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SURFACE
SEISMIC REFLECTION

Surface seismic reflection is an active geophysical method used to reconstruct subsurface geometry by analysing seismic waves reflected at interfaces between materials with different acoustic impedance.
A seismic source generates energy at the surface. The waves propagate through the ground and, when they encounter an interface characterised by an acoustic-impedance contrast between materials, part of the energy is reflected back towards the surface. The reflected signals are recorded by a spread of geophones connected to a multichannel seismograph.
Unlike seismic refraction, which mainly uses the first arrivals of seismic waves, seismic reflection analyses the two-way travel times of reflected waves. This provides information on the lateral continuity of horizons, the presence of discontinuities, layer geometry and, under favourable conditions, buried structures or stratigraphic variations that are not always detectable with other methods.
Sismografo Gea24 Plus

How it works

The physical principle of seismic reflection is linked to the acoustic-impedance contrast between two materials. Acoustic impedance depends on the propagation velocity of the seismic wave and on the density of the material through which it travels.
When a seismic wave reaches an interface between two layers with different properties, part of the energy continues at depth and part is reflected. The time taken by the wave to reach the reflector and return to the surface is called the two-way travel time.

Data processing converts the recorded signals into seismic sections, in which reflectors are interpreted as possible geological interfaces, stratigraphic boundaries, discontinuity surfaces or significant variations in material properties.

When it is used

Surface seismic reflection is suitable when the objective of the survey is to reconstruct subsurface geometry with a high level of lateral detail.
It can be used to:
  • identify and trace reflecting horizons in the subsurface
  • reconstruct stratigraphic geometries and lateral variations
  • map bedrock surfaces or buried geological layers
  • identify faults, fractures or structural discontinuities
  • support geotechnical, environmental and hydrogeological studies
  • integrate refraction, MASW, HVSR or borehole surveys
It is particularly useful when the subsurface model cannot be described solely by velocities increasing with depth, or when the objective is to trace the continuity of one or more reflectors.

Reflection and refraction:
different methods, different information

Seismic refraction and seismic reflection both use artificially generated seismic waves, but they analyse different portions of the signal.
Refraction focuses on first arrivals and makes it possible to reconstruct subsurface velocity models. It is often used to identify bedrock, estimate thicknesses and characterise layers with increasing velocity.
Reflection, by contrast, analyses events reflected from interfaces with an impedance contrast. It is more specifically oriented towards reconstructing horizon geometry, assessing the lateral continuity of layers and identifying discontinuities.
Land streamer

What it produces

A surface seismic reflection survey can produce:
  • high-resolution 2D seismic sections
  • identification and tracing of the main reflectors
  • interpretation of the lateral continuity of horizons
  • identification of discontinuities, faults, stratigraphic variations or anomalous geometries
  • interpretative models that can be integrated with geological, geotechnical or hydrogeological data
  • a technical report describing the acquisition geometry, recording parameters, processing criteria and interpretation
The result is not a simple point measurement, but an interpretative representation of the subsurface, to be read together with the available geological data and the objectives of the survey.

APPLICATION CONTEXTS

- applied geology 

- geotechnical engineering

- civil engineering

- environmental studies

- hydrogeology
- bedrock characterisation 
- identification of discontinuities, faults or stratigraphic variations

- integration with other geophysical surveys

Required equipment

A typical configuration includes:
Geofono verticale 28 / 40 / 50 / 100 Hz
Cavo sismico 24 canali
Energizzatore Isotta

Examples of operational configurations

Operational configurations for a surface seismic reflection survey can be selected according to the number of available channels and the acquisition geometry required by the investigation.
The most common configurations are:
  • 24-channel acquisition, suitable for surface reflection surveys on short profiles, localised investigations and preliminary checks when a simple, quick-to-install configuration is required
  • 48-channel acquisition with 12-channel seismic cables, suitable for surveys requiring more traces and greater lateral continuity, while retaining a modular spread and good flexibility in cable layout
  • 48-channel acquisition with 24-channel seismic cables, suitable for longer profiles and structured campaigns when it is useful to reduce cabling complexity and maintain an orderly, repeatable acquisition geometry
  • 72-channel acquisition with 24-channel seismic cables. Suitable for more extensive multichannel surveys when greater profile coverage and a larger number of traces along the same seismic line are required
  • 96-channel acquisition with 24-channel seismic cables. Suitable for more complex acquisition campaigns when an extended seismic line and greater lateral continuity of the data are required
Depending on the selected layout, all configurations include a multichannel seismograph, seismic cables, high-frequency vertical geophones, a surface seismic source, a trigger system and field-layout accessories.
DISCOVER THE AVAILABLE CONFIGURATIONS
Radio trigger

Interpretation software

Processing seismic reflection data requires dedicated software for trace management, signal processing, velocity analysis and the construction of interpretative sections.
Software packages suitable for seismic reflection include:
REFLEXW
reflection 2D
REFLEXW
complete 2D/3D

Limitations and assumptions

The quality of the result depends on several factors:
  • sufficient impedance contrast between the investigated layers
  • correct geophone-to-ground coupling
  • appropriate choice of seismic source
  • favourable signal-to-noise ratio
  • spread geometry consistent with the required depth and resolution
  • correct data processing and control of non-reflected events
In shallow applications, direct waves, refracted waves, surface waves, ambient noise and air-coupled waves (for example, those generated by the seismic source) may interfere with the reflected signal. For this reason, seismic reflection requires careful survey design and more complex processing than many refraction applications.

Resources and references

Standards
Scientific literature
Software manuals

Technical terminology

  • acoustic impedance
  • seismic reflector
  • reflected events
  • two-way travel time
  • seismic trace
  • acquisition geometry
  • common midpoint / CMP
  • stacking
  • velocity analysis
  • signal filtering
  • migration
  • signal-to-noise ratio