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Page 1: What is Seismic Surveying

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 earth’s 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 Principle

Seismic techniques generally involve measuring the travel time of certain types of seismic energy from surficial shots (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 path perpendicular 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 is refracted (i.e. bent) and/or reflected at 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 of surface waves (or ground roll), and the sound (i.e. the air coupled wave or air blast) at the ground surface all combine to

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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 geophone

Seismic method instrumentation

Both 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.

Method

The 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

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(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 velocity–depth 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.

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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 permit the detection of thin geologic layers, and 4) the apparent dip of the units or layers is less than ten to fifteen degrees. If these conditions are not met, accurate depth information will not be obtained.There are several disadvantages to collecting and interpreting seismic refraction data. Datacollection can be labor intensive. Also, large line lengths are needed — as a general rule, thedistance from the shot, or seismic source, to the geophone stations (or geophone “spread”) mustbe at least three times the desired depth of exploration.

Seismic Refraction Survey Design, Procedure and Quality AssuranceSurvey design is site dependent and must be planned so that the geometry of the geophonespread will allow the target to be resolved. A primary limitation of the refraction method onmany sites is that long refraction traverses are sometimes required. The spacing of the geophonestations within the spread can vary from several feet to tens of feet, depending on the depth ofthe geologic layer and required resolution. A closer spacing of geophones within the spread ischosen when a higher resolution of a shallow target is the objective. Shotpoints should extendalong the entire traverse length and show a redundant sampling of the resolved interfaces. Caremust be taken to maintain quality control on distance measurements. Small differences inhorizontal displacements can cause a considerable change in the interpretation.The geophone stations should lie along as straight a line as possible (for profile data). Deviationsfrom a straight path will result in raypath projection inaccuracies. This will affect theaccuracy of the survey. Also, deviations in elevations will cause errors in the calculations.Shotpoint and geophone elevations must be surveyed using a level or transit if variations inelevation occur along the traverse. These elevations are used in the static elevation correctionsof the refraction data. Elevations to the nearest half-foot are adequate for most purposes.A diligent field procedure will result in optimum results and will eliminate problems whenprocessing and interpreting data. The geophysicist must be aware of any problems encounteredduring the survey, which may degrade the quality of the data. Modification of the originalsurvey plan may become necessary if problems are encountered in the field. The field geophysicistshould fill out an “observers log” listing pertinent information. Seismic Refraction Data Reduction and InterpretationStatic elevation corrections must be made when there are significant changes in topographicrelief along the traverse. Failure to make elevation corrections will simply transfer those differencesin elevation to the interpreted results or otherwise cause errors in the interpreted results.The geophone and shotpoint elevations obtained from the leveling or surveying are used tocompensate for travel-time differences caused by the changes in shotpoint and geophoneelevations. Corrections should also be made when the geophone stations deviate from a straightline.Seismic refraction data can be interpreted graphically or with the aid of a computer. There aremultitudes of interpretation schemes for seismic refraction data, depending upon the methodand desired results. Seismic Refraction Presentation of ResultsThe interpretation should be presented in profile form and in contour map form when a grid ofdata is collected. The contour map should include all information pertinent to the site, includinglocations of buildings, property lines, roads, and other cultural and physical features. Locationsof the traverses should also be indicated on the site map. Traverse sections or profiles shouldinclude details showing fixed positions, labeled interpretations, surface landmarks intersected

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by the traverse, areas of poor data quality, and a vertical time/depth scale.A listing of the seismic data, including the elevation data, time-picks (where applicable), and therespective layer velocities should be included in the report. A brief description of the surveyprocedure, instrumentation, and data reduction and interpretation procedures should also beincluded in the report. If the original survey plan has been altered, the reasons for the alterationshould also be explained in the text. The best report will contain not only the positive results ofthe investigation, but will also detail the limitations and negative results encountered during theinvestigation.

Seismic Reflection

The seismic reflection method records acoutic waves at the surface that are reflected off of subsurface stratigrphic interfaces where changes in the material density and conductive velocity of the acoustic waves are significant. The reflection patterns are described by Snell's Laws of Reflection.

