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A Non-Invasive Approach to Subsurface Exploration

AESI employs state-of-the-art non-invasive geophysical methods to characterize subsurface soils, geologic structure, and groundwater.  Our geophysical studies combine with our hydrogeologic and geotechnical expertise for cutting edge site classification for seismic design, slope stability modeling, landslide studies, groundwater resource and flow models, and more.  Geophysical methods employed by AESI are summarized below:   

Seismic Methods

Seismic Refraction Tomography (SRT)

  • This technique produces a 2D profiling of compressional wave velocity (Vp). Based upon the principle of Snell's law, when a seismic wave encounters a boundary of which there is a contrast in velocity, the wave changes direction (refracts) upon entry into the new medium. A sledgehammer is used to apply energy to the Earth surface and geophones are deployed to record the arrival times of seismic waves at varying distances.

  • Common applications include bedrock depth estimation, geologic characterization for engineering/groundwater/environmental projects, rippability assessment, geotechnical hazard assessment, and identifying faults and fractures.

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Swinging a sledgehammer onto a strike plate simulates energy moving through the Earth's surface. The line of geophones records the arrival of seismic waves at varying distances from the strike plate.

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Surface Waves Methods

  • Multichannel Analysis of Surface Waves (MASW) & Microtremor Array Measurements (MAM) are two techniques that use surface waves to estimate shear wave velocity (Vs). MASW is an active method where energy is applied using a sledgehammer to generate higher frequencies while MAM is a passive method using spatial autocorrelation of lower frequency energy from cultural noise, wind, wave motion, etc. These methods can be combined to provide a more comprehensive look into both the shallow and deep subsurface.

  • Common applications include seismic site characterization (Vs100), geologic characterization for engineering/groundwater/environmental projects, geotechnical hazard assessment, and slope stability assessment. These methods are typically used to create a 1D Vs model, however, multiple 1D profiles can be contoured together to provide a pseudo-2D cross sectional view of the subsurface.

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Horizontal to Vertical Spectral Ratio (HVSR)

  • This technique is a passive method where ambient seismic noise is recorded by a 3-component seismometer which is then used to estimate the fundamental resonant frequency (f0) of a site by comparing the ration of spectral amplitudes recorded on the horizontal and vertical components. f0 is related to the layer thickness (h) and the shear wave velocity (Vs) such that f0 = Vs/4h, so we can use this frequency along with a measured Vs to estimate depth to bedrock. Bedrock depth can also be estimated using a local power-law regression created by measuring f0 with HVSR at locations where bedrock depth is known. 

  • HVSR can be a very cost-effective means of estimating changing subsurface conditions over a site where active methods like seismic refraction tomography (SRT) or electrical resistivity tomography (ERT) may not be feasible.

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Electrical Methods

Vertical Electrical Sounding (VES)

  • This technique provides a 1D measurement of electrical resistivity distribution at depth. Using four electrodes in a Wenner array, current is applied between the outer electrodes and voltage is measured between the inner electrodes. 

  • Common applications include groundwater exploration, mineral exploration, environmental site investigations, and site characterization for engineering projects. This technique is well-suited for point-specific investigations.

Electrical Resistivity Imaging (ERI)

  • This technique produces a 2D profiling of electrical resistivity distribution across a surveyed area. Using an array of 24+ electrodes in various configurations, hundreds to thousands of measurements are taken to create a spatially continuous image of resistivity variations over the surveyed area. 

  • Common applications include groundwater exploration, mineral exploration, environmental site investigations, site characterization for engineering projects, landslide and slope stability studies, infrastructure assessment, and more. This technique is well-suited for small to large site investigations.

Soil Resistivity Measurement

  • This technique is a point specific measurement of soil resistivity using the Wenner four-electrode method (ASTM-G57). The measurement of soil resistivity plays a crucial role in managing the corrosion of structures buried underground. It is employed to estimate anticipated corrosion rates and to formulate the design of cathodic protection systems.

Fall of Potential Grounding Test

  • This test is an evaluation of the capacity of an electrical grounding system using the 3-pole test method (IEE Standard 81). A current is injected into the ground and a potential electrode is placed at varying distances from the grounding system to measure the voltage drop with distance.

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Potential Field Methods

Gravity Method

  • Local variations in the distribution of mass throughout the subsurface cause very small changes in the Earth's gravity field that can be measured using a sensitive instrument called a gravimeter. 

  • This method is typically used for medium to regional scale investigations, gravity surveying has applications such as mineral exploration, locating subsurface voids and/or buried bedrock valleys, and mapping regional geology.

Magnetic Method

  • The Earth's magnetic field has localized variations that are influenced by the presence of magnetic minerals and/or buried metallic objects of ferromagnetic composition (iron, steel, nickel, etc.). 

  • The magnetic method has applications at all scales, from identifying buried tanks/drums on a small property to regional geologic and mineral exploration studies.

Vibration Monitoring

Using the industry leading Instantel Micromate, AESI can provide continuous vibration monitoring during pile driving, blasting, compaction, and other construction activities that may exceed local regulations. AESI is proud to offer a standalone monitoring setup to provide continuous power for extended monitoring projects and LTE network capabilities for automated notifications when vibration levels exceed regulatory levels.

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