What Is Seismic Surveying?Seismic surveys are used to locate and estimate the size of underground oil and gas reserves. Seismic images are produced by generating, recording and analyzing sound waves that travel through the Earth. These sound waves are also called seismic waves. Explosives or vibrating plates generate the waves and a line or grid of geophones records them. Density changes between rock or soil layers reflect the waves back to the surface and the speed and strength that the waves are reflected back indicates what geological features lie below. The oil and gas exploration industry has deployed this evolving technology for decades to help determine the best places to explore for oil and gas.The seismic survey is one form of geophysical survey that aims at measuring the earths geophysical properties by means of physical principles such as magnetic, electric, gravitational, thermal, and elastic theories. It is based on the theory of elasticity and therefore tries to deduce elastic properties of materials by measuring their response to elastic disturbances called seismic (or elastic) waves.A seismic source-such as sledgehammer-is used to generate seismic waves, sensed by receivers deployed along a preset geometry (called receiver array), and then recorded by a digital device called seismograph. Based on a typical propagation mechanism used in a seismic survey, seismic waves are grouped primarily into direct, reflected, refracted, and surface waves.There are three major types of seismic surveys: refraction, reflection, and surface-wave, depending on the specific type of waves being utilized. Each type of seismic survey utilizes a specific type of wave (for example, reflected waves for reflection survey) and its specific arrival pattern on a multichannel record. Seismic waves for the survey can be generated in two ways: actively or passively. They can be generated actively by using an impact source like a sledgehammer or passively by natural (for example, tidal motion and thunder) and cultural (for example, traffic) activities. Most of the seismic surveys historically implemented have been the active type. Seismic waves propagating within the vertical plane holding both source and receivers are also called inline waves, whereas those coming off the plane are called offline waves.General Seismic PrincipleSeismic techniques generally involve measuring the travel time of certain types of seismic energy from surficialshots(i.e. an explosion or weight drop) through the subsurface to arrays of ground motion sensors or geophones. In the subsurface, seismic energy travels in waves that spread out as hemispherical wavefronts (i.e. the three dimensional version of the ring of ripples from a pebble dropped into a pond). The energy arriving at a geophone is described as having traveled a ray pathperpendicular to the wavefront (i.e. a line drawn from the spot where the pebble was dropped to a point on the ripple). In the subsurface, seismic energy isrefracted(i.e. bent) and/orreflectedat interfaces between materials with different seismic velocities (i.e. different densities). The refraction and reflection of seismic energy at density contrasts follows exactly the same laws that govern the refraction and reflection of light through prisms. Note that for each seismic ray that strikes a density contrast a portion of the energy is refracted into the underlying layer, and the remainder is reflected at the angle of incidence. The reflection and refraction of seismic energy at each subsurface density contrast, and the generation ofsurface waves(or ground roll), and the sound (i.e. the air coupled wave or air blast) at the ground surface all combine to produce a long and complicated sequence of ground motion at geophones near a shot point. The ground motion produced by a shot is typically recorded as a wiggle trace for each geophoneSeismic method instrumentationBoth refraction and reflection data are acquired using a seismograph. A seismograph records thearrival of reflected and refracted seismic waves with respect to time. These waves are detected atthe surface by small receivers (geophones), which transform mechanical energy into electricalvoltages. The voltages are relayed along cables to the seismograph, which records the voltageoutput versus time, much like an oscilloscope.There are a variety of seismographs used in the industry. Engineering seismographs are the mostcommon types of seismograph used in ground water pollution site investigations. Each seismographhas different capabilities to handle data that is dependent on the number of channels in theseismograph. Seismographs are available with one, six, twelve, twenty-four or forty-eight channels,or as many channels as desired (usually the number of channels is a multiple of six). Eachchannel records the response of a geophone or array of geophones. Other capabilities of a seismographmay include analog or digital recording, frequency filters, electronic data storage, and signalenhancement hardware.On multichannel systems, geophone stations are located at established distances along the seismiccable; on single channel systems, the geophone is moved to the next station after each shot.Geophones are coupled to the ground, usually by a small spike attached to the bottom of thegeophone. Care must be taken in the placement of geophones; each geophone gives the bestresponse when the axis of the geophone element is positioned vertically with the attached spikedriven firmly into the ground. Geophones are manufactured at different natural frequenciesdepending upon the desired result. High natural frequency geophones (usually greater than 30hertz) are used when collecting shallow reflection data and lower natural frequency geophones areused in refraction surveys. There are many types of seismic sources used to impart sound into the earth. The most commontype of source in seismic investigations is a sledgehammer and strike plate. Other sources include explosives, shot gun shells detonated in shallow augerholes, and various mechanical devices that shake the ground or drop large weights. The types of sources used are dependent on the signal versus noise ratio in the survey area. Noise can come from vehicular traffic, people or animals walking near the geophones, electrical current in the ground (electromagnetic interference which affects the geophone cables), low-flying aircraft, or any sound source. Generally, the noise can be overcome by using a larger source, which effectively increases the signal. Filtering on the seismograph can also reduce noise.Seismic Refraction Seismic refraction is defined as the travel path of sound wave through an upper medium and along an interface (at a critical angle) and then back to the surface as shown in the figure below. The acoustic waves, like light waves, follow Snells's Laws of Refraction.Seismic refraction surveys are commonly used to determine the thickness of unconsolidated materials overlying bedrock (overburden thickness) and depth to the water table. They are used for characterizing the geological framework of ground-water contamination studies and for assessing geologic hazards and archaeologic studies.MethodThe seismic refraction method is based on the measurement of the travel time of seismic waves refracted at the interfaces between subsurface layers of different velocity. Seismic energy is provided by a source (S) located on the surface. Energy radiates out from the shot point, either travelling directly through the upper layer (direct arrivals), or travelling down to and then laterally along higher velocity layers (L1) as refracted arrivals (R1, R2, etc.) before returning to the surface. This energy is detected on the surface using a linear array of geophones. Observation of the travel-times of the refracted signals provides information on the depth profile of the refractor.If external constraints are available, the velocitydepth profile can be transformed into a geological model. The conversion of observed travel times can be carried out using a number of techniques. In simple geological scenarios where fast turn-around of results is a required, a time-intercept approach can be used. For cases with suspected significant lateral heterogeneity the, tomographic inversion approach is recommended.
Seismic Refraction AdvantagesThe seismic velocity of a geologic horizon can be determined from a seismic refraction survey, and a relatively precise estimate of the depth to different acoustic interfaces (which may be related to a geologichorizon) can be calculated. Seismic refraction surveys can be useful to obtain depth information at locations between boreholes or wells. Subsurface information can be obtained between boreholes at a fraction of the cost of drilling. Refraction data can be used to determine the depth to the water table or bedrock. Refraction surveys are useful in buried valley areas to map the depth to bedrock or thickness of overburden.The velocity information obtained from a refraction survey can be related to various physicalproperties of the bedrock. However, rock types have certain ranges of velocities and thesevelocities are not always unique to a particular rock type. For instance, some dolomites andgranites have similar seismic velocities. However, seismic velocity data can allow a geophysicistto differentiate between certain units with divergent seismic velocities, such as shales and granites.Seismic Refraction LimitationsThe seismic refraction method is based on several assumptions. To successfully resolve the subsurface using the refraction method the conditions of the geologic environment must approximate these assumptions. These conditions include the following: 1) the seismic velocities of the geologic layers increase with depth; 2) the seismic velocity contrasts between layers is sufficient to resolve the interface; 3) the geometry of the geophones in relation to the refracting layers will per