Seismic reflection surveys are used for determining the thickness and structure of subsurface geology and are commonly applied in hydrocarbon and mineral exploration, earthquake and tectonic studies, and in the marine enviromant for resolving stratigraphic details (for example, the location and thickness of beach-sand deposits).

Method

Seismic reflection profiling involves the measurement of the two-way travel time of seismic waves transmitted from surface and reflected back to the surface at the interfaces between contrasting geological layers. Reflection of the transmitted energy will only occur when there is a contrast in the acoustic impedance (product of the seismic velocity and density) between these layers. The strength of the contrast in the acoustic impedance of the two layers determines the amplitude of the reflected signal. The reflected signal is detected on surface using an array of high frequency geophones (R1, R2 ,R3, etc.). As with seismic refraction, the seismic energy is provided by a 'shot' (S) on the surface. For shallow applications this may comprise a hammer and plate, weight drop vibroseis, mini-sosie or an explosive charge.

The recorded travel time–amplitude information is used to generate a reflection seismic profile. These data can be transformed into a velocity–structure profile. If external constraints are available, the velocity–structure profile can be transformed into a geological model.

Seismic Reflection AdvantagesThe seismic reflection method yields information that allows the interpreter to discern between fairly discrete layers. The reflection method has been used to map stratigraphy. Reflection data is usually presented in profile form, and depths to interfaces are represented as a function of time. Depth information can be obtained by converting time sections into depth from velocities obtained from seismic

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refraction data, sonic logs, or velocity logs. The reflection technique requires much less space than refraction surveys. The long offsets of the seismic source from the geophones, common in refraction surveys are not required in the reflection method. In some geologic environments reflection data can yieldacceptable depth estimates.Seismic Reflection LimitationsThe major disadvantage to using reflection data is that a precise depth determination cannot bemade. Velocities obtained from most reflection data are at least 10% and can be 20% of the truevelocities.

The interpretation of reflection data requires a qualitative approach. In addition to being morelabor intensive, the acquisition of reflection data is more complex than refraction data.The reflection method places higher requirements on the capabilities of the seismic equipment.Reflection data is commonly used in the petroleum exploration industry and requires a largeamount of data processing time and lengthy data collection procedures. Although mainframecomputers are often used in the reduction and analysis of large amounts of reflection data,recent advances have allowed for the use of personal computers on small reflection surveys forengineering purposes. In most cases, the data must be recorded digitally or converted to a digitalformat, to employ various numerical processing operations. The use of high resolution reflectionseismic methods places a large burden on the resources of the geophysicist, in terms ofcomputer capacity, data reduction and processing programs, resolution capabilities of theseismograph and geophones, and the ingenuity of the interpreter. These factors should becarefully considered before a reflection survey is recommended.

Seismic Reflection Survey Design, Procedure, And Quality AssuranceBecause the seismic reflection method is extremely dependent upon the geology and physicalconditions of the site, a thorough evaluation of the survey area, including a site visit and reviewof all available geologic data, is necessary.There are many different seismic energy sources, geophone and shotpoint array configurations,and survey plans that may be used in a particular investigation. However, there is no “best”survey plan. Due to the many variables in site conditions and reflection survey parameters, eachsite must be evaluated separately. Only a geophysicist with substantial experience in highresolutionreflection seismology is able to prepare such a site-specific survey plan. Experiencecan be substantiated by the presentation of case histories where reflection has been used successfully.Several generalities with respect to instrumentation and field procedure should be followed. Theseismograph should be able to record data digitally, and signal enhancement and filteringcapabilities are often necessary. The geophysicist should choose a seismic source that not onlyimparts a sufficient signal, but also generates a minimum airwave. The seismic sources used inreflection surveys are the same as those used in refraction work. A comparison of various highresolutionseismic reflection sources can be found in the literature (Miller and others, 1896).Shotpoint and geophone locations should be surveyed for elevation control. Elevations shouldbe surveyed to the nearest half-foot. As mentioned in the Seismic Refraction section, the geophonestations should lie along a straight line, with the geophones properly coupled to theground.The field geophysicist should be able to make changes to the initial survey plan if necessary.These changes should be discussed in detail with the State geophysicist prior to implementation.Seismic Reflection Data Reduction and InterpretationSeismic reflection must be corrected for static elevation and normal moveout. In some instances,dip moveout corrections can be applied. Dip moveout corrections are applied in areas where the dip of the reflecting layer is several degrees from horizontal. Seismic Reflection Presentation of ResultsThe final report should present the results of the investigation as outlined above in the Seismic

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Refraction Presentation of Results section.

