Exploration on water covered areas began in the late of 1920 when refraction crew in Louisiana used boats to extend coverage into shallow coastal waters. The first truly marine crews began operating in the Gulf coast in 1946 with 6 channel cable 4 hydrophone/ channel and dynamite source. The field procedure employed during the 1960 utilizing a recording boat which towed the cable to the location. Earlier the acquisition of single fold data were introduced in the oil exploration later on CDP data with multiple foldage came into industry.The advantage of CDP data acquisition.Recording instrument: In seismic field recording we would like to store the maximum information possible. To achieve this the complete recording system from detector through the digital tape unit should have a high dynamic range and a broad flat frequency spectrum. The length of a seismic signal is usually thought of as the record length, but in communication theory the multiplicity (CDP fold ) would also represent an increase in signal length. Dynamite as an energy source produces an input signal having considerable amplitude but very short length. On the other hand a vibrator energy source can produce signals having only limited amplitude bit this is offset by the long length of the input sweep signal. Other surface energy sources such as the thumper also produce a low level signal, but make up for it through multiplicity of recording for vertical stacking.Seismic data acquisition: Seismic method is widely used to map subsurface formation in oil exploration. In oil exploration earlier single fold recording with limited channel were introduced later on multifold CDP method came. The advantages of CDP method are-1. It improve the signal to noise ratio by factor M ,where M is the foldage.2. It attenuate the random noise .3. Attenuation of multiples, reverberations, diffractions and coherent noise is better.4. CDP data provides velocity information at close intervals.

Before starting of seismic survey in the area the following information about the area should be collected and study.1. Geological / interpretation objective of the survey should be understand clearly.2. Topography and logistical constraints should be known by reconnaissance of area.3. Study of earlier survey ( if conducted).4. Study of well data / VSP data should be done (if available).

After study of all the relevant documents / information about the area experimental survey for fixing of parameters should be conducted.PARAMETERS.1. Spread Geometry

(a) Type of spread(b) Near offset.(c) Far offset.(d) Group interval(e) Shot interval.(f) Line spacing(g) Direction of shooting.

Type of spread Type of spread must be decided on the basis of objective of the survey, target of

the depth and type of noises in the area.There are two types of spreads mostly adopted in seismic survey.

1. End on spread.2. Split spread.

(a) Symmetric split spread.(b) Asymmetric split spread.

End on spread method: In this method shot is kept at one end of the receiver spread. Advantages:1. This method is suitable for multiple suppression.2. This method provide better velocity information because of its long offset.3. It is suitable for deep target.4. If dip of the reflector is very high then this method is preferable.

For high dip up dip shooting should be adopted. R1 R2 R3 R4 R5 R96 S1 S2 S3 --

CH. 1 2 3 4 5 96 1st sp 2nd sp 3rd sp -

spread length near offset

far offset

Split spread In this method shot is kept in between the spread .Symmetric spread: In this method the spread is kept at both side of the shot in equal number of channels and offset.Advantage-1. This method is suitable in those areas where multiples are not present.2. For shallow target this method is suitable.3. This method is suitable for horst and graben.4. If dip of the reflector is very less then also this method is preferable.

R1 R2 R3 R48 R49 R96CH 1 2 3 48 49 96

spread Near offset spread

far offset 96 channel symmetric split spread.

Asymmetric split spread In this method number of channels or offset is kept at both side of spread in unequal manner say 1:2 or 2:3 ratio.Advantage

1. If the target is shallow and deep both then this method is suitable.2. If dip of the reflector is more then this method is preferable.

R1 R2 R3 R4 R5 R6 R72 R73 R96CH1 2 3 72 73 96

Spread spread Near offset

Far offset far offset 96 channel asymmetric split spread.

Near trace offset Neat trace offset should be optimum for effect of shot noise. At least single fold coverage should be retained for shallowest target depth. Near trace offset should be decided by the following methods.1. Near offset can be decided from differential move out plot also. in differential move out plot deepest reflector difference of multiple move out time and primary move out time must be 4 to 6 ms. Tm - Tp = 4 to 6 ms3. Near trace offset can be estimated from the noise records. The details is given in the

noise test .

Far trace offset Far trace offset should be large enough for good velocity analysis and better multiple suppression. Far offset should be decided on basis of following methods.1. For good velocity analysis

X = v 2T0/f

Where v = RMS velocity of the deepest reflector. T0 = corresponding two way time. F = corresponding dominant frequency.2. Far offset can be calculated by using the following formula also.

2 2 Xmax. = Tmax. 2T0 Vp Vm 2 2 Vp – Vm

Where Tmax = longest period of multiple reflection to be suppressed at T0 (TWT) Vp = RMS velocity of primary event at the target depth. Vm = RMS velocity of the corresponding multiple.

3. Far trace offset calculation from differential move out plot.

For better multiple suppression Tm - Tp = T0

T0 is the TWT of reflector.

Permissible valve of suppression Tm - Tp = (0.9 to 1.5)T0

2 2 Tm = X / 2T0 Vm,

2 2 Tp = X / 2T0 Vp

Group interval The group interval should be considered by following methods.1. From number of available active channels. Group interval can be decided as per availability of number of channels and required spread length.

Xfar – Xnear X = ------------------- N - 1

Where Xfar = Far trace offset. Xnear = Near trace offset. N = Number of active channels.

3. From spatial aliasing critera.Taking into consideration dip , velocity and frequency of the target horizon group interval should give adequate sampling. The sampling theorem states that the maximum sampling interval should provide two or more data points per cycle for the highest frequency present. In CDP shooting the subsurface in sampled at half the group interval. Keeping these points in view the group interval X for adequate spatial sampling of the highest frequency present in the signal is given by

V X = ------------------------ 4fmax Sin 0

where V = average velocity fmax = maximum frequency of interest.

