237 St. Georges Ave. The seismic refraction survey is a very important geophysical technique used in the investigation of subsurface characteristics. The synthetic model contained a low velocity zone in a depression at the base of the regolith. Seismic Refraction Methods. The key piece of recorded information is the time of the first arrival. For simple situations the velocity can be determined from the change in arrival time as source–Geophone distance changes. The seismic reflection method usually gives better resolution (i.e., makes it possible to see smaller features) than other methods, with the exception of measurements made in close proximity, as with borehole logs (see below). The technique is widely used for rippability assessment of bedrock. Most of the current knowledge about the Earth’s internal constitution is derived from analysis of the time–distance curves from earthquakes. • Seismic exploration is divided into refraction and reflection surveys, depending on whether the predominant portion of … The seismic refraction method • First major geophysical method applied to subsurface investigation of relatively deep oil-bearing geologic structures • No longer the primary method in oil exploration, but has found use for near-surface, high-resolution subsurface investigation The seismic wavemay be generated by an explosion, a dropped weight, a mechanical vibrator, … Most seismic work utilizes reflection techniques. seismic reflection). S-wave velocity measurements are of special interest to engineers because building stability depends on the shear strength of the foundation rock or soil. The velocity values determined from time–distance plots depend also on the dip (slope) of interfaces, apparent velocities increasing when the Geophones are updip from the source and decreasing when downdip. Seismic reflection and refraction methods are major tools in natural hazard assessments. The seismic wave may be generated by an explosion, a dropped weight, a mechanical vibrator, a bubble of high-pressure air injected into water, or other sources. V7L 4T4, Geophysical Investigations of Pipeline Crossings. Changes in the amplitude and waveshape, however, contain information about stratigraphic changes and occasionally hydrocarbon accumulations. Seismic refraction methods Seismic methods are based on measurements of the time interval between initiation of a seismic (elastic) wave and its arrival at detectors. Sources and Geophones are essentially the same as those used in refraction methods. The first wave motion from an earthquake reveals the nature of earth motion involved in the earthquake. Refraction is a geophysical method frequently used for surveying depth to bedrock and investigating groundwater and/or a bedrock water supply queries. Twelve or more geophones are used in a traverse while the seismic pulse is provided by explosives, falling weights or hammer blows. Love waves are another type of surface wave; they involve shear motion. Seismic refraction investigates the subsurface by generating arrival time and offset distance information to determine the path and velocity of the elastic disturbance in the ground. This arrival is the direct wave, or more commonly, the refracted wave which occurs when seismic energy propagates along a geological interface having a sufficiently great velocity contrast. The seismic refraction method involves measuring the shortest time required for an induced seismic pulse to travel from the source location to a series of receivers. 1.2.1 This guide provides an overview of the seismic refraction method using compressional (P) waves. A pattern of shotpoints is then executed within and off the ends of the cable and the seismic wave arrivals for each geophone are recorded in the seismograph. The disturbance is created by shot, hammer, weight drop, or some other comparable method for … The seismic refraction method, due to its versatility, is one of the most commonly used geophysical methods in engineering, mining, groundwater exploration and environmental site investigations. The ease with which a rock can be ripped by a bulldozer relates to the rock’s seismic velocity. The seismic wave is detected by a Geophone on land or by a hydrophone in water. Seismic Refraction Tomography (SRT) The seismic refraction method uses P- and S-wave energy to map vertical and lateral subsurface changes. Seismic surveys for engineering projects on land are mostly undertaken by the refraction technique. The method exploits the behavior of seismic energy at interfaces with different seismic velocities. SEISMIC REFRACTION AND REFLECTION METHODS GEOVision geophysicists conduct high-resolution seismic refraction and seismic reflection surveys in support of a variety of engineering, environmental, and hydrogeologic investigations. Still other varieties of surface waves can be transmitted through low-velocity layers (channel waves) or along the surface of a borehole (tube waves). Another possibility of interpreting seismic refraction data is the refraction-tomography, which is pre-sented in chapter IV. Data are usually recorded on magnetic tape for subsequent processing and display. Earthquakes usually generate several wave modes. Field operations involve laying out a seismic cable with several geophone detectors (usually 12 or 24), at the takeout points on the cable. Such waves are called head waves, and the refraction method involves their interpretation. Introduction to Seismic Method: 2. Transient electromagnetic soundings, resistivity soundings, or multielectrode resistivity surveys provide a means of assessing additional layering information. North Vancouver, B.C. They are employed, for example, to detect faults that may disrupt a coal seam or fractures that may allow water penetration into a tunnel. In this case a shear wave source and shear wave geophones are employed. The instruments used include a seismograph and spread cable of geophones. The assumptions usually made are that (1) each layer is homogeneous and isotropic (i.e., has the same velocity in all directions); (2) the boundaries (interfaces) between layers are nearly planar; and (3) each successive layer has higher velocity than the one above. The analysis of the refraction data is assisted by the use of an integrated suite of programs. By measuring in both directions the dip and rock velocity, each can be determined. Seismic energy is provided by a source ('shot') located on the surface. Seismic measurementsDepending on the travel path of the seismic wave, we differentiate between the seismic refraction method and the seismic reflection method (see also Fig. The concept is similar to echo sounding: seismic waves are reflected at interfaces where rock properties change and the round-trip travel time, together with velocity information, gives the distance to the interface. Information provided by seismic refraction includes compression wave (p-wave) velocities within the investigated subsurface profile. Calculations of depth by the seismic refraction method must be highly qualified for a number of reasons, (Nettleton, 1940, p. 255). 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