J&S has experienced staff and equipment to conduct the following geophysical testing:

• Ground Penetrating Radar (GPR)
• Electrical Resistivity(ER) Surveys
• Seismic Refraction
• Downhole/Crosshole Seismic Survey

Ground Penetrating Radar (GPR)

GPR is a technique typically used for non destructive detection of subsurface features such as underground voids, utilities, rebars and other objects of interest in soil and concrete structures. GPR method is performed by displacing an electromagnetic antenna through the surface of the test area while a periodic pulse is transmitted and received in form of echoes. This electromagnetic wave pulses are transmitted at the antennas center frequencies ranging from 100 and 1500 Mhz. Pulses propagate through the test medium directly under the antenna. Some energy becomes reflected back whenever a change in electric impedance is encountered, such as at a rebar, utility or void. The received echoes are amplified and filtered with specialized GPR software. A distance wheel odometer records scan distance along test path, allowing for the determination of relative location of features from start point. The resulting raw data is obtained in the form of echo amplitude versus time. The material’s dielectric constant is used to define velocity which is then used to convert echo time data to echo depth. GPR has the limitation of not being effective in detection of features on clayey subgrade.

ELECTRICAL RESISTIVITY SURVEYS

This method consists of mapping resistivity changes in the earth which is commonly useful for detection of voids, discontinuities or variations on the ground. . Resistivity is a fundamental parameter of the material that describes how easily the material can transmit electrical current. High values of resistivity imply that the material is very resistant to the flow of electricity, and low values of resistivity imply that the material transmits electrical current very easily. J&S typically uses either the Dipole-Dipole or the Wenner Method.

Dipole-Dipole is useful for detection of discontinuities or voids along a survey line. The dipole-dipole method commonly requires placement of 56 electrode/stainless steel stake on the ground at equally spaced intervals in the order of 1 to 2 meters.

The four point Wenner method is performed by placement of four (4) stakes in the ground at equally spaced intervals to make single measurements of the electrical resistivity values at certain depth. Electrode spacing is varied to obtain values for different depths. This method is commonly used for determination of soil electrical resistivity for grounding (earthing) of cell phone towers, wind turbines, electrical substations among other electrical transmission facilities.

Seismic Refraction (SR) Survey:

Seismic refraction is mostly used to determine depth to bedrock, rock rippability, fault zones and variations in the subsurface conditions. SR surveys are performed by placement of a string of geophones over the ground. A seismic wave is generated and the arrival times are recorded for each seismograph. The data is fed into processing software for the calculation of p wave velocity profiles along the survey line. These profiles are useful for interpreting condition variation between boreholes. ER surveys can be correlated to the published rock rippability charts for different bulldozer sizes. ER survey can also serve as a guide to evaluate if explosives will be required to achieve excavations in rock.

Seismic refraction survey colored profile showing the p wave velocity gradient being correlated to rock weathering horizons

Figure8: Seismic Refraction Survey used for detecting transition of zones with different geology

Borehole Seismic Surveys: Downhole Seismic (DHS) or Crosshole Seismic (CHS):

The DHS test is performed by drilling a borehole to the required tests depth to sample soils and install a PVC casing. The test consists of lowering two (2), three component (triaxial) geophone receivers to each measurement depth and impacting the ground adjacent to the borehole in the vertical and horizontal directions. The arrival times of the shear and compression waves are determined for each measurement depth. The change in arrival times from depth point to depth point will give the local shear and compression wave velocities (Pseudo Interval Velocities). DHS field procedures and computations are made in accordance to ASTM D 7400. J&S has also capabilities of conducting this test with the Cone Penetration Test (CPT) equipment which is equipped with seismic sensor.

A CHS test is a variation of the DHS where two (2) casings are installed and the geophone (receiver) is placed in one of the casings and the seismic source in the other adjacent casing. CHS test measurements of seismic wave velocities are made by recording the corresponding horizontal travelling wave arrival times between source and receiver at different depths.

J&S has completed many of these tests within the past 10 years, mostly with the objective of establishing the average shear wave velocities for seismic site response. These tests can be used to obtain a more accurate determination of the IBC 2009 seismic site classification. It is also used for site specific seismic studies or probabilistic seismic hazard analysis. DHS or CHS data can be used to obtain the required soil dynamic properties for the seismic response modeling and/or seismic soil structure interaction modeling.

Seismic wave time arrival record