SEISMIC SURVEYING METHODS
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Different types of Seismic Method Services
Seismic methods are techniques based on the measurement and study of the propagation of seismic waves. Precisely, seismic methods measure the elastic properties of soils and rocks that are a function of the physical properties such as seismic velocity, density, and shear modulus.
Seismic methods consist of different data acquisition and processing techniques, depending on the project’s goal. A seismograph, a string of receivers (geophones), and a seismic source are the components of a seismic data acquisition system. Each seismic technique can yield new insights into the subsurface. The seismic survey can be performed on the ground, in boreholes, or marine environments.
Cordillera Geo-Services offers the following land seismic methods to solve various problems: refraction (tomography), reflection, MASW, ReMi, and borehole seismic methods (downhole seismic, crosshole seismic, seismic tomography.) Typical applications of these methods include:
- Rippability of bedrock
- Dam and levee site investigations
- Seismic microzonation studies
- Estimation of Vs30 for seismic site classification
- Mapping of clay, sand, and gravel deposits
- Mapping loose soil areas and soft clay areas
- Estimation of overburden thickness
- Delineation of bedrock
- Depth to the water table
- Mapping subsurface voids
- Mapping of low-velocity zones and velocity inversions
- Detection of potential collapse subsurface features
- Mapping of horizontal and dipping interfaces and geological contacts
- Detection of shallow fracture zones, faults, and karst topography
- P-and S-wave velocity mapping for dynamic modulus calculations
- Seismic site characterization for geotechnical and civil engineering projects
- Oil and gas and mineral exploration
Ground Seismic Methods:
- Seismic refraction and seismic refraction tomography: The seismic refraction method involves measuring the shortest time required for an induced seismic pulse to travel from the energy source location to a string of receivers (geophones). From this travel time data, seismic velocities and layer depths can be calculated. Seismic refraction remains the preferred method for accurately mapping the depth to competent bedrock under most conditions. Seismic refraction tomography utilizes seismic (P and S) waves refracted on geological layers to image velocity contrasts of the subsurface. It works similar to the classical seismic refraction methods; however, data acquisition uses more seismic shots.
- Seismic reflection: The method involves inducing a seismic wave into the Earth and recording the seismic waves reflected from sub-surface layers. Seismic reflection is ideal for mapping geology at depths exceeding 50 m. The acquired data can be used to evaluate the lateral continuity of geological layers associated with reflectors.
- Seismic Surface Waves Methods: Rayleigh and Love surface waves travel slower than P and S body waves but carry a considerable amount of energy and cause the most damage to surface structures during earthquakes. Typically, more than two-thirds of the total seismic energy generated by a compression source will be converted to Rayleigh waves, the main ‘ground roll’ component. Particles follow retrograde elliptical paths in vertical planes aligned along the propagation paths. Love waves are horizontally shear waves reflected between the surface and the base of a low-velocity layer. Rayleigh and Love waves decay exponentially in amplitude with depth. They are dispersive, i.e., different frequency components travel at different velocities. The MASW ( Multichannel Analysis of Surface Waves) and ReMi (Refraction Microtremor) are widely known seismic surface waves methods that perform effectively in urban areas with considerable cultural ambient noise.
- MASW: The cost-effective, non-invasive MASW method uses the dispersion properties of Rayleigh surface waves to produce 1D, 2D, or 3D depth profiles of shear wave velocities of soils and bedrock. Active and passive energy sources can be used in MASW surveys. The data are collected on the ground surface without the need for a borehole. Because the shear-wave velocity of earth materials is closely related to their stiffness, the MASW method is ideal for numerous geotechnical and civil engineering problems related to foundation design.
- ReMi: The cost-effective, non-invasive ReMi method also uses the dispersion properties of Rayleigh surface waves to develop average shear wave velocity profiles and 2D images using passive energy sources such as cultural ambient noise (e.g., jogging, vehicles, trains, airplanes, etc.) The vertical component of ambient noise, dominated by Rayleigh waves, is recorded using a seismograph with a geophone array similar to the seismic refraction method. ReMi can characterize a lower velocity horizon underlying a higher velocity horizon (velocity reversal), not possible with standard seismic refraction. ReMi profiles help characterize sites for both engineering investigations and site response evaluations.
Borehole Seismic Methods:
- Seismic borehole tomography provides high-resolution 2D or 3D images of seismic velocities (P and S) between boreholes. The method is used to delineate geological structures, map cavities, weak zones, and determine mechanical soil and rock properties within depths of geotechnical significance. The technique is also applied to characterize the subsurface conditions before infrastructure is built and image time-dependent processes. This method requires at least two nearby boreholes.
- The crosshole seismic method provides a high vertical resolution depth profile of shear wave velocities and compressional wave velocities between boreholes. The method determines dynamic soil parameters, such as shear modulus, Poisson ratio, and Young’s modulus, required by engineers to predict the response of soils to dynamic loading. This method requires two nearby boreholes.
- The downhole seismic test also provides shear wave velocities and compressional wave velocities for geological layers along a single borehole at a lower vertical resolution compared to the crosshole test. Soil dynamic parameters, such as shear modulus, Poisson ratio, and Young’s modulus, can be determined to evaluate the soil’s response to dynamic loading. The downhole test has.