Code of practice for seismic testing of rock mass, Part 1: Within a borehole
1992 Edition

Per IS 13372 Part 1 (1992), seismic measurements inside boreholes primarily involve P-waves (compressional waves) and S-waves (shear waves), including SH waves which are a subtype of S-waves. The standard mandates capturing travel time curves for both P and S waves at various depths and employing geophones or accelerometers to measure particle velocity or acceleration. These wave types are critical for calculating velocity profiles, dynamic elastic moduli, and conducting geotechnical assessments.

The standard recommends placing seismic sources either at the surface or in auxiliary boreholes near the main borehole to improve coupling and bypass weathered layers. For downhole testing, sources can also be located inside the borehole, such as via blasting or falling weight methods. Receivers, typically geophones or piezoelectric accelerometers, are installed firmly against the borehole wall or suspended in water-filled holes. Frequency ranges usually span 10 Hz to 500 Hz for general testing, extending higher for detailed analyses.

Seismic wave travel times are fundamental for calculating rock properties under IS 13372 Part 1. First, travel times of P and S waves are recorded from the source to receivers. Velocities are computed by dividing the distance by travel time. These velocities, combined with rock density, are used to derive dynamic elastic parameters such as Poisson's ratio, rigidity modulus, bulk modulus, and dynamic Young's modulus through established formulas. This procedure provides a non-destructive in-situ evaluation of rock stiffness and mechanical behavior.

According to the code, after each relocation of the seismic source (or both source and receiver if placed in the same borehole), seismic waves should be generated and measurements repeated to ensure data reliability. Similarly, following each movement of receivers within the borehole, seismic wave generation and response recording are required. For S-wave downhole testing, special emphasis is placed on generating and detecting SH waves. Sources are typically placed in boreholes with receivers on the surface nearby or within the borehole above the source, ensuring firm contact or appropriate suspension.

The standard mandates comprehensive reporting that includes borehole details (location, dimensions, inclination, casing), detailed drawings of source and receiver positions with coordinates, equipment specifications including frequency responses, and any deviations from standard procedures. Reports must also present waveforms, time-distance curves, drilling logs, velocity distributions, tabulated dynamic elastic modulus values, and any additional equations or assumptions used. When applicable, geotechnical interpretations considering local geology should be included. Numerical results should be rounded following IS 2:1960 guidelines to ensure clarity and consistency.