Recommendations for seismic instrumentation for river valley projects

Scope of IS 4967: Instrumentation for Seismicity Investigations

• Application:
  Used primarily for seismicity investigations of river valley projects; also applicable to other projects based on magnitude and site conditions (Clause 2.1).

• Instrument Pillars (Clause 2.1.2):

  • Constructed as a single solid block of rich cement concrete.
  • Must rest on unweathered bedrock; if unavailable, on hard foundation.
  • Pillar sections per Fig. 1 (A, B, C) with dimensions in cm.
  • Provide moats around seismograph pillars:
    • Width: 15 cm
    • Depth: 120 cm
    • Filled with dry sand or tar for isolation.

• Accelerograph Specifications (Clause 2.2.3):

  Parameter	Value
  Natural free period	1/15 to 1/20 seconds
  Damping	60 to 70%
  Range	g, ½g, or ⅓g*
  Sensitivity	4 cm paper amplitude at max acceleration

  *Acceleration depends on seismic zone per IS 1893:

  • Zones V & VI: g
  • Zones III & IV: ½g
  • Other zones: ⅓g

• SI Units (for reference):

  • Length: metre (m)
  • Mass: kilogram (kg)
  • Force: newton (N = kg·m/s²)
  • Pressure: pascal (Pa = N/m²)

flowchart TD
    A[Seismicity Investigation] --> B[Instrument Pillars]
    B --> C{Foundation}
    C -->|Unweathered bedrock| D[Concrete Block Pillar]
    C -->|No bedrock| E[Hard Foundation]
    B --> F[Moat: 15cm x 120cm]
    A --> G[Accelerograph]
    G --> H[Natural Period: 1/15 - 1/20 s]
    G --> I[Damping: 60-70%]
    G --> J[Range: g, 

IS 4967 - Instrumentation for Investigations of Seismicity: Key Specifications

1. Accelerograph Specifications (Clause 2.2.3)

2. Recommended Maximum Acceleration by Seismic Zone (Note)

Summary:

• Use a 3-component accelerograph (1 vertical + 2 horizontal).
• Ensure damping between 60-70% for accurate seismic response.
• Adjust accelerograph range based on seismic zone per IS 1893.
• Sensitivity calibrated to produce 4 cm paper amplitude at max acceleration.

flowchart LR
    A[Seismic Zone] --> B{Acceleration Range}
    B -->|Zones V & VI| C[1g]
    B -->|Zones III & IV| D[½g]
    B -->|Other Zones| E[¼g]
    F[Accelerograph] --> G[Natural Period: 1/15 to 1/20 s]
    F --> H[Damping: 60-70%]
    F --> I[Range: Based on Zone]
    F --> J[Sensitivity: 4 cm paper amplitude]

This ensures reliable seismic data for dam safety and design.

IS 4967: Study of Microtremors & Predominant Period of the Site

Key Points from Clauses 2.6 & 3.1:

• Seismograph Installation: On concrete pillars founded on hard rock or consolidated foundation if hard rock is unavailable.
• Microtremor Recording:
  • Use high sensitivity short period seismographs with magnification ~10^5.
  • Record continuously for about 1 week.
  • Take records at different times daily to capture variability.
• Purpose: Determine predominant site period (T₀) and frequency of occurrence.

Predominant Period Estimation (General Engineering Practice)

[ T_0 = \frac{4H}{V_s} ]

Where:

• ( T_0 ) = Predominant period (seconds)
• ( H ) = Thickness of soil/soft layer (m)
• ( V_s ) = Average shear wave velocity of the layer (m/s)

Typical Microtremor Study Procedure:

• Deploy seismograph on stable foundation.
• Collect ambient vibration data (microtremors).
• Analyze frequency spectrum to identify peak frequency ( f_0 ).
• Calculate predominant period: ( T_0 = \frac{1}{f_0} ).

Summary Table: Instrumentation & Data Collection

flowchart TD
    A[Install Seismograph] --> B[Record Microtremors for 1 Week]
    B --> C[Analyze Frequency Spectrum]
    C --> D[Identify Peak Frequency (f₀)]
    D --> E[Calculate Predominant Period T₀ = 1/f₀]
    E --> F[Use T₀ for Site Seismic Response Assessment]

This approach ensures reliable determination of site dynamic characteristics critical for seismic design of dams and foundations.

IS 4967: Permanent Installation of Instruments in Dams for Earthquake Effects

Key Points from Clause 4 (Permanent Installation)

• Objective: To monitor earthquake effects on dam and appurtenant structures continuously.
• Instruments Recommended:
  • Seismographs: High sensitivity, short period (magnification ~10^5).
  • Accelerographs: To record strong ground motion.
  • Piezometers: For pore water pressure changes during seismic events.
  • Strain gauges & displacement transducers: For structural response.

