Crosshole Sonic Logging (CSL) uses compressional seismic waves as the energy source. Seismic waves passing through concrete are influenced by the density and elastic modulus of the concrete. Fractured or “weak” concrete zones lower the velocity of the seismic waves and can, therefore, be detected. In addition, the amplitude of a seismic pulse is affected by these defects although this is not extensively used at the present time. The frequency content of the seismic energy pulse determines the resolution and penetration of the signal. High frequencies have high-amplitude attenuation but can image small targets. Conversely, lower frequencies have less attenuation but image larger targets. The seismic source produces an impulse whose frequency content is usually 30 to 40 kHz.
The CSL method is a “derivative” of the ultrasonic pulse velocity (UPV) test. The basic principle of the CSL test is that ultrasonic pulse velocity through concrete varies proportionally with the material density and elastic modulus. A known relationship between fractured or weak zones and measured pulse velocity and signal attenuation is fundamental for these tests. Research has shown that weak zones reduce velocities and increase attenuations. During the CSL measurements, the apparent signal travel time between transmitter and receiver are measured and recorded. By measuring the travel times of a pulse along a known distance (between transmitter and receiver), the velocity can be calculated as a function of distance over time. If a number of such measurements are made and compared at different points along the concrete structure, the overall integrity of the concrete can be assessed. The first-arrival travel times (FAT) recorded during CSL testing are known as compressional, primary, longitudinal, or P-wave arrivals. The P-wave is the wave having discrete particle motion in the same direction as the wave is moving. The surface of the constant phase, or the surface on which particles are moving together at a given moment in time, is called the wavefront. An imaginary line perpendicular to the wavefront is called a ray path. It is often assumed that a beam of produced ultrasonic energy travels along the ray path (Robert E. Sheriff and Lloyd P. Geldart, 1995).