Interpretation of borehole impulse tests at Haut Glacier d'Arolla, Switzerland

One hundred and sixteen impulse tests were conducted in a dense borehole array at Haut Glacier d'Arolla, Switzerland, during July and August 1995. Impulse-test response pressure oscillations reveal marked variations in three parameters: relative amplitude decay, oscillation frequency and net water-level displacement. Analysis of response signals from water-filled boreholes indicates that (i) responses from individual boreholes are reproducible over some tens of minutes; (ii) responses are similar, but not identical, for slug insertion ("slug tests") and removal ("bail tests"); (iii) signal amplitude decay varies with the depth at which the recording pressure transducer is located in a borehole; and (iv) coherent minima in all three response parameters coincide with the location of a known preferential, subglacial water-flow pathway. This correspondence suggests, first, that water-filled and non-fluctuating boreholes have established a link to the subglacial aquifer at Haut Glacier d'Arolla and, secondly, that locally transmissive basal sediments may be identified by relatively low response-signal frequencies, response-signal decays and net water-level displacements. Impulse testing water-filled boreholes therefore has the capacity to provide information relating to the local hydrogeological properties of subglacial aquifers. In 43 cases, water levels in boreholes adjacent to those being tested were recorded in order to identify possible subglacial linkages. No such connections were detected.