In situ test of a borehole extensometer

Abstract

SummaryStresses in the Earth's crust are known to represent a key element of geodynamical processes at various spatial and temporal scales. Static stresses within lithospheric plates can be estimated on the basis of direct in situ measurements in boreholes or near the surface. Earthquake focal mechanisms yield the same type of information by sampling at greater depths. The quantification of geological deformations happens to be the principal tool to determine possible changes in time of the tectonic stress pattern. These changes can however be recorded only qualitatively and on the geological time scale.In seismic regions it is clear that the regional stress field varies on a time scale defined by the recurrence of earthquakes, say several tens of years. Our goal was to design a robust and cheap instrument which could continuously monitor stress changes at depths large enough to reduce meteorological influences.

A Theoretical Model to Predict Both Horizontal Displacement and Vertical Displacement for Electromagnetic Induction-Based Deep Displacement Sensors

“…At present, methods/instruments for deep displacement monitoring can mainly grouped to three categories. The first one is borehole extensometers, which includes two subcategories: multi-point borehole extensometers [ 19 , 20 ] and wire/cable extensometers [ 21 ]. Although wire/cable extensometers are relatively simple and low-cost devices, displacements measured by them are global ( i.e ., the total changing distance from one point on landslide surface to another fixed point inside the borehole below the slip surface) and the device can neither detect the vertical components and horizontal components of the underground slope movement, nor identify the presence of several slip surfaces, nor obtain the relative displacements at different depths within the sliding mass.…”