The environmental engineering industry has used different types of seismic testing to evaluate subsurface geology since the 1960's.  An early form of the technique, called seismic refraction, was used primarily in rippability studies to determine what type of machinery was required to excavate materials at construction sites.  The refraction technique is limited in its ability to image complex geologic structure or detailed stratigraphy, but because of the relatively low cost to perform a survey, and the ease with which refraction data can be processed, it is still a popular seismic application.

The seismic technique was originally developed by the oil exploration industry.  Almost all of the developments of the technique for shallow-depth, environmental problems were adapted from oilfield practices.  Seismic reflection surveys have been performed in oil exploration to delineate subsurface structure since the 1930's.  The early surveys (2D, single fold, continuous coverage profiling) provided large-scale structural information about the subsurface, but forced oil exploration teams to drill without a completely accurate image of the reservoir (much as is done in environmental engineering today).  As the use of seismic surveys became more accepted and as funds were available for research, the technique evolved until it became an effective way to view and interpret large-scale subsurface geologic structural features.  The advent of the 2D, multi-fold, common-depth-point surveying techniques, along with advances in instrumentation, computers, and data processing techniques, greatly increased the resolution of seismic data and the accuracy of the subsurface images.  However, the technique still yielded little information on the physical properties of the imaged rocks, or the pore fluids within them.

It was not until the introduction of 3D reflection surveying in the 1980's that seismic images began to resolve the detailed subsurface structural and stratigraphic conditions that were missing or not discernable from previous types of data.  Today potential oil reservoirs are imaged in three dimensions, which allows seismic interpreters to view the data in cross-sections along 360° of azimuth, in depth slices parallel to the ground surface, and along planes that cut arbitrarily through the data volume.  Information such as faulting and fracturing, bedding plane direction, the presence of pore fluids, complex geologic structure, and detailed stratigraphy are now commonly interpreted from 3D seismic data sets.

In the environmental engineering industry 2D shallow seismic reflection imaging has been performed to map the overburden-bedrock interface at test sites since the 1970's.  In recent years seismic reflection profiling has been applied to other geotechnical and environmental problems as well. In 1994, RRI performed the first high-resolution 3D seismic reflection survey at a hazardous waste site (Naval Air Station North Island, California).  Since that time, over thirty 3D seismic surveys have been performed by RRI for environmental investigations.

2D/3D seismic reflection surveys provide information that can be essential for characterizing and remediating hazardous waste sites.  Seismic surveys are proficient at mapping potential contaminant migration pathways, determining the presence of subsurface fracture systems, and imaging structural and stratigraphic heterogeneities below a site.  Seismic imaging technology provides valuable information to evaluate groundwater management alternatives.

Method

Seismic reflection imaging is based on the principle that acoustic energy (sound waves) will bounce, or "reflect" off the interfaces between layers within the earth’s subsurface.   This principle is analogous to the process of a human voice echoing off of a building wall.

During a seismic reflection survey acoustic energy is imparted into the earth with a seismic source.  RRI typically uses noninvasive sources on our environmental projects, such as a sledgehammer or a power-assisted weight drop source.  These sources are impacted upon an aluminum strike plate ground surface to create high frequency seismic energy.  After impact of the source the sound waves propagate and spread out along spherical wavefronts in all directions. The usable sound energy travels into the earth (signal), while some energy is lost into the air or along the ground surface (noise).  The figure on the right shows a simplified cross-sectional view of a 2D seismic recording system with some of the signal and noise ray paths associated with a reflection survey.

The earth is characterized by many layers, each with different physical properties.  When sound waves traveling through the earth encounter a change in the physical properties of the material in which they are traveling, they will either reflect back to the surface or penetrate deeper into the earth (where they may again be reflected at another interface).  At a geologic interface some seismic energy is always transmitted while some is reflected.  The acoustic impedance is a measure of how seismic energy will react when it encounters a subsurface layer.  This physical property is closely associated with the density of a layer.  Contrasts in acoustic impedance create seismic reflection interfaces.   Subsurface reflections of seismic energy, therefore, most often occur at the interfaces between lithologic changes (a transition from sediment to rock, for example).   As a result, seismic reflections make it possible to map the stratigraphy below a site.

Areas of structural deformation, such as fractures, can also be observed using seismic reflection.   A fractured rock surface produces different reflections than a continuous rock surface. A coustic energy is diffracted by fractured rock surfaces in much the same way that a visual image is distorted in a shattered mirror.  Identifying diffracted energy patterns is one way in which geologic structures such as faults and fractures can be mapped using seismic reflection surveys.

