Seismic Refraction

Overview

The seismic refraction technique is a classic geophysical method applicable to a variety of engineering and environmental projects.

Common applications include:

• Mapping depth to bedrock and bedrock topography
• Providing elastic properties of the subsurface for engineering design
• Calculating the subsurface velocity profile
• Mapping subsurface water table in sediments
• Identifying fault locations and weak rock zones
• Determine rippability of hard rock prior to construction
• Mapping slide planes of active landslide

The seismic refraction method requires three components: a controlled shot of seismic energy (source), sensors to receive the energy (geophones), and a central data recorder (seismograph) connected via radio links or cabling. The transmitted energy is recorded at each geophone along the seismic line. A hammer blow or explosive charge (the shot) generates a shock wave which travels through the earth by refraction along material boundaries. The energy received at the surface by an array of sensors or geophones is analyzed for structure and velocity.

The time at which the energy is received at the surface is analyzed for structure and velocity. Seismic data may be modeled using either layer-based or tomographic techniques. Each method has its own strengths and weaknesses, and data collection parameters, thus should be determined prior to data collection to meet the objectives of the project.

Data is typically presented in simple cross section highlighting the interface between soil and hard rock. In complex geologic environments, 3D refraction data (3D tomography) reduces ambiguity and allows integrated models to be created combining the seismic data with borehole and other ground-truth data.

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Geologic Model

Geologic materials which increase in velocity with depth. Layered and non-planar boundaries between strata of differing velocities are sensed.

Requirements

A. A reasonable source:
sledge hammer and metal plate OR
accelerated weight drop OR
explosives OR
vehicle-weighted plank and sledge hammer

B. Low ambient noise: stay away from:
all-night gravel processors
interstate highways
busy airports

C. Surface access:
offset shots beyond ends of lines
geophone emplacements (pavement, snow, etc.)

D. Ground truth:
drill holes
well logs
outcrops
water table location

Considerations

Low velocity layers within higher velocity milieu are not detected. Water table location may not be detected. High speed stringers may be mistaken for bedrock. Ambiguity: velocity vs. structure tradeoff.

Logistics

Crew size usually 2 or 3 people. Portable (no vehicles), if source can be carried. Line location and actual elevations by surveying or GPS.

Deliverables

Plan maps, travel-time curves, seismic cross-sections, geologic interpretation of seismic cross-sections and narrative description of work done.