Measuring instruments | Pasisrl

SEISMIC
REFRACTION

Seismic refraction is a geophysical method based on the analysis of the arrival times of artificially generated seismic waves refracted at the interfaces between subsurface layers characterized by different propagation velocities.
The analysis mainly focuses on compressional waves (P-waves) and, where possible, shear waves (S-waves).
The seismic energy, generated on the ground surface by a seismic source, propagates:
  • within the shallowest layer, as direct arrivals
  • at depth along higher-velocity layers, generating refracted waves that emerge again at the surface at greater distances
Signals are recorded using a linear deployment of seismic sensors  (geophones).
By repeating the excitation at different positions along the survey profile, it is possible to reconstruct the geometry and velocity characteristics of the refractor layers, allowing the geometry of the bedrock to be identified.
The Vp and Vs velocity values obtained from the seismic-stratigraphic model also make it possible to estimate the main elastic parameters of the investigated materials, providing useful information for the geotechnical and geomechanical characterization of the bedrock.
Sismografo Gea24

Seismic refraction tomography

Seismic refraction tomography uses the arrival times of seismic waves, or first arrivals, recorded along a profile to reconstruct a subsurface velocity model through travel-time inversion.
Unlike conventional refraction, tomographic processing makes it possible to describe lateral variations in ground properties more effectively, providing 2D velocity sections and, where applicable, 3D extensions.

When it is used

Refraction tomography is suitable when it is necessary to:
  • reconstruct the geometry of the bedrock or a rigid substratum in the presence of significant lateral variations
  • identify fractured, weathered or low-velocity zones
  • analyze complex lithological contacts that cannot be represented by 1D models
  • improve the interpretation of conventional refraction in contexts that are not ideally stratified
  • support geological and geotechnical studies during preliminary or final design phases
  • calculate the elastic parameters of the ground using Vp and Vs measurements
In many projects, refraction tomography is integrated with other methods, such as MASW and HVSR, to obtain a more complete characterization.

Why tomography rather
than standard refraction

Conventional refraction is often based on simplified models and assumptions of regular stratification.
Refraction tomography, through the inversion of arrival times, makes it possible to obtain a more continuous velocity model that better reflects real site conditions, especially when:
  • the subsurface shows lateral heterogeneities
  • the interfaces are not planar
  • the geometry of the contacts is complex
Stendimento Land Streamer

What it produces

A refraction tomography survey produces:
  • a 2D P-wave velocity section (Vp) and, where applicable, information on S-waves (Vs)
  • iso-velocity maps or profiles
  • interpretations of the main lateral variations, consistent with the available geological evidence
  • a technical report including:
    1. acquisition geometry and main parameters, such as number of channels, spacing and profile length
    2. type of source
    3. trigger mode
    4. quality control (QC) criteria for first-arrival picking

APPLICATION CONTEXTS

- foundation studies for civil works and infrastructure 

- preliminary assessments and subsurface characterization in urbanized areas 

- quarrying and mining contexts 
- environmental sites, including filled areas, landfills and contaminated sites 
- hydraulic works and checks on substratum continuity

Required instrumentation

The typical configuration includes:
Geofono orizzontale 4,5 Hz
Cavo sismico 24 canali
Piattello di battura onde orizzontali

Examples of operational configurations

For seismic refraction surveys, the instrumental configuration may vary according to the number of channels, the investigation depth and the objectives of the survey.
Below are some examples of operational configurations, representative of the most common solutions:
  • 24-channel configuration, suitable for standard surveys and medium-depth investigations
  • 48-channel configuration with 12-channel cables, suitable for longer spreads and higher resolution along the profile
  • configurazione a 48 canali con cavi da 24 canali, utilizzata per acquisizioni più estese e layout ottimizzati
  • Configuration over 48 channels (72, 96 or multiples) with 24-channel cables, when a higher resolution and/or length profile is required.
Each configuration includes the seismograph, geophones, seismic source, trigger systems and spread accessories.
DISCOVER THE AVAILABLE CONFIGURATIONS
GEA24 con 3DLG

Interpretation software

Tomographic processing requires dedicated software for first-arrival picking and travel-time inversion.
ZONDST2D
2D tomographic interpretation of refracted-wave data and, where supported, also reflected-wave data.
RAYFRACT
Tomographic inversion of arrival times and generation of velocity models with quality control tools.
INTERSISM
Management and interpretation of seismic data, analysis of arrival times and support for interpretation phases for educational purposes or simple applications.

Limits and assumptions

Like any methodology, refraction tomography has limits related to site conditions and acquisition geometry. In particular:
  • the quality of the result depends on coverage, (number of channels, spacing and spread geophone deployment geometry), as well as on the ability to record reliable first arrivals
  • insufficient energy can limit the investigation depth
  • environmental noise and poor geophone coupling can reduce signal quality
  • stratigraphic contexts with velocity inversions can limit the resolution of the tomographic model and require integration with other methods

Resources and references

Standards
Scientific literature
Articles using PASI instrumentation
Software manuals

Technical terminology

  • Vp, P-wave velocity
  • Vs, S-wave velocity
  • first arrivals / first breaks
  • travel‑time inversion
  • velocity model
  • ray tracing / eikonal equation 
  • regularization / smoothing