Comparison between Seismic reflection and refraction

The differences between seismic refraction and reflection are summarized in the table below.

Seismic Method Comparison

Refraction Reflection

Typical TargetsNear-horizontal density contrasts at depths less than ~100 feet

Horizontal to dipping density contrasts, and laterally restricted targets such as cavities or tunnels at depths greater than ~50 feet

Required Site Conditions

Accessible dimensions greater than ~5x the depth of interest; unpaved greatly preferred

None

Vertical Resolution 10 to 20 percent of depth 5 to 10 percent of depth

Lateral Resolution ~1/2 the geophone spacing ~1/2 the geophone spacing

Effective Practical Survey Depth

1/5 to 1/4 the maximum shot-geophone separation

>50 feet

Relative Cost $N $3xN to $5xN

Note that in situations where both could be applied, seismic reflection generally has better resolution, but is considerably more expensive. In those situations, the choice between seismic reflection and refraction becomes an economic decision.  In other cases (e.g. very deep/small targets) only reflection can be expected to work.  In still other cases, where boreholes or wells are accessible, neither refraction, nor reflection may be recommended in favor of seismic tomography.

T-X Diagram

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The interpretation of underground structure from refraction results relies on ray-path analysis. The ray path is identified from a travel-time graph of arrival times vs. distance from source. This sometimes called a T-X diagram.

The technique is basically to inspect the T-X diagram and identify/guess the most likely underground structure from which it arises. Values are then picked off the T-X diagram and converted into structure parameters such as depth, etc using the assumed geometry of the ray path. Thus we need to know how T-X diagrams arise.

A refraction T-X diagram is based on the first arrival at each geophone. This is either picked off the geophone output (manually or in software) or is automatically recorded by a cut-off timer. The T-X diagram is thus a graph of first arrival times against distance from source.

Reflection time-distance plots

Consider a source (shot point) at point A with geophones spread out along the x-axis on either side of the shot point.

A raypath from A to C or A to E is: 2√h2+¿¿

The travel time, t, is the raypath divided by the velocity, V1, or:

t=√x2+4 h2

V 1

Rearranging: V 1

2t 2

4 h2 − x2

4 h2 =1

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This is the equation of a hyperbola symmetric about the t axis. The travel time plot for the direct wave arrivals and the reflected arrivals are shown in the following plot. The first layer is 100 m thick and its velocity is 500 m/s. The intercept of the reflected arrival on the t axis, t i, is the two-way zero

offset time and for this model is equal to 400ms. At large offsets the hyperbola asymptotes to the direct wave with slope 1/V1.

In most seismic reflection surveys the geophones are placed at offsets small compared to the depth of the reflector. Under this condition an approximate expression can be derived via:

t 2=4 h2

V 12 + x2

V 12

which can be rewritten as;

t=2hV 1

¿

or since 2hV 1

=ti t=t i¿

Since x

V 1t i is less than 1, the square root can be expanded with the binomial expansion. Keeping

only the first term in the expansion the following expression for the travel time is obtained:

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t=t i[1+ 12( xV 1 ti

)2] This is the basic travel time equation that is used as the starting point for the

interpretation of most reflection surveys.

Refraction time-distance plots

A typical ray path for an incident ray refracted at the critical angle is made up of the lines ABDE shown in the figure below. The incident ray at the critical angle, AB, yields a reflection BC and generates the head wave which propagates along the interface. The wave front of the head wave generates waves which return to the surface along rays which leave the interface at the critical angle, e.g path DE in the figure. The refraction arrivals consequently begin at the same time as the reflected wave on path ABC. Subsequent refraction arrivals are delayed by their travel time along the interface at the velocity of the lower medium.

