Seismic Surface Waves
- Accurate
placing/surveying of sensors/soundings,
- Appropriate frequency/wavelength content is recorded
- Modeling
or Calculating the velocity distributions at depth.
Seismic Surface Wave Techniques
Surface wave surveys are a geophysical technique for modeling the vertical shear-wave (Vs) velocity profile
of the subsurface. Traditional techniques for determining subsurface Vs profiles include down-hole and cross-hole seismic techniques.
However, modern seismic surface wave techniques do not require laboratory testing or a borehole, offering a cost-effective alternative for estimating Vs.
Seismic surface wave techniques can be divided into two broad categories: active and passive. Active techniques include the Spectral Analysis of Surface Waves (SASW) and Multi-channel Analysis of Surface Waves (MASW) techniques. Passive techniques include array microtremor and refraction microtremor (ReMi) techniques. Zonge Geosciences has extensive experience applying both MASW and microtremor techniques where subsurface material properties are needed. The critical ingredients for successful surface wave studies include:
Seismic surface wave techniques actually measure Rayleigh-wave phase velocity, rather than shear-wave velocity. At some sites with complicated geology or circumstances, clarifying this distinction can be critical. However, a primary component contributing to phase velocity is shear-wave velocity. Rayleigh waves are dispersive, which means that different wavelengths of the surface wave will have different phase velocities. Sampling different wavelengths allows the geophysicist to create a Rayleigh-wave dispersion curve. The frequency and phase velocity information contained in the dispersion curve may be used to model a corresponding shear-wave velocity profile. Rayleigh waves are dispersive, which means that different wavelengths of the surface wave will have different phase velocities. The phase velocity, VR, depends primarily on the material properties (VS, mass density, and Poisson’s ratio) over a depth of approximately one wavelength. Waves of different wavelengths, l, sample different depths. Longer wavelengths (lower frequencies) are more sensitive to the elastic properties of the deeper subsurface. Shorter wavelengths (higher frequencies) are more sensitive to shallower layers. A plot of phase velocity as a function of frequency is called a dispersion curve. This dispersion curve is modeled to generate a corresponding shear-wave velocity profile.
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When both vertical and lateral variation of Vs is needed, a 2D profile may be constructed by generating a series of 1D dispersion curves, modeling them individually and combining the inverse models into a 2D plot of Vs versus depth. The use of a seismic “landstreamer” allows the geophone array to be towed along the survey profile at a fixed spacing, greatly reducing the set-up time required to obtain each 1D sounding. The vertical resolution of such a profile is identical to the 1D method; horizontal resolution is a function of the reciever array length and the spacing between 1D soundings. 2D surface wave surveys are most often performed using a Land Streamer as shown in the figure below:
A photograph of a typical Landstreamer can be seen here in FIGURE 4.
Here, The receivers are 4.5Hz vertical-component geophones that are fixed onto a nylon strap, utilizing base-plates instead of spikes for mechanical coupling to the ground. The geophone spacing can vary depending on survey objectives, but is normally 1.5 meters (5ft). This system allows for rapid acquisition of data to produce a series of 1D surface wave soundings. This is acheived by dragging the receiver array forward to each sounding location along a profile rather than manually re-planting each geophone for every sounding. 2D MASW data acquisition is further automated by utilizing a vehicle-mounted impact source (i.e., a weight drop system) and towing the landstreamer behind the same field vehicle.
Additional considerations are important in a well designed surface wave survey. These include avoiding large topographic features that affect the propigation of surface waves and the end modeling results, identifying the desired target depth (helps determine best survey parameters such as geophone spacing), lateral extent of an expected target/anomaly (helps with survey line layout/design), and the importance of geologic input (allows for velocity-calibrated interpretations of results).
Accurate placing/surveying
of sensors/soundings
The sounding locations (center of each spread) are routinely surveyed (leveled) relative to an
absolute known elevation point or to a relative elevation point. This allows for accurate comparrison/plotting of 1D and 2D models.