Installation Guidelines

• Instruments should be placed at critical locations:

  • Foundation
  • Dam body (upstream, downstream faces)
  • Abutments
  • Adjacent ground

• Instruments must be protected against environmental effects and easily accessible for maintenance.

Typical Parameters & Specifications

Suggested Procedure for Microtremor Study (Clause 3.1)

• Record microtremors continuously for about one week.
• Use high sensitivity short period seismographs.
• Take records at different times of the day to capture variability.
• Analyze to find predominant period and frequency of occurrence.

Summary Mermaid Diagram

flowchart TD
    A[Start: Site Selection] --> B[Install Instruments]
    B --> C{Instrument Types}
    C --> D[Seismograph]
    C --> E[Accelerograph]
    C --> F[Piezometer]
    C --> G[Strain Gauge]
    D --> H[Record Microtremors]
    E --> I[Record Strong Motion]
    F --> J[Measure Pore Pressure]
    G --> K[Measure Strain]
    H & I & J &

IS 4967: Design & Construction of Seismological Observatory Buildings

Key Specifications:

• Functional Rooms:

  • Photographic Room: 3 m × 2.5 m (Clause 2.5c)
  • Store Room: 3.5 m × 3 m (Clause 2.5d)
  • Bath & W.C. (Clause 2.5e)

• Temperature Control:

  • Instrument rooms must maintain diurnal temperature variation ≤ 5°C (Clause 2.1.3).

• Structural Support:

  • Seismograph and accelerograph pillars rest on basement rock or compact ground.
  • Pillars constructed with cement concrete blocks or concrete/brick masonry with cement mortar.

Layout & Electrical Provisions (Fig. 1):

Design Considerations:

• Earthquake Resistance: Follow criteria from the revised 1975 earthquake-resistant design standards.
• Instrument Installation: Subsidiary observatories house specific instruments per Clauses 2.3.1 to 2.3.3.
• Material: Use cement concrete or brick masonry with cement mortar for durability and stability.

Summary Diagram:

graph TD
  A[Basement Rock/Compact Ground]
  B[Seismograph Pillar]
  C[Accelerograph Pillar]
  D[Instrument Room]
  E[Photographic Room (3x2.5 m)]
  F[Store Room (3.5x3 m)]
  G[Bath & W.C.]

  A --> B
  A --> C
  B --> D
  C --> D
  D --> E
  D --> F
  D --> G

For detailed layouts and functional requirements, refer to Appendix A and Fig. 1 of IS 4967.

IS 4967: Other Requirements for Instrument Installation

Key Specifications (Clause 2.1.2 & related)

• Instrument Pillars:

  • Constructed as a single solid block of rich cement concrete.
  • Should rest on unweathered bedrock; if unavailable, then on a hard foundation.
  • Provide moats around seismograph pillars:
    • Width: 15 cm
    • Depth: 120 cm
    • Filled with dry sand or tar to isolate vibrations.

• Instrument Room (Clause 2.1.3):

  • Diurnal temperature variation inside the room must be limited to ≤ 5°C to ensure instrument accuracy.

• Recommended Instruments (Clause 4.5):

  • Silica tube extensometer
  • Tilt meters (water tube & horizontal pendulum types)
  • Seiche recorders

Summary Table: Pillar Moat Dimensions

Conceptual Layout (Pillar + Moat)

graph TD
    A[Instrument Pillar] --> B[Concrete Block]
    B --> C[Rest on Bedrock or Hard Foundation]
    A --> D[Moat around Pillar]
    D --> E[Width: 15 cm]
    D --> F[Depth: 120 cm]
    D --> G[Filled with Dry Sand or Tar]

Notes:

• Use rich concrete mix for durability and vibration isolation.
• Ensure stable foundation to avoid measurement errors.
• Control temperature fluctuations in instrument rooms for precision.

For detailed layouts and functional requirements, refer to Appendix A of IS 4967.

IS 4967: Key Specifications for Siting of Observatories

• Geological Considerations (Clause 2.1.1):
  Select sites based on stable geological structures and favorable soil characteristics to minimize seismic noise and ground vibrations.

• Observatory Distribution (Clause 2.1):

  • Set up 5 observatories: 1 main + 4 subsidiary.
  • Spacing: Maximum 70 km between observatories.
  • Main observatory near the proposed site; subsidiaries around it.

• Instrument Room Temperature Control (Clause 2.1.3):
  Maintain diurnal temperature variation ≤ 5°C inside instrument rooms to ensure measurement accuracy.

