Seismic Method
AGS uses varying seismic methods to map geologic and hydrogeologic features within the subsurface. These methods include refraction, reflection, multichannel analysis of surface waves (MASW), spectral analysis of surface waves (SASW), passive shear wave velocity (pVs also known as ReMi), and downhole seismic. The data can be acquired on land and in a marine environments and is used to determine:
- Overburden or Landfill Thickness
- Bedrock Configuration in Two or Three Dimensions
- Stratigraphic or Lithologic Mapping
- Dip and Orientation of Geologic Layers
- Fault, Fracture, Weathered, and Shear Zone Presence
- Continuity of Confining Layers
- Depth to Groundwater
- Elastic Properties of Sediments and Bedrock (P– and S-wave velocities)
- Pathways of Groundwater Flow
- Paleochannel Delineation
- Void and Sinkhole Detection
- Bedrock Rippability
- Landslide Evaluation
- Vibration Monitoring
The seismic method is based upon the transmission of seismic waves into the subsurface and the recording, measurement, analysis, and interpretation of the resulting waveforms. Detailed cross-sectional images are generated that provide information on the depth, thickness, and velocity of the target horizons (bedrock, water table, lithologic units), and their lateral variations along the line. Various filtering techniques and post-acquisition processing methods can be implemented to enhance the resolution characteristics and interpretability of the data sets. Seismic information can be obtained within a depth range of five feet to several thousand feet.
Advantages
Seismic methods provide a detailed, cross-sectional image of the subsurface. The data can indicate the presence of important geological features and their physical characteristics (orientation, seismic velocity, density). Three dimensional depth-to-target contour plots can be generated. The data collection is non-destructive and non-intrusive.
Limitations
Depending on the target, seismic methods may require more field and office time to complete than other geophysical methods. Cultural noise vibrations can adversely affect the seismic data quality.