The horizontal placement of geophones/soundings is also controlled to precisions of one foot. Controlling geophone locations
in plan and level enables a precise geologic model and interpretation.
Appropriate frequency/wavelength content is recorded
Recording instrumentation used by Zonge Geosciences, Inc.
allows for a broad band of frequencies to be recorded, ultimately allowing for a wide range of survey designs and
target depths of investigation and/or vertical resolution depending on the application or desired results.
Modeling or calculating the
velocity distributions at depth
An iterative modeling process is used to generate S-wave velocity models for each sounding.
During this process an initial velocity model is generated based on general characteristics of
the dispersion curve. The theoretical dispersion curve is then generated using the 1-D
modeling algorithm (fundamental mode Rayleigh wave dispersion module) and compared to the
field dispersion curve. Adjustments are then made to the thickness and velocities of each layer
and the process repeated until an acceptable fit to the field data is obtained. For 2D surveys, a series of 1D
models are first created, and then these models are used in conjunction with survey data to create XYZ files that can be gridded and plotted as 2D cross-sections.
An example of a typical 2D Vs model is shown below:
The theoretical model used to interpret the dispersion data is based on an assumption that the survey medium is laterally homogeneous beneath the survey array, and consists of horizontally layered homogeneous materials. While these experimental assumptions are not expected to be true at any given field site, the results of surface wave testing provide a good “global” estimate of the material properties along the array. The results may be more representative of the site than a borehole “point” estimate (Martin, et al., 2005).
Typically, we utilize passive seismic data (when possible) to extend the depth model of the Vs model when MASW alone is not capable of reaching target depths of 50 ft. Passive seismic techniques are more sensitive to longer wavelengths than MASW and therefore may be more accurate estimators of Vs at depth. However, passive data is typically less-sensitive to the shorter wavelengths needed to model the shallow depth. Therefore, when MASW and passive seismic data overlap, typically the MASW data is preferred for interpretation of the shallow Vs model, and passive data for the deeper Vs model.
In some instances shot/receiver coupling is not optimum, or sufficient cultural noise is present to make phase velocities and/or dispersion picks inaccurate. As a result, some of the individual records (or entire soundings) may not be used in the analysis. This is partially overcome in active surface wave studies by recording several records for each sounding. In passive studies, only receiver coupling is a concern, and random noise is actually the desired signal. As a result, passive techniques such as ReMi offer a good approach to processing data acquired at noisy sites such as urban environments.
Additional Considerations
Because the two factors of velocity and structure are intrinsically
related in surface wave theory, their independent determination from surface wave
surveying alone is impossible. The ambiguity is that layer depth/thickness can be
traded for velocity differences over a broad range of velocity-structure
pairs.
Seismic Surface Wave Surveys are Cost
Effective
A seismic surface wave survey is a cost-effective approach for finding the depth
to bedrock. Typical applications include unconsolidated alluvium over
competent bedrock. Variation of +/- 10% of depth with lateral extent
of 50% of the depth are routinely resolved. The water table/fuid saturation doesn't impact shear wave propigation
and therefore has no effect on the modeled velocity distributions. This offers a good alternative to standard refraction
where the water table ocasionally obscures/distorts the modelled depth to bedrock.
Difficult refraction problems can often be resolved
by the use of surface wave soundings.
The water table is transparent to surface waves and even low-velocity layers can be
detected/resolved (and is often the project objective) using surface wave techniques. Furthermore, since
surface waves account for approximately 90% of energy in a seismic record, the signal-to-noise ratio is often far
superior to that of p-waves which are utilized in standard refraction surveys. This means that surface wave surveys
can offer more reliable results in noisy environments.
Zonge Geosciences, Inc. (Zonge) has performed surface wave studies with various geophone spacings. We use modern digital seismographs for recoding data and state-of-the-industry software for analysis.
Please furnish us with the rock types, depths (including a range), and any strength or velocity information (blow counts, etc.) that you may have. A well planned survey can be a cost effective means of geologic investigation.
Zonge Geosciences, Inc. will be happy to respond
to your questions!! Contact Us