Seismic methods record the movement of vibrations through the ground with their speed and path telling us something about the structure, strength and/or stability of the subsurface.
All seismic systems require three components: a source (either user-generated or naturally occurring vibrations), sensors and cabling to detect the seismic waves as they move through the subsurface (geophones on land, hydrophones in water) and an instrument (seismograph) to record that motion for later analysis.
When you have a need for a non-destructive investigation technique for subsurface conditions, seismics can be a good alternative to provide a more comprehensive picture compared to traditional intrusive methods, such as digging and drilling.
Bedrock depth detection and topography mapping, fracture and fault zone identification, cavity detection and mapping, and lithology (subdividing rock sequences into individual lithostratigraphic units for the purposes of mapping and correlation between areas) can all be undertaken with seismic methods.
Map ice thickness, discontinuities within the ice (such as crevasses) and bedrock topography of temperate glaciers (excluding snow). Most often seismic surveys give a clearer picture than GPR, as GPR data will be more affected by the water content in the ice.
Some form of energy source transmits elastic waves through the ground, which are then recorded by a seismograph via a set of geophones or hydrophones. The speed and path of the waves is controlled by the density, elasticity, and structure of the ground. By recording the movement of these waves with our seismic equipment and analysing their speed and nature, a model of the sub-surface structure can be produced. The seismic velocities of individual layers can help to predict the type of material present as well as its strength and/or stability. Typically, faster velocities indicate stronger or more stable materials.
Depending upon the type of accessories and survey procedure used, different types of motion can be recorded. Common modes of seismic survey include refraction, reflection, surface wave and borehole all of which are discussed briefly, below.
Refraction has a relatively simple field procedure and is well suited to mapping bedrock location, topography, and strength as well as general geological sequencing. The processing is also relatively straightforward and only looks for the first seismic arrival at each sensor. It relies on the fact that seismic velocity of the layers being investigated increases with depth, thus energy that has travelled through deeper layers will ‘overtake’ energy travelling through slower shallower layers and arrive first at more distant geophones.
Inversion software is used to find a combination of layer thicknesses and velocities which provides modelled data close to the ‘real-world’ recorded data.
A rule of thumb for designing a refraction survey is that the cable spread should be 3 – 5 times longer than the depth required, but consideration should also be given to the energy available from the source.
Unlike refraction work, where a velocity model is the primary output, with reflection survey the aim is to create a direct representation of the buried structures and stratigraphy – something akin to a GPR profile but with the potential to image much more deeply.
Reflection survey does not require seismic velocity to increase with depth; any significant velocity contrast will cause a reflection and a drop in velocity will just alter the phase of the reflection.
To produce a single profile, records from a large number of shots and/or receiver positions are required and reflection processing requires numerous steps and higher level of competency to get a good result. Inversion processes can also be used to determine layer velocities.
The rule of thumb for depth is that the cable spread should be 1 – 2 times longer than the depth required.
With surface wave methods, again, the aim is to generate a model of velocity variation rather than a reflection-style, detailed image of stratigraphy. The field methodology will vary depending upon the method employed. But the output is generally a 1D, 2D or 3D model of shear wave velocity which gives an excellent indication of a material’s stability and can be used in widely-published formulae to generate quantitative engineering parameters.
For active surface wave methods, the rule of thumb for depth capability is that the cable spread should be 1 – 2 times longer than the depth required.
A key requirement for collecting good data is a suitable seismic source; one which is powerful enough to generate detectable waves all along the survey spread and to the required depth. The frequency signature of the source is also important: something which generates more high-frequency energy will result in greater detail, but the signal will be attenuated (absorbed) much faster; a low frequency source will provide good depth penetration, but resolution is compromised. With vibration sources (as opposed to impact sources like a hammer or weight-drop) it is possible to define the specific vibration frequency, or even choose a “sweep” of frequencies from high to low. Use of a vibration or impact source defines “active” methods, but in “passive methods” the source is background vibrations originating from both natural and human motion (such as tides, distant tectonic activity, construction, traffic). These are very low frequency but very powerful vibrations, typically used in surface wave studies.
Geophones will need to be matched to the task in hand and are normally defined by their orientation and their “Hz” rating, which defines the frequencies they are most sensitive to. As with sources, low frequency units respond well to energy that has likely travelled further into the ground, whilst their higher frequency counterparts will be able to deliver better detail. Some methods need lower frequency units, such as any surface wave analysis, whilst others like reflection can benefit from those with a higher rating. As for orientation, refraction studies normally require vertically mounted sensors, reflection surveys often use horizontally orientated units, and some methods use geophones that have multiple sensors mounted in opposing orientations. For hydrophones, which measure pressure change rather than motion, the only choice is active or passive. Passive units simply send the signals generated by the vibrations passing through them, whilst active systems amplify the signal at the sensor before transmitting back along the spread cable.
The layout of the investigation will determine whether the result is a sounding (showing velocity variation only with depth – “1D”) or something more comprehensive which also reveals velocity variations along a profile (“2D”) or across an area (“3D”).
The ABEM Terraloc Pro 2 is a versatile seismograph that can be used with any type of seismic accessory for the widest range of applications.
For a definitive measure of individual layer velocities, or to accurately map layer interfaces, it may be necessary to undertake borehole seismic measurements. There are three main configurations, namely down-hole, cross-hole and tomography:
Water-borne surveys use hydrophone strings running horizontally within the water column or laid across the bed of the river, lake or sea. Specialist sources such as boomers, sparkers or even explosive charges, set-up pressure waves in the water which translate to vibrations in the underlying sediments and geology. The range of methods and analysis are similar to those used onshore.