Code of practice for seismic testing of rock mass, Part 2: Between the borehole

IS 13372 Part 2: Scope - Key Specifications & Tables

Scope Summary:

• Defines seismic testing between boreholes for rock mass evaluation.
• Covers apparatus, measurement methods, data reporting, and evaluation.
• Applicable to seismic wave velocity measurement (P, SH, SV waves).

Key Tables & Formulas

Table 1: Seismic Sources for Crosshole Measurement

Apparatus Requirements (Clause 4.1)

• Seismic Source: Select based on wave type (see Table 1).
• Seismic Receiver:
  • Geophone (velocity-sensitive, 10 Hz–2 kHz)
  • Piezoelectric accelerometer (acceleration-sensitive, 10 Hz–60 kHz)
  • Hydrophone (pressure-sensitive, water-filled boreholes only)
• Receiver Array: Triaxial (1 vertical + 2 horizontal) for wave identification.
• Data Acquisition System:
  • Amplifier (Gain 70–130 dB, freq. 4 Hz–2 kHz)
  • Signal enhancement for noisy environments
  • Timing device (0.1, 1, or 10 ms intervals)
  • Time break unit (separate channel)
  • Recorder (analog/digital visual record)

Reporting Requirements (Clause 7)

Reports must include:

• Borehole details: location, length, diameter, inclination, casing.
• Drawings: seismic source/receiver positions with coordinates.
• Equipment specs & measurement methods.
• Waveforms for each source-receiver pair.
• Average propagation velocities between points.
• Dynamic elastic modulus tabulation.
• Equations and assumptions used.
• Geotechnical interpretation if required.

Velocity & Elastic Modulus Relation (General)

[ E_d = \rho V_p^2 (1 + \nu)(1 - 2\nu) / (1 - \nu) ]

Where:

• (E_d) = Dynamic elastic modulus
• (\rho) = Rock density
• (V

IS 13372 Part 2: Key Definitions, Formulas & Tables

1. Definitions (Clause 2.0)

• The standard defines terms related to seismic testing between boreholes.
• Key terms include seismic source types, receivers, wave velocities, and dynamic elastic parameters.

2. Seismic Source Types (Table 1, Clause 4.1)

3. Dynamic Elastic Parameters (Clause 6.4)

Given:

• ( P ) = density of rock (kg/m³)
• ( V_p ) = P-wave velocity (m/s)
• ( V_s ) = S-wave velocity (m/s)

Formulas:

[ \textbf{Dynamic Poisson's ratio, } \nu = \frac{(V_p/V_s)^2 - 2}{2((V_p/V_s)^2 - 1)} ]

[ \textbf{Dynamic modulus of rigidity, } G_d = P V_s^2 = \frac{E_d}{2(1 + \nu)} ]

[ \textbf{Dynamic bulk modulus, } K_d = P \left(V_p^2 - \frac{4}{3} V_s^2 \right) = \frac{E_d}{3(1 - 2\nu)} ]

[ \textbf{Dynamic Young's modulus, } E_d = 2 G_d (1 + \nu) = 3 K_d (1 - 2\nu) ]

4. Reporting Velocity (Clause 2.5, Table 7)

• Typical velocity for reporting: 3.5 km/sec (3,500 m/s)

5. Seismic Testing Setup (Clause 4.1)

• Use triaxial receivers (vertical + two horizontal at right angles).
• Frequency range: 10 Hz to 2 kHz (general purpose).

IS 13372 Part 2: Principle of Test Method (Seismic Testing Between Boreholes)

Key Concept:

• Measure seismic wave velocity between boreholes by generating waves at a source borehole and recording arrival times at receiver boreholes.
• Compare measured velocities with expected values to evaluate rock mass properties (Clause 6.6).

Apparatus & Sources (Clause 4.1 & Table 1):

Receivers:

• Geophone: Voltage ∝ particle velocity (10 Hz–2 kHz), natural freq < ½ predominant wave freq.
• Piezoelectric accelerometer: Voltage ∝ acceleration (10 Hz–60 kHz), natural freq ≥ 2× predominant wave freq.
• Hydrophone: For water-filled boreholes, voltage ∝ water pressure (10 Hz–60 kHz).

Basic Formula for Velocity:

[ V = \frac{L}{t} ]

• V: Wave velocity (m/s)
• L: Distance between source and receiver (m)
• t: Travel time of seismic wave (s)

Reporting Requirements (Clause 7):

• Borehole details (location, length, diameter, inclination)
• Seismic source & receiver positions and coordinates
• Equipment specs & frequency characteristics
• Waveforms and computed average velocities (see Fig. 3)
• Dynamic elastic modulus tabulation
• Equations and assumptions used
• Geotechnical interpretation considering geology

Summary Diagram:

flowchart LR
    A[Seismic Source in Borehole] --> B[Seismic Wave Propagation]
    B --> C[Receivers in Other Boreholes]
    C --> D[Record Arrival Times]
    D --> E[Calculate Velocity V = L/t]
    E --> F[Evaluate Rock Mass Properties]

Note: Use a triaxial array of receivers (vertical + 2 horizontal) for wave identification. Frequency band typically 10 Hz–2 kHz for general crosshole tests.