During a seismic reflection survey high-speed digital data recording systems (seismographs) and acoustic sensors (geophones) are used to measure reflected sound waves.  Compressional waves (p-waves) are a type of seismic waves.  Compressional waves are so named because the wavefronts propagate through the earth mechanically; when one particle moves and compresses the next particle.  The figure on the left (A) shows the wavefront of sound waves impinging on a geophone.  The particle motion in the earth moves the geophone body, which houses a magnet within a suspended coil inside the geophone.  This action produces an analog voltage signal that is proportional to the ground motion (B).  The seismograph then digitizes the analog signal by breaking the signal into discrete time samples, and creates a digital level (a numeric value) for the amplitude of the signal during that time sample (C).  The data in RRI surveys are digitized to 21-bit resolution, which means the analog geophone signal is broken into 2²¹ or 2,097,152 levels.  We analyze the final processed wavelet (D), which is the result of the post-survey data reduction process, and is a high resolution, distortion free representation of the subsurface.

3D Seismic Surveys

Since 2D data collection occurs along a line of receivers, the resultant image represents only a section below the line.   Unfortunately this method does not always produce a clear image of the geology.   2D data can often be distorted with diffractions and events produced from offline geologic structures, making accurate interpretations difficult.

Because seismic waves travel along expanding spherical wavefronts they have surface area.  A truly representative image of the subsurface is only obtained when the entire wave field is sampled.  A 3D seismic survey is more capable of accurately imaging reflected waves because it utilizes multiple points of observation. A grid of geophones and seismic source impact points are deployed along the surface of the site in a 3D survey.  The result is a volume, or cube, of seismic data that was sampled from a range of different angles (azimuth) and distances (offset), as shown schematically in the figures below.

RRI uses a data processing technique called 3D migration, which takes advantage of the multiple observation points provided by a 3D survey, to greatly increase the horizontal resolution of 3D seismic data.  The 3D migration process collapses diffraction patterns caused by points and edges found in the subsurface (such as fractures and faults), which can dramatically improve the seismic image.   The figure to the left illustrates the improvement of 3D data quality in contrast to 2D data, and was taken from the work of French (1974).  French collected seismic data over a model with 2 anticlines and a fault scarp.  Thirteen lines of data were collected, the results from Line 6 are shown.  The raw, 2D data shows anomalous effects from neighboring structures.  Diffraction patterns from the fault block (red) and both anticlines (green & yellow) are apparent in the data, making the section confusing and also incorrect.  The image is improved with 2D data migration.   Anticline #1 is correctly imaged since Line 6 passed over its crest.  However, Anticline #2, which is visible on the section, is not actually beneath Line 6.  Also, the fault scarp was imaged with the wrong slope.  Only the 3D migrated section accurately delineates the true geology.  This experiment exhibits a relatively simple geologic system.  Sites with fractures, faults, and complex structural geology produce a much more confusing seismic image that only 3D migration can help clarify.

Because oil reservoir exploration is a spatial, 3D problem, 3D seismic data collection, processing, and imaging has been advanced by all major oil companies.  Today new oil reserves are very rarely located without the use of 3D imaging.  Attempting to locate contaminant migration pathways or potential DNAPL accumulations are just two examples that present similar 3D problems for the environmental industry.

Vertical Seismic Profiles

A Vertical Seismic Profile (VSP) is a geophysical field test that measures accurate seismic velocity values for exact depth intervals beneath a site.  RRI uses VSPs to aid the processing and interpretation of 2D/3D surface seismic data collected at a site.  Stratigraphic information from boring logs combined with seismic wave travel time information measured during the VSPs provide data to correlate borehole geology with the surface seismic data so accurate geologic interpretations of the seismic images can be made.

The figure on the right shows a general schematic diagram, which identifies the components of the recording system for the VSP.

Soil and rock units are inherently heterogeneous and anisotropic, and as such, they differ in their ability to transmit and reflect seismic signals.  Physical characteristics such as mineral content, bulk density, degree of cementation, and pore fluid content and properties all impact the rate at which seismic signals travel through any volume of subsurface media, be it soil or rock.  Prior to collecting VSP data, the exact depth to features present on a seismic image can only be assumed based on general estimates of seismic velocity values for the types of soils or rocks known or thought to be present beneath the site.   VSPs provide the means to calibrate or "tie" the 2D & 3D surface seismic data to correct physical depths.  Stratigraphic information from boring logs along with seismic travel times measured from the surface down to any soil/rock feature, or other contact of interest, provide data to correlate borehole geology with surface seismic data.

Advantages & Disadvantages of 3D Seismic Imaging Technology for Environmental Investigations

NOTE:  Definitions to seismic-related terms and more thorough discussions regarding seismic data acquisition, processing, and interpretation techniques can be found here, in the Technical Memoranda section

References:

French, W.S. (1974), Two-dimensional and three-dimensional migration of model-experiment reflection profiles; Geophysics, v.39, p. 265-277.