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The equation for the travel time to an arbitrary point on the surface is the sum of the travel times along AB, BD, and DE. The first and third times are identical so:

t=t AB+ tBD+tDE

t=2 ABV 1

+ BDV 2

Using the geometry imposed by Snell’s Law this becomes:

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t= 2 hV 1cosθc

+x−2h tan θc

V 2

Since θc is determined via the velocities, sin θ=V 1

V 2, then the equation can be rewritten in terms of

velocity as: (note cos θc=√V 2

2−V 12

V 1 and tanθc=

V 1

√V 22−V 1

2 )

This is the equation of a straight line with slope 2V1 and an intercept on the t axis,

.

This is the mathematical intercept; there are no refracted arrivals at distances less than AC or at times less than the reflection travel time for the ABC path.

The velocities can be determined directly from the travel time plot as the inverse of the slopes of the direct and refracted arrivals so the depth can be determined from the intercept time via:

The distance AC at which the first refraction arrives, called the critical distance, x c , can be obtained from:

so

Finally it can be seen from the time-distance plot that there is a distance after which the refracted arrivals come before the direct arrivals. This occurs at the crossover distance, xcross, when the refraction and direct waves have equal travel times, i.e when

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This is another useful equation for determining h. In practice with real data it is usually found that projecting the refracted arrivals back to the t axis to find the intercept time is more accurate than estimating where the crossover distance is.

The refraction arrivals from shot points at each end of a survey line over a dipping interface are shown in the following figure:

The arrivals at geophones down dip from shot point A come at progressively later times than their horizontal interface counterparts so that the slope of the arrival curve is steeper. The apparent velocity obtained from the plot, Vapp down dip, is less than V2. The apparent up dip velocity obtained with geophones up dip from shot point B is greater than V2. The travel times from A to B and from B to A, the reciprocal times, must be the same. Refraction surveys must be shot in both directions. Arrival times taken in only one direction and interpreted as being taken over a horizontal interface may yield erroneous results if the interface is dipping.

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The equations for the travel times for a dipping interface, and for multiple layers with dipping or horizontal interfaces, are derived analytically

in Telford et al.(1990) and they present a useful collection of expressions for finding the depths and dips for up to three layer models.

A particularly useful result for small dips is that

where Vd and Vu are abbreviations for the down dip and up dip apparent velocities respectively.

General expressions have been derived for the travel times for any number of layers with accompanying equations for depths and true velocities but the quality of the field time-distance data makes it difficult to identify intercept times or cross over distances for more than a few refraction arrival segments. A better approach which leads into general methods of interpreting seismic data is to use a numerical technique to generate arrivals in model of an arbitrary medium and then by a process known as inversion adjust the parameters of the model to match the observed data.

In summary the principal advantage of the refraction method over the reflection method is that it depends only on measuring the first arrival times on a seismic time trace. There is no problem separating the refracted arrival from other arrivals as there is in picking reflection events. Problems or disadvantages are:

i) there is no evidence in the travel time plot for an intermediate layer(s) of lower velocity than the layers enclosing it. Interpretation in this case, which assumes a progressive increase in layer velocity with depth, will be in error.

ii) there are situations where, even with increasing velocity in successive layers, a refraction arrival segment may be masked by a deeper higher velocity earlier arriving segment.

iii) the surface distribution of geophones must extend to distances of several times the anticipated depth of the refractor in order to identify the crossover distance and to determine the slope of the refractor arrival plot.

iv) at the large off-sets required by iii) the arrivals may be very weak and impractically big shot energies may be required

References:

Html://parkseismic.com/Whatisseismicsurvey.

http://appliedgeophysics.lbl.gov/seismic/seismic_23.pdf

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http://www.enviroscan.com/html/seismic_refraction_versus_refl.html

http://www.state.nj.us/dep/njgs/geophys/seis.htm

FPSM 2005 Chapter 8 Geophysical Techniques


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