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Seismic Refraction Testing (SRT)

Seismic refraction is an active geophysical method based on the principle governed by Snell’s Law of refraction. This permits the reconstruction of geometries and thicknesses of the subsurface, depths of substrata (within the limits of seismic source penetration), and eventual lateral discontinuities, via the study of elastic waves generated on the surface.

Seismic waves (P-waves) energy is generated using an acoustic source at predetermined locations (S). The travel times of acoustic energy that has been critically refracted at horizons of interest (L1) is recorded at predetermined receiver locations (R1, R2, etc.). The recorded travel time information is used to generate a velocity–structure profile of the subsurface along the length of the refraction profile.

Modified from “Introduction to Geotechnical Geophysics – N. Anderson and N. Croxton, Circular – Number E-C130, October 2008”.

SRT (P-waves) can be carried out in a variety of terrains and will suit multiple purpose.

12, 24 or 48 geophones are placed on the ground using 75mm tapered spikes to ensure good coupling with the surface. The geophones are spaced based on the required depth and resolution of the investigation and connected via a seismic cable to a seismograph.

Seismic energy source is generated along the seismic alignment and can vary based on the depth of penetration required and the terrain/environment. Types of seismic sources used by Scurbat are sledgehammer, mechanised/accelerated Seismic Weight Drop (SWD) system, “Betsy” gun and explosive charges.

Example 1: Seismic Refraction Testing (SRT) tomography section with correlation of simplified geotechnical boreholes.

Example 2: Seismic Refraction Testing (SRT) investigation to assist in the assessment of rippability based on the “Caterpillar Rippability Chart”. To facilitate the evaluation of rippability, the standard seismic velocity profile has been converted to a “traffic light” profile based on velocities specified by the client.

What are the most common uses of Seismic Refraction?

Rippability assessment
Depth to rock and soil profile
Rock and soil characterisation
Fill delineation
Geological structures / Features Detection / Delineation
e.g., voids/cavity, boulder floaters, faulting and fault zone, dykes, palaeochannels
Land compaction assessment (pre and post)

Seismic Refraction FAQs

What is a seismic refraction testing?

SRT is carried out for the sole purpose of knowing the P-waves seismic velocities of materials.

To achieve this on land, a series of detectors (i.e., geophones) are placed on the surface at known distances and seismic waves, via sledgehammer, explosives, or seismic weight drop, are also generated at known distances. Both geophones and seismic energy source are connected to a seismograph, which is basically an extremely sophisticated chronometer. Once the seismic waves are generated at the surface, the seismograph starts counting the time the waves take to travel through the medium, refract, and arrive to the geophones. By knowing the time and the distance travelled by the seismic waves we can calculate the velocities. Velocity = Distance / Time…simple

What is the difference between seismic refraction or reflection survey?

Refraction: waves are refracted as they travel through a material.
Reflection: waves reflect off an interface between two different materials.

The primary difference, from an end result point of view, is that refraction will provide P-waves velocity distribution of the different layers, which will qualify the condition of the subsurface, and reflection will provide a clearer image of the stratigraphy and structures but without direct correlation to strength.

How accurate is seismic refraction testing?

SRT can be very accurate. This is dependent mostly on the geometry of the array, e.g., total length, geophones, and shots spacings, number of shots locations per given spread and the seismic source power. The latter is particularly important if resolution at depth is required, the bigger the “boom” the deeper the search.

How much does a seismic refraction survey cost?

The costs for a SRT survey are determined by the size and complexity of the site, and the project requirements. Therefore, it is difficult, and misleading, to provide an exact dollar value without knowing the particulars. A very indicative cost for the fieldwork component can vary from $2,500 to $6,500 per day. This excludes mobilisation and demobilisation, which can vary by 10s of thousands of dollars, Australia is big! Also, it excludes processing and reporting, which depends on the amount of data acquired.

Reach Out and Let’s Discuss Your Project

No matter how simple or complex is your project, we can assist in many ways…a technical explanation? A practical field implementation? A budget discussion? Or a full project planning and execution? Just ask away.
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