Phil Sirles

Case study: Shear wave velocity measurements before and after dynamic compaction of cohesionless soil deposits

Crosshole shear wave velocity measurements were used to evaluate the effectiveness of dynamic compaction of foundation soils at the Jackson Lake damsite, Wyoming. This investigation method quantitatively assesses the change with depth in the low strain elastic properties of the soils after dynamic compaction. Shear wave velocity measurements from three sites show that shear wave velocities increased within the upper 35 feet (10.7 meters) by approximately 28 percent. Generally, the fine grained silty sand layers showed slightly greater velocity increases than layers with higher percentage of gravels. The investigation demonstrates the ability of crosshole seismic testing to identify changes with depth in the elastic properties of cohesionless soil deposits due to dynamic compaction.

ELECTRICAL GEOPHYSICS FOR DEEP TUNNEL DETECTION AT A GOLD MINE REMEDIATION SITE

Surface and crosshole geophysical investigations, including time-domain dipoledipole resistivity and frequency domain Mise-a-la-Masse (MALM) surveys, were conducted on the Captain Jack Project, an EPA Superfund remediation site which includes the Big Five tunnel. The Mise-a-la-Masse survey used the main access tunnel as a transmitting electrode and recorded data on the surface above the tunnel using a pole-dipole configuration. The objectives of the surface geophysical investigation are two-fold: 1) Geologically characterize the host-rock and region in which the mineralized fault zone and associated mine tunnel system lies; and, 2) Determine the location of the mineralized zone and the associated tunnels, if possible.

Joint Use of Seismic Reflection and Time‐Domain Electromagnetic Methods to Aid Ground Water Modeling

High resolution seismic reflection and time-domain electromagnetic geophysical surveys were conducted in support of subsurface characterization and ground water modeling at a site in Wyoming contaminated with TCE. Two kilometer-long seismic profiles reveal several discontinuous cemented layers within the overburden sediments, and also show evidence of several faults that could be correlated between the lines. Time-dome electromagnetic (EM) data were collected over the same two lines as the seismic, plus four perpendicular tie lines. The EM results show areas of higher resistivity that correlate to the interpreted faults from the seismic sections. In addition, there is evidence that the EM data are delineating the contamination plume, represented by an area of increased resistivity relative to areas where the contamination is not detected in boreholes (i.e., background). This is despite expectations that the contamination concentrations present at the site were too low to be detected with surface geophysical methods. The combination of the two geophysical techniques allow for conclusions about the subsurface structure and presence of groundwater that could have an effect on the vertical and horizontal contaminant transport. The discontinuous layering, the faults, and the extent of the contamination interpreted from the seismic and electromagnetic surveys would be very difficult to interpret from borehole data alone. These data were most helpful to design and complete groundwater modeling for the site. No full paper available.

USING ERT TO LOCATE A HISTORICAL MINE TUNNEL

Existing tunnels in historical mine sites often serve as conduits for acid mine drainage thus, delineating them can be important in remediating these sites. In a recent field study, electrical resistivity tomography was successfully used to locate a tunnel in a former mining district in the Rocky Mountains of Colorado. Prior to the ERT survey, a combination of historical mine data and surface geophysical surveys were used to find the approximate location of the tunnel. This culminated in drilling of boreholes on both sides of the location of the tunnel. Cross-borehole ERT was then used to provide a final estimate of the tunnel location. Surface-to-hole ERT surveys were conducted near a second, known portion of the tunnel to determine the feasibility of finding the tunnel using only a single borehole. In addition to the ERT surveys, an electrode was placed inside the entrance to the tunnel and used to perform mise-a-la-masse (MALM) surveys with receiving electrodes at the surface and in the borehole.