• Main Observatory Vault Size (Appendix A, Clause 1.1.1):

  • Vault dimensions: 8 m × 5 m.
  • Partitioned by a full-height wall into two rooms (see Fig. 1A in IS 4967).

Summary Table: Observatory Setup

flowchart LR
    A[Main Observatory] -->|≤70 km| B[Subsidiary Observatory 1]
    A -->|≤70 km| C[Subsidiary Observatory 2]
    A -->|≤70 km| D[Subsidiary Observatory 3]
    A -->|≤70 km| E[Subsidiary Observatory 4]

This layout ensures comprehensive seismic data collection with controlled environmental conditions.

IS 4967 - Key Specifications for Instruments

Instrument Pillars (Clause 2.1.2)

• Constructed as a single solid block of rich cement concrete.
• Rest on unweathered bedrock or hard foundation if rock unavailable.
• Include moats:
  • Width: 15 cm
  • Depth: 120 cm
  • Filled with dry sand or tar to isolate vibrations.

Accelerograph Specifications (Clause 2.2.3)

Note: Recommended max acceleration by seismic zone (IS 1893-1966):

• Zones V & VI: g
• Zones III & IV: ½g
• Other zones: ⅓g

Units (SI Units)

• Length: meter (m)
• Mass: kilogram (kg)
• Time: second (s)
• Force: newton (N) = 1 kg·m/s²
• Pressure: pascal (Pa) = 1 N/m²

flowchart LR
    A[Instrument Pillar] --> B[Concrete Block on Bedrock]
    B --> C[Moat: 15 cm width, 120 cm depth]
    C --> D[Filled with Dry Sand or Tar]
    E[Accelerograph] --> F[Natural Period 1/15 to 1/20 s]
    E --> G[Damping 60-70%]
    E --> H[Range g, ½g, ⅓g]
    E --> I[Sensitivity: 4 cm amplitude]

This ensures stable, vibration-isolated foundations and sensitive seismic measurements per IS 4967.

IS 4967: Instrumentation for Subsidiary Observatories

Key Specifications (Clause 2.3)

Subsidiary observatories must be equipped with the following instruments:

• 2.3.1 to 2.3.3 Instruments:
  These typically include:
  • Seismographs (sensitive to local seismic activity)
  • Accelerographs (to record ground acceleration)
  • Auxiliary recording devices (for data logging and transmission)

Structural Requirements (Clause 1.1.2)

• Subsidiary observatories are smaller than main observatories.
• Must ensure adequate protection and stable mounting of instruments to avoid noise from structural vibrations.

Main Observatory Vault (for comparison)

• Size: 8 m x 5 m
• Partitioned into two rooms by a full-height wall (Clause 1.1.1)
• Designed for minimal vibration and environmental control.

Typical Instrument Mounting Considerations:

• Instruments fixed on isolated concrete slabs or vibration-damping mounts
• Vault walls and floors designed for rigidity and thermal stability

Summary Table: Instruments for Subsidiary Observatory

flowchart LR
    A[Subsidiary Observatory] --> B[Seismograph]
    A --> C[Accelerograph]
    A --> D[Data Logger]
    B & C & D --> E[Data Collection & Analysis]

Note: For detailed mounting dimensions and instrument specifications, refer to IS 4967 Clause 2.3 and related annexures.

IS 4967: Time Recording and Synchronization Key Points

• Timing Device (Clause 2.2.3.2):
  A timing device must provide the time base for records, ensuring accurate synchronization.

• Accuracy Requirement (Clause 2.4):
  Time measurement accuracy must be within ±0.1 second.
  Use electronic clocks for time recording and as power sources for recording drums.

• Structural Setup (Clause 2.1.2):
  Instrument pillars must be solid concrete blocks on unweathered bedrock or hard foundation, with a moat 15 cm wide and 120 cm deep filled with dry sand or tar to isolate vibrations.

Recommended Specifications Summary

Conceptual Diagram of Instrument Pillar and Moat

graph TD
    A[Instrument Pillar] -->|Resting on| B[Unweathered Bedrock or Hard Foundation]
    B --> C[Moat: 15 cm wide, 120 cm deep]
    C --> D[Filled with Dry Sand or Tar]

Note: Synchronization of time signals at observatories is critical for consistent seismic data recording. Use SI units for all measurements as per IS 4967.

IS 4967: Installation of Seismograph Pillars on Concrete

Key Specifications (Clause 2.1.2 & 2.6)

• Pillar Construction: Rich cement concrete block, solid single block.
• Foundation: Pillars must rest on unweathered bedrock or, if unavailable, on a hard consolidated foundation.
• Moat: Around the pillar, a moat 15 cm wide × 120 cm deep is mandatory, filled with dry sand or tar to isolate vibrations.
• Location: Away from vibration sources like heavy traffic or machinery.

