There are many geophysical techniques that may be used to provide important site condition information for engineering and infrastructure applications. Geophysical methods can provide small and large-scale structural information about the ground and its engineering properties. One of the primary benefits of a geophysical survey is that the engineer or scientist can obtain information for large volumes of ground. Compared to traditional excavation or boring investigations, geophysical methods can be used to quickly and non-invasively provide valuable information such as Vs30 values, bulk modulus values, accurate utility locations, and more. This is a far more practical and less costly strategy than other more commonly performed methods.
Seismic refraction is a technique used to map contrasts in seismic velocity, or the speed at which seismic energy travels through given layers of soil and rock. This parameter typically correlates well with rock hardness and density and may also be used to recognize major fractures in the rock. Higher seismic velocity is generally indicative of harder, more intact material, while slower seismic velocity indicates softer and/or fractured material. Refraction surveys are best performed in areas of competent soil/rock to reduce signal attenuation, and in places where the density of the soil and rock is assumed to increase with depth. The Geode Exploration Seismograph is the tool of choice for seismic refraction surveys, and therefore rippability, fracture analysis, and weathered layer analysis and quantification.
The measured seismic velocity is compared against the Caterpillar Company’s published Ripper Performance charts to assist in choosing the correct equipment for excavating a given site. Refraction is a relatively broad-brush technique – it looks at gross velocity differences, and it is unlikely the test results will detect more than 3-4 individual velocity layers. More information about seismic refraction, including a tutorial and survey design worksheet, can be found by following these links.
- Seismic Refraction Exploration for Engineering Site Investigations
- A Vertical Array Method for Shallow Seismic Refraction Surveying of the Sea Floor
- High Resolution Refraction Data Acquisition and Interpretation
- Near-Surface Seismic Refraction Surveying Field Methods
- Seismic Refraction Tutorial
- Refraction Survey Design Spreadsheet
As with seismic refraction, shear wave velocities increase with material density. Therefore, dense materials generally have higher shear wave velocities than do softer materials. However, refraction surveys are limited by Snell’s law, which states that refraction only occurs when density increases with depth. Density may not always increase with depth and is especially true in places where dense caliche may be intercalated with sandy material or in places where dense volcanic material overlies older, softer material.
Shear wave propagation is not constrained by Snell’s law, therefore this is not a limitation for shear wave surveys. Both low and high density materials may be observed regardless of their, whereas in a refraction survey, less dense materials are rendered invisible if they occur beneath more dense materials. In locations where multiple weathering horizons are suspected, or in places where caliche or volcanic rock is present, shear wave surveys may provide more accurate results than refraction surveys. Shear wave surveys may also be more reliable in unconsolidated sediments, where it may not be possible to perform refraction surveys due to high attenuation of the seismic signal. The Atom Passive Wave Seismograph is the seismograph of choice for passive shear wave surveys
Seismic refraction is a technique used to characterize subsurface geologic structure, including depth to bedrock by measuring contrasts in seismic velocity. Bedrock generally has a higher acoustic impedance than overburden, making seismic refraction a useful tool for fault investigation. Refraction occurs as seismic waves travel away from a source, and cross a boundary between geologic layers of differing acoustic impedance. The method can provide information on faulting in two ways, either by identifying the lateral location of faults and/or by characterizing strata that may be offset, tilted, or folded by the fault.
The seismic refraction method is best performed in locations with rock or a dense soil or ground surface, and away from cultural noise. The method relies on the precise measurement of travel times between the source (such as a hammer and aluminum plate) and the geophones. The Geode Exploration Seismograph is ideal for fault investigation, with the capability of expanding up to 1000 channels for both 2D and 3D surveys depending on your field geometry.
Identifying the soil-bedrock interface is important in many types of applications. Knowing the condition of the subsurface materials is essential for those involved in excavation or foundation design. Additionally, information about the subsurface profile is important to those placing utilities, as utility installation is much easier in soil or weathered bedrock than it is in hard bedrock and utilities are frequently placed in locations where less excavation is required.
In locations where bedrock depth is estimated to be relatively shallow, shear wave surveys can provide a good approximation of that depth. These surveys are best performed in areas where the surface material is soft or unconsolidated and where the depth to bedrock is assumed to be less than 20 meters. Below 20 meters, data points obtained via shear wave surveys may be too sparse to accurately assess conditions for typical wired seismographs. The Atom Passive Wave Seismograph is able to collect active and passive waves, and with a nodal system, the spacing between Atoms is completely customizable. As a result, the Atom Seismograph is able to collect higher resolution data and deeper depths.
