• Continuous, rapid, accurate & precise record of borehole lithology;
• Gamma ray + dynamic probing combinations;
• Borehole – Borehole correlation;
• Dynamic Probehole – Dynamic Probehole correlation.

INTRODUCTION

Natural gamma ray logging measures the natural radiation emitted by the strata adjacent to the borehole or probe-hole wall, by means of an ultra slim ‘sonde’ lowered down a borehole. Contained within the sonde, a sodium iodide scintillation crystal and photo-multiplier tube are used to detect and amplify the radiation. The resultant electronic signal is passed up to surface acquisition and processing software, via steel armoured logging cable, where the log data are outputted & plotted in real-time.

The most commonly occurring gamma emitting radioisotopes are Potassium 40, and to a lesser extent, the nuclides in the Uranium 238 and Thorium 232 decay series. These isotopes are present in most clay minerals and as a result, the gamma ray log may be used as an indicator of clay content.

METHOD & TECHNIQUES

• The gamma ray logging system may be run in conventionally drilled,    window or windowless sampled boreholes.

• The ultra slim gamma ray sonde enables dynamic probed boreholes    to be logged. This may follow insertion of PVC tubing or, in unstable    ground conditions; for instance loose sands below the water table,    the sonde is lowered down the bore within specially developed    hollow dynamic probing rods.

APPLICATION

The gamma ray logging–dynamic probing combination, opens up the potential of a powerful, yet rapid and economic technique producing a continuous record of the lithological profile, together with corresponding penetration resistances. By using published empirical correlations, quantitative and semi-quantitative geotechnical parameters can be assigned at any point in the depth profile.

The gamma ray log may also be used to fill in the horizons where poor sample recovery has resulted in gaps and uncertainties in the recorded soil profile. In addition, when combined with dynamic probing, it may provide an invaluable check on SPT results where borehole disturbance has yielded suspect ‘N’ counts, notably in loose granular deposits prone to piping.

A particular advantage of the gamma ray log is that it is able to resolve bed thicknesses of only a few centimetres with excellent repeatability. It is therefore a powerful tool when used for the correlation of probeholes and boreholes. This has implications for Geoenvironmental studies, for example in the detection of minor aquifers, aquicludes and the refinement of the site conceptual modeland can be used in environmental studies, to detect minor aquifers and aquicludes aiding the development of a detailed site conceptual model.

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