Pillar Dimensions (Fig. 1 Summary)

• Typical pillar cross-section: Concrete block resting on basement rock or compact ground.
• Pillar height and width as per site requirements but must ensure stability and isolation.

Important Notes

• Concrete mix should be rich (high cement content) for durability.
• Moat acts as a vibration isolator.

Summary Table: Pillar Installation

flowchart TB
    A[Seismograph Pillar] --> B[Rich Concrete Block]
    B --> C[Foundation]
    C --> D{Foundation Type}
    D -->|Unweathered Bedrock| E[Direct Rest]
    D -->|No Bedrock| F[Hard Consolidated Foundation]
    A --> G[Moat (15cm x 120cm)]
    G --> H[Filled with Dry Sand or Tar]
    A -.-> I[Away from Vibration Sources]

This ensures stable, vibration-isolated installation per IS 4967 for accurate seismic data.

IS 4967: Accelerograph Placement in Dams

Key Specifications (Clause 2.2.3)

Note on Acceleration based on Seismic Zones (IS 1893-1966):

Accelerograph Placement (Clause 4.1)

• Base: In the gallery near the foundation.
• Top: At the top of the dam.
• Mid-height: If dam height > 100 m, install one near mid-height.
• Location Selection: Avoid background seismic noise from dam appurtenant works.

Additional Recommendations (Clause 3.1)

• Record microtremors for about a week using high sensitivity short period seismographs (magnification ~10⁵).
• Take records at different times daily to identify predominant periods and frequency of seismic events.

graph TD
    A[Base (Gallery near foundation)] --> B[Mid-height (if height > 100m)]
    B --> C[Top of Dam]
    style A fill:#f9f,stroke:#333,stroke-width:2px
    style B fill:#bbf,stroke:#333,stroke-width:2px
    style C fill:#fbf,stroke:#333,stroke-width:2px

This setup ensures comprehensive seismic monitoring across the dam height with sensitive instruments tailored to the seismic zone.

IS 4967: Additional Instrumentation Recommendations - Key Points

1. Instrument Pillar Construction (Clause 2.1.2)

• Material: Rich cement concrete as a single solid block.
• Foundation: Rest on unweathered bedrock or hard foundation if rock unavailable.
• Dimensions & Details: Refer to Fig. 1 (pillar section).
• Moat: Surround pillars with a moat approx. 15 cm width × 120 cm depth, filled with dry sand or tar to isolate vibrations.

2. Units & Quantities (SI Units)

3. Additional Recommendations

• Permanent installations should consider seismicity, microtremor studies, and predominant period of dam sites.
• Instrumentation must minimize external noise and ensure accurate seismic data collection.

Summary Diagram: Instrument Pillar & Moat Setup

graph TD
  A[Instrument Pillar] --> B[Rich Concrete Block]
  B --> C[Rest on Bedrock or Hard Foundation]
  A --> D[Moat around Pillar]
  D --> E[15 cm Width, 120 cm Depth]
  E --> F[Filled with Dry Sand or Tar]

For detailed dimensions and installation, refer to Fig. 1 of IS 4967.

IS 4967: Key Formulas, Tables & Construction Details for Instrument Pillars and Observatory

1. Instrument Pillars (Clause 2.1.2 & Fig. 1)

• Material: Rich cement concrete as a single solid block.
• Foundation: Rest on unweathered bedrock or hard foundation if rock unavailable.
• Dimensions: Refer to Fig. 1 sections A, B, C (all dimensions in cm).
• Moat:
  • Width: 15 cm
  • Depth: 120 cm
  • Filled with dry sand or tar to isolate vibrations.

2. Typical Room Dimensions (Clause 2.5)

• Photographic room: 3 m × 2.5 m
• Store room: 3.5 m × 3 m
• Office room: 4.5 m × 3.5 m
• Instrument vault: 4.5 m × 4.5 m

3. SI Units & Derived Quantities (For Design & Measurement)

4. Construction Notes

• Pillars must be monolithic concrete blocks.
• Moats reduce ground vibration transmission.
• Concrete or brick masonry with cement mortar for walls.
• Foot-lights fixed 15 cm above floor level.

graph TD
  A[Instrument Pillar] --> B[Concrete Block]
  A --> C[Foundation on Rock or Hard Ground]
  A --> D[Moat (15cm x 120cm) filled with sand/tar]
  E[Observatory Rooms] --> F[Photographic Room (3x2.5 m)]
  E --> G[Store Room (3.5x3 m)]
  E --> H[Office Room (4.5x3.5 m)]
  E --> I[Instrument Vault (4.5x4