Seismic methods are based upon the transmission of seismic (acoustic) waves into the subsurface and recording the resulting waveforms at a distance after traveling through the earth. Seismic refraction is the most commonly used technique in engineering, geotechnical and environmental investigations and includes the evaluation of either compressional or surface wave arrivals.
The two main types of seismic surveys are Refraction and Reflection. The seismic refraction method is based on the measurement of the travel time of seismic waves refracted at the interfaces between subsurface layers. Observation of the travel-times of the direct and refracted signals provides information on the depth and dip of the refracting layer. The Generalized Reciprocal Method (GRM), or a Delay Time Method are the classical methods of processing seismic refraction data; Seismic tomography is an inverse modeling method used to process seismic refraction data that can provide more detailed velocity information and better resolution of steeply dipping beds; as a result it is the preferred method of processing when working in karst terrain or when higher resolution data is desired.
The multichannel analysis of surface waves (MASW) method is used to determine variations in surface wave velocities with increasing distances and wavelengths. MASW data can be collected as line or randomly oriented data around an area of concern. Data from these measurements are used to model shear wave (Vs) velocities in the subsurface. This information is then used to infer rock/soil types, stratigraphy and soil characteristics such as density and the dynamic shear modulus as a function of depth. One advantage of MASW techniques is the ability to identify low velocity layers beneath high velocity layers in the subsurface.
Seismic reflection profiling involves the measurement of the two-way travel time of seismic waves reflected back from subsurface layers. Processing of reflection data usually is more labor intensive than refraction data because of the need to improve the signal to noise ratio, but provides detailed information on the depths, thickness, heterogeneities and velocities of the target horizons.
Survey design starts with understanding project objectives, surface conditions and the underlying geology in the area of concern. Base decisions such as method type, survey geometry, geophone selection, and proposed processing sequence are conceptualized at that time. This conceptualized operational sequence is then brought into the field for testing.
Proper field techniques are critical to the collection of quality seismic records.
The chief field concerns are to provide an energy source capable providing signal to adequate depths and recording the data in a manner that maximizes the signal-to-noise ratio. Delta Geophysics has a variety of energy sources ranging from the typical sledgehammer and metal plate to elastic wave generators of varying sizes. You can be assured that your project will be energized!
Signal-to-noise ratio and signal energy are addressed by Delta personnel as the first step in field procedures. These procedures include a number of tests to optimize the field parameters such as line length, geophone and shot-point spacing, energy source, and filter settings. Once these parameters are selected they are continuously monitored during data acquisition and modified as warranted to meet changing Site conditions. Delta’s extensive experience in Seismic acquisition and their attention to detail assures that you will get the best data possible from your project Site.
Seismic methods can provide continuous subsurface information that would not be otherwise obtainable, help target more expensive drilling investigations, and can aid in value engineering projects that involve mass excavation, stormwater planning in karst terrain, sinkhole repair, and foundation design.