Horizontal-to-Vertical Spectral Ratio (HVSR) Test in Bangladesh
The Horizontal-to-Vertical Spectral Ratio (HVSR) Test, also known as the H/V Spectral Ratio method, is a passive seismic geophysical investigation technique used to evaluate the frequency characteristics of near-surface geological materials and identify significant subsurface impedance contrasts. Unlike active seismic methods such as seismic refraction, seismic reflection, MASW, downhole seismic testing, and cross-hole testing, Horizontal-to-Vertical Spectral Ratio (HVSR) generally uses ambient seismic vibrations or microtremors that are naturally present at the site. These vibrations may originate from traffic, machinery, human activity, wind, industrial operations, oceanic activity, and natural seismic sources.
A three-component seismic sensor records ground motion in:
• Two horizontal directions
• One vertical direction
The recorded signals are transformed into the frequency domain, and the ratio between the horizontal and vertical spectral amplitudes is calculated.
A simplified expression is:
HVSR = Horizontal Spectral Amplitude / Vertical Spectral Amplitude
The resulting Horizontal-to-Vertical Spectral Ratio (HVSR) curve can show peaks at particular frequencies. These peaks may provide information about the site's predominant or resonance frequency and can help identify subsurface interfaces associated with strong changes in seismic properties.
Horizontal-to-Vertical Spectral Ratio (HVSR) is particularly useful for rapid site characterization, seismic site-response studies, sediment-thickness estimation, geological investigations, micro-zonation, and earthquake-related hazard assessment.
However, Horizontal-to-Vertical Spectral Ratio (HVSR) results require careful interpretation. A peak in an HVSR curve should not automatically be interpreted as a specific soil layer or bedrock depth without supporting geological, geotechnical, or geophysical information.
QC LAB SOLUTION can provide professional Horizontal-to-Vertical Spectral Ratio (HVSR) surveys and passive seismic investigation services in Bangladesh for engineering, geological, seismic-hazard, infrastructure, and site-characterization projects.
What Is Horizontal-to-Vertical Spectral Ratio (HVSR)?
The Horizontal-to-Vertical Spectral Ratio (HVSR) is a method of analyzing the frequency content of ambient ground vibrations. A three-component sensor records:
• North-South Horizontal Component
• East-West Horizontal Component
• Vertical Component
The recorded time histories are divided into suitable time windows and transformed into frequency spectra. The horizontal spectral components are combined to produce a representative horizontal spectrum. This is then compared with the vertical spectrum. The resulting ratio is plotted against frequency.
A typical HVSR graph contains:
• Frequency on the X-axis
• H/V spectral ratio on the Y-axis
A significant peak may indicate a frequency at which the horizontal ground motion is amplified relative to the vertical component. This frequency is commonly referred to as the predominant frequency or fundamental resonance frequency, depending on the site and interpretation.
Principle of HVSR Testing
The fundamental concept behind HVSR is that subsurface structures can modify seismic-wave propagation. Consider a site consisting of:
Soft Sediments
over
Stiff Soil / Weathered Rock
over
Competent Bedrock
A strong impedance contrast between the softer upper material and underlying material can influence the propagation and amplification of seismic waves. Ambient vibrations interact with the subsurface structure. The three-component sensor records the resulting ground motion. The horizontal and vertical frequency spectra are calculated. Their ratio produces the HVSR curve.
A simplified relationship is:
H/V(f) = Horizontal Spectrum(f) / Vertical Spectrum(f)
where f is frequency.
The frequency corresponding to a clear and reliable HVSR peak can provide an estimate of the site's predominant frequency.
Why Is HVSR Testing Important?