Shear wave velocity is an important parameter for estimating how the earth will behave during an earthquake and is used to determine Vs30 (Vs100). Shear wave velocities can be measured in one of two ways - either active or passive surface wave surveys. The active method, also known as Multi-Channel Analysis of Surface Waves (MASW) requires a source, such as a hammer and steel plate, to produce the shear waves for measurement. The passive method, or Microtremor Array Measurement (MAM), is a measurement of ambient waves and does not rely on a source. The math and physics behind both techniques is essentially the same; the difference lies in the wavelengths, which determine the depth of investigation. Lower frequency (longer wavelength) waves obtained from passive MAM techniques allow for deeper penetration. Waves produced using an active source have a higher frequency and provide information closer to the surface. Passive microtremor measurements may be coupled with active source data to achieve the necessary depth of investigation.
The Atom Passive Wave Seismograph is used for passive or MAM measurements for a site. The unit consists of multiple one-channel Atom Acquisition Units and 2 Hz geophones. A major benefit of the Atom is that you can customize your array, including line spacing, geometry, and system size. Once the units are turned on, they acquire a GPS lock and automatically start recording. After the test, data is downloaded using a WiFi connection. The Atom can be used for surveys up to 1 km in depth. The Atom is an easy system to use and can be deployed in a fraction of the time of a traditional Vs30 survey.
Shear wave velocity is an important parameter for estimating how the earth will behave during an earthquake and is used to determine Vs30 (Vs100). Shear wave velocities can be measured in one of two ways - either active or passive surface wave surveys. The active method, also known as Multi-Channel Analysis of Surface Waves (MASW) requires a source, such as a hammer and steel plate, to produce the shear waves for measurement. The passive method, or Microtremor Array Measurement (MAM), is a measurement of ambient waves and does not rely on a source. The math and physics behind both techniques is essentially the same; the difference lies in the wavelengths, which determine the depth of investigation. Lower frequency (longer wavelength) waves obtained from passive MAM techniques allow for deeper penetration. Waves produced using an active source have a higher frequency and provide information closer to the surface. Passive microtremor measurements are typically coupled with active source data to achieve the necessary depth of investigation.
It is no wonder that over 2,700 Geode Exploration Seismograph's have been sold. It is the most versatile and flexible seismograph systems available. Small and lightweight enough to pack in your suitcase, it expands easily for full-scale 2D and 3D surveys at a cost your bottom line will love. When you are not using the Geode for reflection, refraction, MASW/MAM, or tomography surveys, use it for monitoring earthquakes and other passive sources.
The U.S. Army Corp of Engineers (USACE) estimates that in the US alone, flood losses have steadily increased to nearly $6 billion annually. Furthermore, levees protect at over 1/3 of US communities with at least 50,000 people and more than 14 million people live behind levees. Despite the importance of these structures, a large number of existing levees are at least fifty years old, with many of them over 75 or even 100 years old. Levees may become compromised by a number of physical and hydrologic processes, and over time the possibility of failure increases. Public welfare depends on maintaining safe levees, so having a reliable means for evaluating them becomes imperative.
Shear wave velocity is a non-destructive and easily obtainable method for estimating a soil’s shear strength and by extension, levee condition. Traditional evaluation methods are expensive and involve invasive borings with heavy trucks or equipment. Additionally, borings cannot provide continuous information, as samples are taken at discrete locations. The Atom Passive Wave Seismograph is an easy-to-use seismograph which can be used the evaluating the shear wave velocity (Vs) of a levee. The unit consists of multiple one-channel Atom Acquisition Units and 2 Hz geophones. Research results show that the Atom, a cableless seismic acquisition unit deployed in a linear array, can simply and quickly estimate the shear wave velocity of a levee to a depth of 50 meters.
OhmMapper Resistivity Meter
Investigating the integrity of flood containment dikes and levees is a high-priority engineering & geotechnical task in flood zones around the world. Geophysical resistivity surveys using the OhmMapper can provide valuable information about the structure of the dike-levee. Very often, dikes and levees were built with local material scooped up along the river bank or with waste materials from multiple sources. Levee damage can result from a number of factors but two very common culprits are unconsolidated material underlying the levee and weak areas in the levee building material itself. For example, if a portion of the levee is built on top of a sand and gravel paleochannel, that paleochannel can wash out during flood conditions - undercutting the levee. If a portion of the levee is built with clay soils and another portion with highly permeable sandy-loamy soils, the sandy-loamy soils have a high risk of collapsing or washing out in extreme flood conditions.
