Geophysics HM

The Refraction Seismic methodology is based on transmitting and recording seismic waves refracted on the boundary of stratigraphic layers and structures with sufficient density contrast to generate the refraction of the seismic waves along the interface. The type of seismic waves used for this technique are compressional and shear waves, which are detected by geophones and recorded by high precision ADC at the surface. The propagation of the seismic waves depends on the elastic properties of soils and rocks which makes the methodology an effective tool for lithological differentiation and estimation of the elastic and mechanical properties of the materials. The data is presented in time series, hodographs, seismic cross-sections with the identified refractor layers and velocity models.

This technique is widely used in geological, geotechnical  and hydrogeological investigations for bedrock topography profiling, water table depth estimation, rock integrity and fracture index evaluation. Refraction is a low cost methodology that provides detailed coverage when compared to conventional techniques such as drilling and laboratory testing. The primary applications are bedrock depth and stratigraphic layer profiling, oil elastic property estimation, rippability and rock quality assessment. The technique is successfully deployed in construction projects, quarries, landfill planning and development, groundwater management and infrastructure suitability and risk assessment. Seismic refraction surveys are routinely conducted during preliminary site investigations to provide a rapid assessment of seismic soil properties, depth to rock, configuration of the rock surface, and an indication of the relative integrity of foundation materials. Rocks and soils with low seismic velocity (usually < 2000 m/s) are easy to excavate with commercial available machinery.

For a proper design of the seismic data acquisition the geophysicist should have knowledge of the expected  bedrock depth, the thickness of the stratigraphic layers and the overburden extent over the survey area. This will dictate the length of the geophone spread to be used for mapping the layers of interest, the spacing between the geophones, the expected time of the arrival of the seismic wave at each geophone and the locations, magnitude and number of energy sources and offsets. The first step in processing/interpreting refraction seismic data is to pick the arrival times of the seismic signal. A plot is then made showing the arrival times against distance between the shot and geophone, called a time-distance graph. The time distance plot shows the waves arriving at the geophones directly from the shot and the refracted waves that arrive ahead of the direct waves. The refracted waves have traveled a sufficient distance along the higher speed refractor (bedrock) to overtake the direct wave arrivals. One of the most common methods of refraction interpretation is Generalized Reciprocal Method (GRM). Two-dimensional section results are derived from this technique, where the rock and soil velocities are estimated for each layer. 

Compressional and shear wave velocities are estimated from the first breaks of the refracted seismic waves using time lapse hodographs and interactive inversion programs. Seismic velocity is a function of density, porosity, mineral composition, and the degree of cementation and consolidation, fracturing and weathering. The physical properties of soils and rocks are estimated with seismic refraction for geological, geotechnical and construction suitability assessment. The elastic moduli (Poisson’s ratio, bulk’s modulus, rigidity modulus and Young’s modulus), competence scales (material index, concentration index and stress ratio, and density gradient) and soil bearing capacity are key parameters for geotechnical investigations. Elastic and mechanical parameters are used for estimating the deformation in soil and rocks supporting large infrastructure foundations such as bridges, tunnels and buildings.

The seismic refraction method may be useful to map the thickness of overburden, sand and gravel deposits if the base rocks have higher velocity than the sand and gravel. The seismic refraction method provides reasonably accurate depth estimates of the topography of the bedrock. The refraction seismic method is used to measure the depths and velocities of subsurface layers. It is particularly useful for mapping the depth and topography of the bedrock surface.  It can also be used to find the elastic properties of these layers, which are useful for engineering purposes.  Using the velocity of the bedrock, the rippability of the bedrock can be determined along with an estimate of the size of machine required. 

Advantages

When compared to other surface geophysical methodologies such as GPR, MAG and EM,  the refraction seismic is more expensive and time consuming; although this technique is irreplaceable in engineering geology and other geotechnical applications considering the high degree of resolution and accuracy obtained in the estimation of the physical properties of the rock and soils and the details of the stratigraphic sections, bedrock profile and earth velocity models. The main advantages of the system are:

• Good lateral and vertical resolution of stratigraphic layers
• Estimation of lateral and vertical variations of the elastic properties and density of the soils and rocks
• Detailed cross sections of stratigraphic layers, bedrock, water table and underground features such as voids and sinkholes
• Simple array and field deployment
• Small number of energy sources and geophones compared to reflection seismic
• Simple processing and interpretation