Understanding the dynamic characteristics of the ground is important for earthquake engineering and infrastructure development. Two sites may have similar surface conditions but very different subsurface structures. For example:
Site A: Shallow competent bedrock
Site B: Thick soft sediment over deep bedrock
During earthquake shaking, these sites may respond differently. The second site may have a substantially lower fundamental frequency and potentially stronger amplification at particular frequencies. HVSR can therefore help investigate:
• Site resonance
• Predominant frequency
• Sedimentary basin characteristics
• Approximate sediment thickness
• Subsurface impedance contrasts
• Seismic site response
• Geological structure
It can also be used efficiently over many measurement points, making it useful for microzonation and urban-scale surveys.
Market Growth and Industry Trends in Bangladesh
Bangladesh has experienced rapid urbanization and infrastructure development, particularly around major cities and industrial regions.
Large numbers of:
• Residential buildings
• Commercial buildings
• Industrial facilities
• Bridges
• Roads
• Elevated infrastructure
• Metro and railway systems
• Power facilities
• Special economic zones are being developed.
At the same time, earthquake hazard and seismic site response are important considerations for engineering projects in Bangladesh. Traditional geotechnical investigation provides essential information about:
• Soil type
• SPT N-values
• Groundwater
• Bearing capacity
• Settlement
• Stratigraphy
However, dynamic site characterization can provide additional information that conventional boreholes may not capture efficiently. This creates an opportunity for passive seismic techniques such as HVSR.
HVSR surveys can be particularly attractive where a project requires measurements at many locations because the equipment is relatively compact and the method generally does not require active seismic sources. Potential applications in Bangladesh include:
• Urban seismic microzonation
• Building-site investigation
• Infrastructure corridors
• Geological mapping
• Basin studies
• Earthquake engineering
• Site-response characterization
Horizontal-to-Vertical Spectral Ratio (HVSR) is the
refore best considered as part of an integrated geotechnical and geophysical investigation program.
HVSR and Earthquake Engineering
One of the most important applications of HVSR is the evaluation of a site's dynamic characteristics. Every site has a characteristic response to seismic excitation that depends on factors including:
• Shear-wave velocity
• Soil stiffness
• Density
• Layer thickness
• Damping
• Geological structure
A simplified estimate of fundamental site frequency for a relatively simple layered soil profile can be represented as:
f₀ ≈ Vs / 4H
where:
• f₀ = fundamental frequency
• Vs = representative shear-wave velocity
• H = thickness of the sedimentary layer
This is a simplified one-dimensional relationship and should not be applied blindly to complex geological conditions. Horizontal-to-Vertical Spectral Ratio (HVSR) can provide an estimate of the predominant frequency, while independent information such as MASW, downhole testing, boreholes, or geological data may help estimate the associated subsurface structure.
Key Components of an HVSR Survey
A professional HVSR survey generally involves:
1. Three-Component Seismic Sensor
The sensor records ground motion in:
• X direction
• Y direction
• Z direction
2. Data Acquisition System
The system records the seismic signals at an appropriate sampling frequency.
3. GPS/GNSS
Used to determine the measurement location.
4. Field Computer or Controller
Used for:
• Instrument configuration
• Recording
• Data checking
• Survey documentation
5. Processing Software
Software is used to:
• Select time windows
• Remove poor-quality sections
• Calculate spectra
• Calculate H/V ratios
• Smooth spectra
• Generate HVSR curves
• Identify significant peaks
6. Accessories
These may include:
• Battery systems
• Tripod or sensor base
• Orientation equipment
• Measuring tools
• Field notebook
• Environmental monitoring equipment
Types of HVSR Investigation
1. Single-Point HVSR Measurement
A single measurement is performed at a selected location.
This can be useful for:
• Preliminary site assessment
• Building sites
• Rapid geological investigation
• Initial seismic characterization
2. Multiple-Point HVSR Survey
Measurements are performed at multiple locations.
This is more useful for understanding spatial variation.
The resulting predominant-frequency values can be plotted spatially.
3. HVSR Microzonation Survey
A grid of measurement locations can be established across an urban or regional area. The resulting information can help develop maps showing variations in:
• Predominant frequency
• Site response characteristics
• Geological conditions
HVSR for Basin or Sediment-Thickness Investigation
In suitable geological settings, predominant frequency can be correlated with sediment thickness using appropriate velocity information. This can help investigate:
• Sedimentary basins
• Buried bedrock
• Valley structures
• Geological boundaries
HVSR Test Working Procedure
Step 1: Project Objective Review
Before fieldwork, the survey objective should be clearly established.
