Rock velocity

Measurements of the velocity of sound through rock are widely used in geophysics and engineering to study the structure, composition and physical state of the crust. For example, laboratory measurements of compressional (Vp) and shear (Vs) wave velocity in rocks at elevated pressures may be used to interpret crustal velocities obtained by seismic refraction in terms of petrology because velocities are very sensitive to rock composition. Similarly, differences between laboratory and actual formation velocities can be used to estimate formation porosity and the formation velocities themselves are used to make time-to-depth conversions in seismic reflection surveys. Other applications of such data include the prediction of seismic reflectivity at lithologic boundaries in the crust and the estimation of the engineering properties of rocks. To provide a quantitative basis for such applications, measurements of Vp and density were made at elevated confining pressures ranging from 10-600 MPa on 61 igneous, metamorphic and sedimentary rock samples from the Slave Province. The map below displays the location of the study area within the Slave Province.

Velocities were measured in a 1 GPa pressure vessel at the Geological Survey of Canada, High Pressure Laboratory at Dalhousie University in Halifax, Nova Scotia on 2.5-6.0 cm long x 2.5 cm diameter minicores using the pulse transmission technique. 1 MHz lead zirconate transducers mounted on backup electrodes were used to send and receive P-waves through the samples. To prevent the pressure medium from invading the sample during the pressure runs, the minicores were sheathed in thin copper foil and the entire sample/transducer/electrode assembly was enclosed in neoprene tubing. Once the sample assembly was sealed in the pressure vessel and the pressure raised, the sending transducer was excited by a high voltage spike from a pulse generator and the time of flight to the receiving transducer was measured at specified pressure intervals using a state of the art digital oscilloscope. The velocity was then calculated from the time of flight and the length of the sample. The accuracy of the measurements is estimated to be 0.5%. All samples were measured dry at room temperature. In samples with no visible fabric, Vp was measured in only one arbitrary direction. For samples with a pronounced fabric, velocities were typically measured in several directions, with the propagation and vibration directions aligned parallel and normal to foliation, bedding and lineation. For rocks with more than one measurement average velocities where used to plot the value at a site.

Velocities reported for a confining pressure of 200 MPa are displayed on the map below. The velocity data set are available for download [XLS, 46.6 kb] and contain the sample number, latitude, longitude, rock-type, density (g/cc) and compressional velocity (km/sec).