The compressional (Vp) and shear (Vs) wave velocities of over 1000 rock samples from around the world are presented at elevated confining pressures for use in basic research and seismic and engineering applications. The samples include a wide variety of igneous, metamorphic and sedimentary rocks from the continental and oceanic crust, plus a large suite of ores provided by the mining industry. The velocity measurements were made at room temperature and hydrostatic confining pressures ranging from 10 Mpa to 1 Gpa using the pulse transmission technique. Velocities were measured parallel and perpendicular to foliation and lineation in samples with strong fabrics in order to determine anisotropy, and some samples were measured both wet and dry, bringing the total number of runs entered in the database to about 2800. Densities are given for all samples and modal analyses are given for 20% of the samples.

Measurements of the velocity of sound through rock are widely used in geo-physics 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, the Geological Survey of Canada (GSC), in conjunction with Dalhousie University, has measured the acoustic properties (Vp, Vs, density) of over 1000 rock samples at hydrostatic confining pressures ranging from 10 to 1000 MPa at the GSC/ Dalhousie High Pressure Laboratory located in Halifax, Nova Scotia. The samples examined consist largely of igneous, metamorphic and sedimentary rocks and ores from Canada, but also include samples of oceanic and continental crust from around the world (see Figures 1 and 2 for generalized sample locations). The database from these studies is presented below together with a description of the lab facilities and methods used and a brief summary of the data.

The GSC/Dalhousie High Pressure Laboratory is equipped with an array of pressure vessels that can be used for pressure-curing and velocity measurements. The principle vessel (Figure 3) is a 7 ton, double-walled steel vessel with a 40 cm long x 10 cm diameter working cavity which can operate to a hydrostatic confining pressure of 1.4 GPa or 200,000 psi, making this the largest pressure vessel in the world with this service rating. The pressure medium consists of light hydraulic oil pumped into the working cavity by means of a two-stage intensifier. Electrical communication to samples in the working cavity is maintained through 8 insulated cone feed-throughs in the vessel's end closure, allowing as many as six samples to be measured at a time. The system, which was built by Harwood Engineering with funding from the Natural Sciences and Engineering Research Council of Canada, is housed in a double-reinforced concrete vault and monitored from control panels in an adjacent laboratory (Figure 4).

Velocities are measured on 2.5-6.0 cm long x 2.5 cm diameter minicores using the pulse transmission technique of Birch (1960) and Christensen (1985). 1 MHz lead zirconate transducers mounted on backup electrodes are used to send and receive P-waves through the samples and 1 MHz lead zirconate titanate transducers mounted on electrodes are used for S-wave measurements. To prevent the pressure medium from invading the sample during the pressure run, the minicores are sheathed in thin copper foil and the entire sample/transducer/electrode assembly is enclosed in neoprene tubing. Once the sample assembly is sealed in the pressure vessel and the pressure is raised, the sending transducer is excited by a high voltage spike from a pulse generator and the time of flight to the receiving transducer is measured at specified pressure intervals using a state of the art digital oscilloscope. The velocity is then calculated from the time of flight and the length of the sample. The accuracy is estimated to be 0.5% for Vp and 1 % for Vs.

In a typical pressure run (Figure 5), the velocity increases rapidly from 0 MPa to about 200 MPa in response to the closure of microcracks and linearly at higher pressures in response to the intrinsic elastic properties of the minerals in the rock. Since velocities at any given pressure are typically higher during decompression than during compression and the decompression cycle is more representative of insitu conditions, the velocities reported below were read from decompression curves. All samples were measured dry (D) at room temperature and a small number were re-measured wet (W) after saturation in water to simulate insitu conditions as closely as possible. In samples with no visible fabric, Vp and Vs were 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 (p) and normal (n) to foliation (F), bedding (B) and lineation (L).

Compressional wave velocities are presented along with densities, lithologies and sample locations for over 1000 rock samples in the GSC/Dalhousie Rock Properties Database. In addition, shear wave velocities are listed for approximately a quarter of the samples and anisotropy was measured in many samples, bringing the total number of velocity runs to approximately 2800.

As can be seen in Figures 6 and 7, which respectively show Vp and Vs versus density at a confining pressure of 200 MPa for all sample runs in the database, the rocks examined display a broad range of velocities and densities. While some of the range in velocity is due to anisotropy or differences in water saturation, most of the velocity variation and all of the density variation is due to mineralogy. Most of the rocks with densities < 3 g/cc are silicate rocks which tend to increase in velocity with density. The large field to the right of the silicates is occupied by ores in which the velocities are governed by variations in the bonding and mean atomic weight of the constituent ore minerals and their dilution in silicate or carbonate host rocks.

• Excel spreadsheet [XLS, 1.3 Mb]
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Birch, F., 1960. The velocity of compressional waves in rocks to 10 kilobars, Part 1, J. Geophys. Res., 65, 1083-1102.

Christensen, N.I., 1985. Measurements of dynamic properties of rock at elevated temperatures and pressures. Am. Soc. for Testing and Materials, Spec. Pub. 869, 93-107.

• Scientific Authority
  Matthew Salisbury
  Geological Survey of Canada - Atlantic
  Bedford Institute of Oceanography
  P.O. Box 1006, Dartmouth, N.S.
  Canada, B2Y 4A2
  
• Senior Technologist
  Robert Iuliucci
  Department of Earth Sciences
  Dalhousie University, Halifax, N.S.
  Canada, B3H 3J5

• Geological Survey of Canada
  • GSC Atlantic
• Dalhousie University: Earth Sciences