Examples include:
• Determining predominant site frequency
• Investigating sediment thickness
• Supporting seismic hazard studies
• Mapping spatial variation
• Supporting earthquake-resistant design
• Geological characterization
Step 2: Site Reconnaissance
The field team evaluates:
• Ground condition
• Traffic
• Machinery
• Buildings
• Power infrastructure
• Construction activities
• Surface vibration
• Accessibility
• Potential sources of cultural noise
This is important because HVSR uses ambient vibrations and is therefore sensitive to environmental conditions.
Step 3: Measurement Point Selection
Measurement points are selected according to the investigation objectives.
For a large survey, points may be arranged using:
• Regular grid
• Geological boundaries
• Engineering zones
• Existing borehole locations
• Road corridors
• Structural areas
Step 4: Sensor Installation
The three-component sensor is placed firmly on the ground.
Good coupling between the sensor and ground is important.
The sensor should be protected from:
• Physical disturbance
• Direct contact by personnel
• Wind-induced movement
• Unstable surfaces
Step 5: Sensor Orientation
The horizontal components should be appropriately oriented and documented.
The exact procedure depends on the instrument and survey methodology.
Step 6: Instrument Configuration
The field team establishes suitable:
• Sampling frequency
• Recording duration
• Gain/settings
• Time synchronization
• Sensor configuration
Recording parameters should be selected according to the target frequency range and project objectives.
Step 7: Ambient Vibration Recording
The sensor records ambient ground vibrations. No large active seismic source is normally required. During recording, the field team should minimize unnecessary movement near the sensor. Vehicles and machinery should also be documented because they can influence the recorded signal.
Step 8: Field Quality Control
The recorded signal should be reviewed.
The field team checks:
• Signal quality
&
bull; Excessive noise
• Sensor stability
• Frequency content
• Recording duration
• Instrument operation
• Data completeness
Additional recordings may be collected if the first record is affected by excessive noise.
HVSR Data Processing
Processing is one of the most important stages of HVSR analysis.
1. Data Preparation
The recorded three-component time histories are imported into processing software.
The data are checked for:
• Missing records
• Instrument problems
• Timing errors
• Excessive disturbances
2. Window Selection
The continuous record is divided into smaller time windows.
Poor-quality windows may be rejected.
Examples of undesirable events include:
• People walking close to the sensor
• Vehicles passing directly beside the sensor
• Construction impacts
• Sudden mechanical vibrations
3. Fourier Spectral Analysis
The selected time windows are transformed into the frequency domain.
This provides the frequency spectrum of each component.
4. Horizontal Component Combination
The two horizontal components are combined using an appropriate method.
For example, the horizontal amplitude may be represented by an average or geometric combination of the two horizontal spectra, depending on the adopted processing procedure.
5. H/V Calculation
The horizontal spectrum is divided by the vertical spectrum.
This produces the H/V spectral ratio.
6. Smoothing
The H/V curve may be smoothed using an appropriate smoothing procedure. The objective is to identify stable spectral features rather than short-duration fluctuations.
7. Peak Identification
A significant and reliable peak is evaluated. Important parameters may include:
• Predominant frequency
• Peak amplitude
• Peak stability
• Bandwidth
• Directional consistency
• Window-to-window consistency
8. Interpretation
The final HVSR curve is interpreted together with:
• Borehole logs
• SPT results
• MASW results
• Seismic refraction
• Downhole seismic testing
• Geological maps
• Groundwater information
• Existing geotechnical data
Example of HVSR Interpretation
Suppose an HVSR survey produces a clear peak at:
f₀ = 2.0 Hz
This indicates a significant site response feature around 2 Hz.
