To schedule a geophysical seismic surveying job with Subtronic Corporation, please contact us.
Geophysical seismic surveys have been used since the advent of geophysics, and can be used to determine various properties of the subsurface, such as:
- Determine rock and soil characteristics
- Map soil, rock, mineral, and groundwater structures
- Investigate geological hazards such as faults and landslides
- Map landfill and hazardous waste boundaries
Introduction to Geophysical Seismic Surveys
The propagation of sound waves through the Earth are called seismic waves, and a number of different geophysical methods depend on sound waves to observe various characteristics of the subsurface. The seismic methods Subtronic uses are seismic refraction, seismic reflection, and multi-channel analysis of surface waves (MASW).
Geophysical seismic equipment collects and records data on the velocities of seismic waves propagating through the earth. The different types of seismic waves, P-waves (primary), S-waves (shear), and surface waves are all recorded. Geologic and man-made structures can be identified as long as the structures are of a sufficient scale and produce a strong enough velocity contrast from the surrounding subsurface. The velocities of the subsurface are able to be calculated from the recorded arrival times of the waves and the measured distance they arrive at.
Seismic refraction relies on the refraction of seismic waves as they travel through the subsurface as governed by Snell’s Law.
Seismic refraction is usually only applicable where the seismic velocity of the geologic layers of the subsurface increase with depth. Seismic refraction might result in incorrect results when higher velocity materials may overlie lower velocity materials.
Additionally, seismic refraction surveys require an overall length approximately 4 to 5 times the depth to the area of interest (e.g. the top of bedrock). As a result of the long survey lines needed to collected information at depth, it is uncommon to use seismic refraction to map layers which occur at depths greater than 100 feet. Depths greater than 100 feet are possible, but the required survey lengths can quickly exceed site dimensions, and the energy required may become unfeasible. Multiple techniques (time-term inversion, generalized reciprocal method, and tomography) can be used to interpret seismic refraction data, but a 2D cross-sectional model visually showing the velocity changes of the subsurface vertically and horizontally is the end result for all the methods.Typical rock velocities, from Bourbie’, Coussy, and Zinszner, Acoustics of Porous Media, Gulf Publishing.
The seismic refraction time-term inversion method is a quick data analysis approach which will accurately provide information such as depth to bedrock as long as the seismic velocity of site increases with depth. The seismic refraction generalized reciprocal method, yields similar results to time-term inversion but uses a different approach. Seismic refraction tomography is the most advanced seismic refraction method, its advantages including high model resolution and the ability to model a seismic velocity gradients, typically shortcomings of other seismic refraction methods.
Seismic refraction surveys are limited by the strength of the source energy used (sledgehammer, shotgun, impact hammer), time requirements, and the models generally assume that velocity increases with depth, which is not always the case.
Seismic reflection relies on the reflection of seismic waves as they travel through the subsurface. Reflected seismic energy never is a first arrival, and therefore must be carefully identified and analyzed within a complex dataset of overlapping seismic wave arrivals. Software filtering of reflection data is a requirement to yield good results, and as a result the processing time for a seismic reflection survey is much greater than for a seismic refraction survey. With depths less than ~50 feet, the seismic reflections of interest overlap with the much higher amplitude surface waves and air blasts, making seismic reflection impractical/impossible at shallow depths. Reflections from depths greater than ~50 feet arrive after the surface waves and air blasts have arrived, and seismic reflection data is much easier to interpret as a result.
Seismic reflection can be used for many environments where seismic refraction is impractical and/or ineffective. Seismic reflection can be performed in environments with velocity inversions or low velocity zones, can measure very deep density contrasts, usually has superior lateral resolution compared to seismic refraction, and can use much less shot energy and a shorter survey length than would be required for a comparable seismic refraction survey.
Seismic Refraction versus Seismic Reflection
Compared to seismic refraction, seismic reflection is a higher cost technique and it is only practical for depths greater than ~50 feet. For situations where seismic refraction or seismic reflection could be used, seismic reflection generally yields higher resolution data, but seismic reflection is more expensive. For those instances, the choice between which technique to use is based off of the quality of data needed and the funding available.
Sinkhole Seismic Exploration Project
In January 2015, Subtronic was contracted to investigate the presence of possible sinkholes at a facility in California. Using Geometric’s Geode G24 Exploration Seismograph with 24 14Hz geophones, Seismic refraction tomography was used to create subsurface velocity profiles to a depth of ~40 feet along four survey lines.
Map depicting the four survey lines and the interpreted low velocity zones .
Seismic refraction tomography model of survey line 1 with no contouring. Two low velocity zones are circled in black. Low velocity zone (LVZ) 1 begins at a depth of 79ft and goes to a depth of 65ft. In the x position, LVZ1 begins at 75ft and continues to 105ft. LVZ2 begins at a depth of 84ft and goes to a depth of 70ft. In the x position, LVZ2 begins at 188ft and continues to 214ft. For LVZ’s 1 and 2, there are no large velocity differences between the LVZ and the subsurface above, but horizontally and below the LVZ the velocity differences are apparent. The velocity difference between LVZ’s 1 and 2 and the surrounding subsurface is ~800ft/s.
Seismic refraction tomography model of survey line 1 with contouring.
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