IS 13372 Part 2: Apparatus for Seismic Testing

Key Apparatus and Specifications (Clause 4.1):

Typical Frequency Band:

• General crosshole: 10 Hz – 2 kHz
• Detailed studies: Higher frequencies required

Table 1: Seismic Sources for Crosshole Testing

IS 13372 Part 2 - Seismic Test Procedure Key Points

Apparatus (Clause 4.1)

• Seismic Sources (Table 1):

• Receivers:

  • Geophone: voltage ∝ particle velocity, freq. range 10 Hz–2 kHz, natural freq < 0.5 × predominant freq.
  • Piezoelectric accelerometer: voltage ∝ acceleration, freq. range 10 Hz–60 kHz, natural freq ≥ 2 × predominant freq.
  • Hydrophone: voltage ∝ water pressure, freq. range 10 Hz–60 kHz (water-filled boreholes only).
  • Use triaxial arrays (1 vertical + 2 horizontal receivers at right angles) for wave identification.

• Data Acquisition System:

  • Amplifier gain: 70–130 dB, flat freq. response 4 Hz–2 kHz.
  • Signal enhancement for noisy areas.
  • Timing device: time marks at 0.1, 1, or 10 ms intervals.
  • Time break unit: separate channel for wave generation instant.
  • Recorder: analog/digital visual record (electromagnetic oscillograph preferred).

Reporting (Clause 7)

Reports must include:

• Borehole details (location, length, diameter, inclination, casing).
• Layout drawings with source/receiver coordinates.
• Equipment specs and frequency characteristics.
• Waveforms for all source-receiver pairs.
• Average propagation velocities (see Fig. 3 schematic).
• Dynamic elastic modulus tabulation.
• Equations and assumptions used.
• Geotechnical interpretation if required.

Formula for Velocity (basic):

[ V = \frac{D}{t} ]

• (V) = seismic wave velocity (m/s)
• (D) = distance between source and receiver (m)
• (t) = travel time of wave (s)

Simplified Mermaid Diagram of Setup

graph LR
  A[Seismic Source] --> B[B

IS 13372 Part 2: Calculation and Interpretation Summary

Key Points from Clause 6 (Calculation and Interpretation):

• Signal Recording: Signals from receivers must be recorded on magnetic media or displayed visually with adequate amplitude and travel time for accurate reading.
• Velocity Calculation: Average seismic wave velocities are computed between shot points and receiving points, including between boreholes.
• Rock Properties: Compare measured velocities with specimen velocities to evaluate rock mass properties.
• Dynamic Elastic Modulus: Compute and tabulate dynamic elastic modulus values from seismic velocities.

Important Formulas & Tables:

Reporting Requirements (Clause 7):

• Borehole details (location, length, diameter, inclination)
• Positions and coordinates of seismic sources and receivers
• Measurement methods and equipment specs
• Waveforms and average velocities (see Fig. 3 schematic)
• Dynamic elastic modulus tabulation
• Equations and assumptions used
• Geotechnical interpretation considering geology

Schematic Concept (Fig. 3):

flowchart LR
    A[Shot Point] -->|Seismic Wave| B[Receiver Point]
    B -->|Travel Time t| C[Calculate Velocity V = L/t]
    C --> D[Compare with Specimen Velocity]
    D --> E[Evaluate Rock Mass Properties]

Summary: Use travel time and distance to calculate seismic velocities, then derive dynamic elastic modulus to interpret rock mass properties. Report comprehensive data including borehole info, equipment, waveforms, velocities, and analysis assumptions.

IS 13372 Part 2: Reporting of Results (Clause 7)

The report for seismic testing between boreholes must include:

• Borehole details: Location, length, diameter, direction, inclination, casing/cement details.
• Drawings: Positions of seismic sources and receivers with coordinates.
• Equipment & Methods: Description, specifications, and frequency characteristics.
• Waveforms: For each source-receiver pair.
• Average propagation velocities: Between shot and receiver points (see Fig. 3 schematic).
• Dynamic elastic modulus: Tabulated computed values.
• Equations & assumptions: Used beyond the standard method.
• Geotechnical interpretation: Geological context and analysis (if required).

Key Formula for Dynamic Elastic Modulus (E_d):

[ E_d = \rho V_p^2 (1 + \nu)(1 - 2\nu) / (1 - \nu) ]

Where:

• ( \rho ) = density of rock mass
• ( V_p ) = P-wave velocity
• ( \nu ) = Poisson's ratio

Table: Seismic Source Types (from Clause 4.1)

Fig. 3 Concept: Average Velocity Calculation

flowchart LR
    A[Shot Point] -->|Seismic Wave| B[Receiver Point]
    B -->|Travel Time t| C[Calculate Velocity V = Distance / t]
    C --> D[Tabulate Average Velocities]

Note: Use magnetic recording for signal accuracy (Clause 5.5). Include time break signals for precise travel time measurement.

This structured reporting ensures consistency, clarity, and facilitates geotechnical interpretation of seismic test results.