If independent investigation indicates an average shear-wave velocity of:
Vs = 400 m/s
A simplified quarter-wavelength relationship gives:
H ≈ Vs / 4f₀
Therefore:
H ≈ 400 / (4 × 2)
H ≈ 50 m
This could suggest an approximate 50 m thickness of the relevant sedimentary layer if the assumptions of the simplified model are appropriate.
It should not be treated as a direct measurement of bedrock depth.
Actual interpretation requires geological and velocity informatio.
HVSR Results and Reporting
A professional HVSR report can contain:
Project Information
• Project name
• Client
• Site location
• Survey date
Survey Methodology
• Instrument
• Sensor type
• Recording duration
• Sampling frequency
• Measurement locations
Site Information
• Coordinates
• Ground elevation
• Site photographs
• Environmental conditions
Processing
• Window selection
• Frequency processing
• Horizontal-component processing
• Smoothing
• Quality-control criteria
Results
• HVSR curves
• Predominant frequencies
• Peak amplitudes
• Measurement locations
• Frequency maps where applicable
Interpretation
• Possible subsurface interfaces
• Site resonance characteristics
• Spatial variation
• Correlation with geological information
Recommendations
Recommendations should be based on the project's objectives and should clearly distinguish measured data from interpreted parameters.
QC LAB SOLUTION Contribution to HVSR Testing
QC LAB SOLUTION can support clients in Bangladesh with professional passive seismic investigation services. Our HVSR workflow can include:
Project Planning
We evaluate the project's:
• Geological setting
• Engineering objective
• Target frequency range
• Survey area
• Required measurement density
Field Survey
Our field team can perform:
• Measurement-point positioning
• Sensor deployment
• Ambient vibration recording
• Field documentation
• Quality control
Data Processing
Recorded data can be processed to generate:
• Spectral curves
• H/V ratios
• Predominant frequency estimates
• Quality indicators
• Spatial frequency maps
Integrated Interpretation
Where available, HVSR results can be correlated with:
• Borehole data
• SPT
• MASW
• Seismic refraction
• Downhole seismic testing
• Geological information
Reporting
QC LAB SOLUTION can provide an organized technical report containing methodology, field information, processed results, interpretation, limitations, and recommendations.
Working Steps on Field
A typical HVSR field operation can follow this sequence:
Site reconnaissance
↓
Measurement point selection
↓
GPS coordinate recording
↓
Ground preparation
↓
Three-component sensor installation
↓
Sensor orientation
↓
Instrument setup
↓
Ambient vibration recording
↓
Field signal QC
↓
Additional recording if necessary
↓
Data backup
↓
Move to next point
↓
Repeat survey
↓
Final field data verification
The simplicity of the physical setup allows many measurement points to be surveyed efficiently when site conditions are suitable.
Applications of HVSR Testing
1. Seismic Site Characterization
HVSR can help identify predominant frequencies associated with local geological conditions.
2. Earthquake Hazard Assessment
The method can contribute to understanding potential site-response characteristics.
3. Urban Seismic Microzonation
Multiple HVSR measurements can be used to investigate spatial variations across cities.
4. Building Site Investigation
HVSR may provide supplementary information for evaluating dynamic characteristics of a building site. It should be integrated with the project's geotechnical investigation.
5. Sediment Thickness Estimation
Where suitable velocity information is available, predominant frequency can help estimate sediment thickness.
6. Geological Mapping
Changes in HVSR characteristics between locations may help identify geological boundaries or variations.
7. Basin Studies
HVSR surveys can contribute to the characterization of sedimentary basins.
8. Infrastructure Development
The technique can support preliminary and supplementary investigation for:
• Roads
• Bridges
• Railways
• Metro systems
• Industrial facilities
• Airports
• Ports
• Large buildings
Advantages of HVSR Testing
1. Passive Method
Horizontal-to-Vertical Spectral Ratio (HVSR) generally uses ambient seismic vibrations and does not require a large active seismic source.
2. Non-Destructive
The method causes minimal physical disturbance to the site.
