paper describes the construction and calibration of the extensometer. The extensometer was installed for monitoring vertical ground movements due to hydro-meteorological processes on the high loess wall of the Danube River at Dunaföldvár, Hungary. The efficiency of the temperature compensation of the instrument was investigated in detail on the basis of the measured data series.

Assessment of long-term structural movements in a historic cliffside construction through Lomb-Scargle spectral analysis of unevenly spaced time records: the Punta Begoña Galleries (Getxo, Spain)

“…Our records show an evident relationship with temperature. In the case of the most superficial devices, crackmeters, the records display reversible opening-closing seasonal movements characteristic of the materials thermal response to ambient temperature variations (Mentes 2012;Colombero et al 2018). The extensometers, which measure displacements relative to internal points in the rock massif, show a more complex behavior, with the highest elongations occurring cyclically during the coldest months.…”

Soil Stratum Tides

“…For example, ground‐ and space‐based geodetic techniques (precise leveling, satellite imagery, satellite navigation) are capable of detecting minute deformation, but their measurements are limited to land‐surface elevation changes (Amelung et al., 1999; Shirzaei & Bürgmann, 2018). In contrast, extensometers can be deployed in vertical boreholes at discrete locations below the ground surface, to observe the compaction of key substrata (Mentes, 2012). To map subsurface deformation in higher spatial detail, Brillouin distributed fiber‐optic sensors have been introduced to complement borehole extensometry (Zhang et al., 2018), with the added benefits of corrosion resistance and immunity to electromagnetic interference.…”

Microanchored fiber-optic DSS in boreholes allows strain profiling of the shallow subsurface

“…These ground-based or remotely sensed techniques have proved to be effective in mapping large-scale ground motions 5 , but they do not allow for subsurface deformation profiles to be obtained. Drilling is a common means to determine lithology; by installing extensometers in drilled boreholes, deformations occurring at certain depths below the ground surface can be observed 6 .…”