A particle-size classification of sand and gravel deposits as a basis for end-use assessment
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Drilled boreholes generally are the only feasible means to access the subsurface for the<br>emplacement of downhole electrodes for most hole-hole and hole-surface resistivity<br>experiments. However, the very existence of the borehole itself creates the potential for<br>significant noise due to the inevitable conductivity contrast that develops between the borehole<br>walls and the formation. Borehole effects develop whenever a current source is placed in a<br>drilled borehole. Borehole geometries may range from nearly perfect cylinders to highly,<br>irregular, rugose holes in consolidated rock, to relatively minor, collapsed, disturbed zones in<br>caving sediments. Boreholes in non-caving formations generally are filled with artificial<br>materials to afford crucial, electrical continuity between downhole electrodes and the borehole<br>walls. Filled boreholes form cylindrically shaped heterogeneities that create significant noise<br>due to preferential current flow up and down the conductive columns. Selected conditions are<br>simulated with a finite difference model to illustrate the significance of borehole effects on holehole<br>and hole-surface mise-à-la-masse electrical potentials near a current electrode. Preliminary<br>modeled results of borehole effects for electrical resistance tomography (ERT) suggest the<br>existence of an inhomogeneity between the two boreholes which may cause misinterpretation of<br>the data, depending on the resistivity contrast of the borehole fill and earth material.
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Electrical Resistivity Tomography
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ABSTRACT In the past, the doorstopper method has been used successfully for determining in situ tectonic stresses. Here an example is given about the application of the borehole slotter in the domain of tectonic stress determinations, which is an innovative strain relief technique without overcoring. Data are presented that test the reliability of the strain measurements by the borehole slotter under well‐known stress conditions, that indicate the reproducibility of borehole slotter strain data compared with those from doorstopper measurements, and that demonstrate the utility of the borehole slotter even for the determination of weak tectonic strains. In northern Switzerland the borehole slotter was used in a horizontal borehole under lithostatic loading conditions only, in a rock slope area with very heterogeneous stress conditions, and in a vertical borehole where tectonic stress should be dominant. The results show that the borehole slotter is a reliable technique, which allows precise determination of in situ strains. In addition, borehole slotter measurements are less time‐consuming and therefore less costly than most other stress‐measuring techniques—an important aspect in a future of reduced research budgets. Summary The borehole slotter is a strain relief technique, using a recoverable strain sensor that need not be glued to the rock surface and need not be overcored. It is quicker to use than most other stress or strain measuring techniques, especially in comparison to standard overcoring techniques (Fig. 14). The borehole slotter supplied convincing results with respect to principal stress orientations, in good agreement with: (i) principal stress orientations related to lithostatic stress conditions (Dittingen 2 site), (ii) palaeostress orientations during the main stage of Jura folding only 13 to 7 Ma (Laufen 1 site), (iii) the results of neighboring door‐stopper measurements (Dittingen 1 site). The borehole slotter tests do not supply stress magnitudes directly. Thus, the elastic properties of the rock mass have to be determined independently. Stress ratios from borehole dotter tests in Dittingen 2 are realistic. In conclusion, the borehole slotter is a fully developed probe to measure in situ strains in boreholes. It allows the reliable determination of a large number of data within a short time even in weakly stressed rocks. This makes the borehole slotter a powerful tool for the determination of contemporary regional tectonic stress fields.
Strain (injury)
Overburden pressure
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