3. Portable Equipment
Modern three-component seismic sensors can be relatively compact and easily deployed.
4. Rapid Data Collection
A large number of measurement points can potentially be surveyed efficiently.
5. Useful in Urban Areas
Because it generally does not require active seismic sources, HVSR can be suitable for some urban environments where active-source methods may be difficult.
6. Predominant Frequency Information
The method can provide useful information about site resonance characteristics.
7. Spatial Mapping
Multiple measurements can be converted into spatial maps of predominant frequency and related parameters.
8. Complements Other Investigations
Horizontal-to-Vertical Spectral Ratio (HVSR) can be combined with:
• MASW
• Seismic refraction
• Downhole testing
• Boreholes
• SPT
• Geological mapping
Limitations of HVSR Testing
HVSR is powerful but should not be overinterpreted.
1. HVSR Does Not Directly Measure Soil Strength
It does not directly provide:
• Bearing capacity
• Cohesion
• Friction angle
• Settlement
• SPT N-value
These require conventional geotechnical investigation.
2. HVSR Peak Does Not Automatically Equal Bedrock Depth
A predominant frequency can be associated with a subsurface impedance contrast, but interpreting that contrast as bedrock requires supporting evidence.
3. Cultural Noise
Traffic, machinery, construction activities, and human movement can influence measurements.
4. Complex Geological Conditions
Three-dimensional geological structures, dipping layers, lateral changes, and multiple impedance contrasts can produce complex HVSR curves.
5. Multiple Peaks
A curve may contain multiple peaks. Not every peak necessarily represents the fundamental site frequency.
6. Directional Effects
Strong directional characteristics can complicate interpretation.
7. Lack of a Clear Peak
Some sites may not produce a clear or stable HVSR peak. This does not necessarily mean that the subsurface has no structure; it may indicate that the method is not providing a reliable resonance signature under the measurement conditions.
8. Need for Independent Verification
For engineering decisions, HVSR should preferably be interpreted together with independent information such as boreholes and seismic velocity measurements.
HVSR vs MASW
|
Feature |
HVSR |
MASW |
|
Method |
Passive seismic |
Usually active surface-wave method |
|
Main measurement |
H/V spectral ratio |
Surface-wave dispersion |
|
Main output |
Predominant frequency |
Vs profile |
|
Active source required |
Normally no |
Usually yes |
|
Direct Vs profile |
No |
Yes, through inversion |
|
Large-area rapid survey |
Excellent potential |
Good |
|
Urban application |
Often convenient |
May require source operation |
|
Site resonance |
Useful |
Can support through Vs model |
|
Sediment thickness |
Can be estimated with supporting Vs |
Can support velocity/depth modeling |
HVSR vs Seismic Refraction
|
Feature |
HVSR |
Seismic Refraction |
|
Source |
Ambient vibration |
Active seismic source |
|
Main output |
H/V frequency response |
P-wave velocity/depth |
|
Field setup |
Relatively simple |
More extensive |
|
Bedrock mapping |
Possible with supporting data |
Often useful |
|
Site resonance |
Strong application |
Not primary objective |
|
Urban environment |
Often suitable |
Can be difficult due to source/noise |
|
Geological interpretation |
Requires correlation |
Requires correlation |
Quality Assurance and Quality Control
Reliable Horizontal-to-Vertical Spectral Ratio (HVSR) results depend strongly on data quality. Important QA/QC practices include:
Instrument Verification
The seismic sensor and recording system should be checked before deployment.
Sensor Stability
The sensor should be installed on a stable surface.
Ground Coupling
Proper sensor-ground contact should be maintained.
Environmental Observation
The field team should document:
• Traffic
• Machinery
• Construction
• Weather
• Nearby vibration sources
Recording Duration
The recording duration should be appropriate for the target frequency range and project objective.
Window Quality
Disturbed windows should be identified and appropriately treated.
Repeat Measurements
Repeat measurements can be useful when the initial record
is questionable.
