Seismic Reflection & Refraction Services
GeoSearches Inc. employs seismic reflection as a geophysical method to reveal what’s under the surface. This technique uses the principles of seismology and reflected seismic waves to estimate the properties of Earth's subsurface. It requires a controlled source of seismic energy, such as dynamite, a specialized air gun, or a seismic vibrator—commonly known by the trademark name Vibroseis. By recording the time it takes for a reflection to arrive at a receiver, we can estimate the depth of the feature responsible for generating the reflection.
Seismic Refraction
Seismic refraction is a geophysical principle governed by Snell's Law. Used in the fields of geotechnical engineering, engineering geology, and exploration geophysics, seismic refraction traverses (seismic lines) are performed using an energy source and geophone(s) and/or seismograph(s) in an array. The seismic refraction method uses the refraction of seismic waves on geologic layers and soil/rock units to characterize the subsurface geologic structure and geologic conditions.
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The method depends on the fact that seismic waves have differing velocities in different types of rock or soil and that these waves are refracted when they cross the boundary between different types (or conditions) of rock or soil. This method determines the general soil types and the approximate depth to bedrock or strata boundaries.
The refraction microtremor method combines the ease and urban utility of microtremor array techniques with the shallow accuracy of the MASW technique and the operational simplicity of the SASW technique. By recording urban microtremors on a linear array of many lightweight seismometers, we can achieve fast and easy field data collection without needing the time-consuming heavy source required for MASW and SASW work. By retaining all the original seismograms and applying a time-domain velocity analysis technique (as in MASW), the analysis described here can separate Rayleigh waves from air waves, body waves, and other coherent noise. Transforming the time-domain velocity results into the frequency domain allows for the combination of many arrivals over a long period and easy recognition of dispersive surface waves.
The seismoelectric method (aka the electroseismic method or seismo-electric) is based on the generation of electromagnetic fields in rocks and soils by seismic waves. Although the method is not reported to detect groundwater flow, it is able to measure hydraulic conductivity, which is related to permeability and, therefore, to the potential for groundwater flow.
The seismic refraction method is based on the measurement of the travel time of seismic waves refracted at the interfaces between subsurface layers of different velocities. Seismic energy is provided by a source (“shot”) located on the surface. For shallow applications, this typically comprises a plate and hammer, weight drop, or small explosive charge (blank shotgun cartridge). Energy radiates out from the shot point, traveling either directly through the upper layer (direct arrivals) or travelling down and then laterally along higher velocity layers (refracted arrivals) before returning to the surface. This energy is detected on the surface using a linear array (or spread) of geophones spaced at regular intervals. Beyond a certain distance from the shot point (known as the cross-over distance), the refracted signal is observed as a first-arrival signal at the geophones (arriving before the direct arrival). Observation of the travel times of the direct and refracted signals provides information on the depth profile of the refractor.
Shots are deployed at and beyond both ends of the geophone spread to acquire refracted energy as first arrivals at each geophone position. Data are recorded on a seismograph and later downloaded to a computer to analyze the first-arrival times from each shot position to the geophones. Distance versus travel time graphs are then constructed, and velocities calculated for the refractor and overburden layers through analysis of the T-minus and direct-arrival graph gradients. Depth profiles for each refractor are produced by an analytical procedure based on consideration of shot and receiver geometry, calculated velocities, and measured travel times. The final output includes a velocity model of the subsurface and a depth profile of the refractor layers.
Shots are deployed at and beyond both ends of the geophone spread to acquire refracted energy as first arrivals at each geophone position. Data are recorded on a seismograph and later downloaded to a computer to analyze the first-arrival times from each shot position to the geophones. Distance versus travel time graphs are then constructed, and velocities calculated for the refractor and overburden layers through analysis of the T-minus and direct-arrival graph gradients. Depth profiles for each refractor are produced by an analytical procedure based on consideration of shot and receiver geometry, calculated velocities, and measured travel times. The final output includes a velocity model of the subsurface and a depth profile of the refractor layers.
The primary application of seismic refraction is to determine bedrock structure and the depth to bedrock. Due to the dependence of seismic velocity on the density and elasticity of the material through which the energy is passing, seismic refraction surveys provide a measure of material strength and can consequently be used to help assess rock quality and rippability. The technique has been applied successfully to map the depth to base of backfilled quarries, the depth of landfills, the topography of groundwater, and the thickness of overburden.
Contact us today to learn how seismic reflection and refraction can help your project.