High clay content soils are conductive. Sandy, gravelly, or loamy soils are more resistive. Mapping changes in the body of the levee or along the foot of the levee will point to those more resistive areas (sands, gravels, loosely compacted soils) as potential risk areas that need to be closely monitored or strengthened to reduce the risk of levee collapse.
The OhmMapper Resistivity Meter can be towed along the crest or foot of the levee to provide a 2-D view of changes in resistivity with depth along the length of the levee. Since the OhmMapper does not use stakes in the ground to make its measurements, it can do detailed resistivity mapped up to 10 times faster as it is pulled along the length of the levee.
Voids may be classified as natural (i.e. karsting), or man-made (i.e. tunnels, mining excavations, or anthropogenic dissolution features) and may form a significant geological hazard. Seismic shear wave velocity surveys are one way to identify these voids. In general, ground density and shear wave velocity are expected to increase with depth. However, air or fluid-filled voids will attenuate and decrease the velocity of the shear wave compared to that of an otherwise homogenous strata. The reason behind this is that neither air nor water have a shear modulus, meaning that when a shear force is applied, fluids flow away from it.
In the case of a void, shear waves will do one of two things – either deflect around the anomaly or stop at the anomaly, resulting in an average velocity decrease, signal attenuation, or the absence of seismic ray coverage. In some cases, a high shear wave velocity halo may be present around a low-velocity zone. Often void detection surveys are performed along the road or in developed areas, thus making the Atom Passive Wave Seismograph an ideal choice. The Atom does not rely on a spread cable, so units may be placed in the area of interest, either linearly or in some other configuration with or without regular spacing. Eliminating the spread cable eliminates drawbacks one typically experiences with other seismic surveys, such as laying expensive cables across driveways and active roadways or across sidewalks, thus becoming tripping hazards in urban environments.
Downhole or crosshole seismic testing provides high precision interval velocities of seismic waves. These methods are frequently used for critical projects such as power plants or when the required test depth exceeds those obtainable through surface methods. For void detection, these vertical seismic methods can provide information such as the size and vertical position of a void. Seismic energy travels either around a void or is stopped by the void. Slowing or attenuation of the seismic signal may indicate the presence of a void. The Geode Exploration Seismograph was designed form the start to be able to collect downhole, uphole, and crosshole data easily. Both downhole and crosshole methods work best when used in conjunction with other methods, such as surface seismic methods or detailed geologic mapping.
We designed the DHA-7 Downhole Hydrophone Array, to be used along with the Geode, for collecting high-resolution seismic borehole testing. The DHA may be used to measure either primary or shear wave velocities in either downhole or crosshole configurations.
Seismic refraction is a technique used to characterize subsurface geologic structures such as stratigraphic boundaries or faults. Refraction occurs as seismic waves travel away from a source and cross a boundary between geologic layers of differing acoustic impedance. Because a fault will often offset materials of widely varying impedance values, seismic refraction is a useful tool for fault investigation. The method can provide information on faulting in two ways, either by identifying the lateral location of faults and/or by characterizing offset and tilted or folded strata by the fault.
Seismic refraction is a precise measurement and requires recording accurate travel times between the source (such as a hammer and aluminum plate) and the geophones. The Geode Exploration Seismograph, ideal for fault investigation, comes as 8-, 12- 16-, and 24-channels, with the capability of expanding up to 1000 channels for both 2D and 3D surveys.
MASW is another technique useful in fault investigation. Just like the P-waves used in refraction, seismic shear wave velocity is affected by the acoustic impedance of a material. For example, a soft weathered claystone will have a lower impedance and thus a slower shear velocity than an unweathered basalt. As is often the case in faulting, materials of varying acoustic impedance may be offset against each other.
Another source of varying impedance may be caused by breakdown or alteration of material near the fault surfaces. This zone is often crushed and weathered, or may alternatively be silicified and indurated, with creates a significant change in the shear modulus as compared to the material farther away from the fault surface.
MASW is often used to determine subsurface structure such as lateral fault location, offset, or dipping beds. Data may reveal a fault as a low-velocity zone sandwiched between two higher velocity zones. Additionally, on one side may be a higher or more steeply dipping bed than on the other. In cases where silicification occurs, the fault zone may appear harder than the surrounding material. The Atom Passive Wave Seismograph along with our SeisImager/SW Seismic Software, is a powerful and easy-to-use software package which can be used for fault investigation. Another benefit of the Atom system is the ability to perform both 2D and 3D surveys, depending on your Atom placement.