Horizontal Component Review
The two horizontal components should be evaluated for consistency.
Peak Stability
A peak should be assessed for stability rather than selected simply because it is the highest point on a curve.
Independent Correlation
Where possible, Horizontal-to-Vertical Spectral Ratio (HVSR) results should be correlated with boreholes and seismic velocity data.
Site and Health Safety
Although Horizontal-to-Vertical Spectral Ratio (HVSR) is generally a low-impact survey technique, field safety remains important.
Potential hazards include:
• Road traffic
• Construction equipment
• Open excavations
• Uneven ground
• Electrical infrastructure
• Underground utilities
• Heavy machinery
• Industrial environments
• Weather conditions
Field personnel should use appropriate PPE, including:
• Safety helmet
• High-visibility vest
• Safety footwear
• Gloves where appropriate
When working beside roads or active construction sites, appropriate traffic and site-control procedures should be implemented.
The passive nature of HVSR does not eliminate normal field safety requirements.
Future of HVSR Services in Bangladesh
The use of passive seismic techniques may increase as Bangladesh continues to develop major infrastructure and strengthen earthquake-risk assessment.
Potential future applications include:
Urban Microzonation
Large-scale HVSR networks can help investigate spatial variations in site frequency across urban areas.
Earthquake Engineering
Horizontal-to-Vertical Spectral Ratio (HVSR) can contribute to regional site-response databases.
Integrated Geophysical Investigation
Future projects may increasingly combine:
HVSR + MASW + Seismic Refraction + Downhole + Borehole + SPT
to develop more reliable subsurface models.
GIS-Based Mapping
HVSR results can be integrated into GIS platforms to create:
• Predominant-frequency maps
• Geological maps
• Site-response maps
• Urban hazard maps
Automated Processing
Modern software can automate many stages of:
• Window selection
• Spectral calculation
• H/V computation
• Curve smoothing
• Peak identification
• Quality classification
Digital Site Characterization
HVSR data can eventually be incorporated into digital geological and engineering models together with borehole and seismic velocity data.
Why Choose QC LAB SOLUTION for HVSR Testing?
Choosing the right investigation methodology is important because Horizontal-to-Vertical Spectral Ratio (HVSR) results can vary significantly depending on site conditions, noise, sensor deployment, recording parameters, and interpretation. QC LAB
SOLUTION focuses on a project-specific approach. Our potential service workflow includes:
• Site assessment
• Survey planning
• HVSR field measurement
• Three-component seismic recording
• Field quality control
• Spectral analysis
• H/V curve generation
• Predominant-frequency assessment
• Spatial mapping
• Integrated interpretation
• Technical reporting
For complex projects, HVSR can be combined with other geophysical and geotechnical investigation methods to improve subsurface interpretation.
The Horizontal-to-Vertical Spectral Ratio (HVSR) Test is a useful passive seismic technique for investigating the dynamic characteristics of a site through analysis of ambient ground vibrations.
Its principal output is the H/V spectral ratio as a function of frequency, which can help identify significant frequency peaks associated with site response and subsurface impedance contrasts.
The technique offers several practical advantages: it is generally non-destructive, portable, relatively rapid, and does not normally require an active seismic source. These characteristics make it particularly attractive for urban areas, seismic microzonation, preliminary site characterization, and large numbers of measurement points.
However, HVSR should not be treated as a standalone substitute for geotechnical investigation. A predominant frequency does not automatically provide a unique soil profile or bedrock depth. Reliable engineering interpretation requires consideration of geological conditions and, where appropriate, correlation with boreholes, SPT, MASW, seismic refraction, downhole seismic testing, and other investigation methods.
For infrastructure and engineering projects in Bangladesh, HVSR can therefore be a valuable component of an integrated seismic and geotechnical site investigation program.
QC LAB SOLUTION can provide professional HVSR survey and passive seismic investigation services in Bangladesh for construction, infrastructure, geological, earthquake-engineering, and site-characterization projects.
