Spatial variability in the flow of a valley glacier: Deformation of a large array of boreholes

Measurements of the deformation of a dense array of boreholes in Worthingto n Glacier, Alaska, show that the glacier moves with generally bed-parallel motion. Strain in the 200 m deep valley glacier is constant near the surfac e but follows a nonlinear vertical profile below a depth of about 120 m. By a depth of 180 m, the octahedral strain rate reaches 0.35 yr(-1). The thre e-dimensional velocity field shows spatial complexity with significant devi ations from plane strain, despite relatively simple valley geometry in the vicinity of the 6 x 10(6) m(3) study volume. No evidence was found for time -varying deformation or movement along localized shear planes. Observations were made by repeatedly measuring the long-axis geometry of 31 closely spa ced boreholes over a 70 day period, and three additional holes after 1 full year of deformation. The holes were spaced 15 to 30 m apart. Installation and measurement of such a large number of boreholes required the developmen t of a semiautomated hot water drilling system that creates straight and ve rtical boreholes with uniform walls. The equipment and procedures enables b orehole profiles to be measured without the use of hole casing. Inclinometr y measurements collected in the holes were processed, analyzed for error, a nd visualized as a fully three-dimensional data set. The new methods offer unique insight into small-scale spatial and temporal variations in the patt ern of flow in a valley glacier.