0 = the maximum true dip at the target horizon. Dip can be taken from a time contour map, dip meter log or an available migrated section in dip direction.

4. From fresnel zone criteria.Reflected energy represents sampling not from a single point but from an area on the reflecting surface. This area is called first fresnel zone. Its radius R is given by

D R = ------- 2

where D is the depth of the reflecting energy. In terms of velocity, zero offset TWT and frequency, the above formula may be modified to.

T0 R = 0.5V ------- f

group interval should be so as to include at least four CDP points per fresnel zone which will make the reflector well sampled.

Four CDP points or three CDP intervals = 2R

One CDP interval = 2R/3 Thus group interval = 2* CDP interval = 2*2R/3 = 4R/3the fresnel zone radius should be calculated for the shallowest reflector of interest.

Shot interval Shot interval should be chosen so as to cover the required CDP foldage. The optimization of CDP foldage is desired for obtaining the best data quality. It is observed that beyond a particular foldage, there in marginal improvement in the data quality. At those places boosting of signal results boosting of noise also. the maximum foldage can be analyzed from the old stacked sections with different foldage.

The formula of CDP fold is

NG M = -------- 2S

where M is the foldage N = number of channels G = group interval S = shot interval

Line spacing Line spacing should be decided as per the smallest anomaly of interest. Grid of lines should be equal to or less than the diameter of the spaced at closer interval and strike lines can be spaced at larger interval.Direction of shooting If the dip in the area is more then up dip shooting is preferable. The wave traveled updip suffers less scattering and arrives at all the receivers. If multiple problem is very high in the area then down dip shooting is preferable but it requires long spread.

INSTRUMENT PARAMETERS1. Record length.2. Sample interval.3. Lowcut filter.4. Highcut filter.5. Notch filter.Record length Two way reflection time is a function of the average velocity and the depth of

the object. While deciding record length time for migration window must be taken into consideration.Sampling interval Smaller sampling interval is used for shallow investigation and high resolution data. Longer sampling intervals are used for deep events. Generally 2 ms and 4 ms sampling intervals are being used in oil exploration.Low cut filter Geophones provide a filter effect with a low cut frequency in the vicinity of natural frequency of the geophone. Very often noise is much stronger than the signal at these low frequencies and a low cut filter is used to supplement the filter effect of the geophones. And the dynamic range of the instrument. If the target depth is deeper then cut off frequency may be kept at a lower value. Normally in seismic survey 8 Hz to 12 Hz with 18 db/octave to 36 db/octave are used.High cut filter High cut filters are used to attenuate frequencies above the Nyquist frequency which will alias as a result of the sampling process.Notch filter A notch filter is used to attenuate 50 Hz power line pickup in the seismic band.EXPERIMENTAL WORK Before starting regular survey work the following experiments are to be conducted.1. Up hole survey2. Shallow refraction survey3. Noise testUphole survey Up hole surveys are conducted in the area to know the thickness of weathering layer, velocity of weathering layer and velocity of sub weathering layer. The thickness of weathering layer helps us to decide the depth of shot hole and calculation of static

corrections. The velocities of weathering layer and sub weathering layer are used to calculate the static corrections. The optimum depth of shot hole should be selected just below of weathering layer to avoid the low velocity weathering layer and to avoid multiple noise. Up hole shooting should be conducted in all area at regular interval preferably at the crossing of lines so that to have the better control.

Procedure: A hole of approximately 2 to 3 times of the expected thickness of weathering layer is drilled. Single geophone may be planted at distances of 3 m, 5 m and 10 m from hole. Shots are taken of equal amount of charge. Signal is recorded with first break for all the offset.Analysis: - Calculate time T0 with zero offset for different depth.- Make time versus depth plot for all the offsets using first breaks.- Calculate velocities of weathering layer sub weathering layer and thickness of

weathering layer.- When area is covered with up hole survey the contour map of thickness of weathering

layer, velocities of weathering layer and sub weathering layer can be prepared. Which help us for deciding depth of shot hole for regular shooting and calculation of static corrections.

Calculation : -Calculate T0 for shots at different depth. 2 2 T0 = Tx . D / X + D -Plot depth versus time curve. -Calculate thickness of weathering layer velocities of weathering layer and sub weathering layer. - The up hole should be covered in all the area. - Prepared the contour maps for thickness of weathering layer velocity - of weathering layer and velocity of sub weathering layer.

Use of up hole data.- The thickness of weathering layer is used to decide the depth of shot holes during

regular production work.- All three , weathering layer thickness, weathering layer velocity and sub weathering

layer velocity are used for calculation of static corrections also.

SHALLOW REFRACTION SURVEY Shallow refraction survey is also conducted to know the thickness of weathering layer, velocities of weathering and sub weathering layer. The area where drilling of holes is not possible like in desert area and hills, at those places shallow refraction surveys are conducted.

Field parametersNo. of channels = 24/48Channel spacing = taperedNo. of geophones = bunching of geophone stringsShot depth = 0.5 / 1.0 mCharge size = approximately 125 gm / 250 gm or as per requirementNo. of shots per spread = 2 (one at each end of spread)Spread and near offset = to be tested to achieve objective.Procedure: - Spread is laid and shots are taken at each end. The record is taken with first break time. Then X – T plot is made for both end.- compute the velocities of different layers from the slops obtained from the plot.- Compute weathering layer thickness using formulas given below.

S1 R1 R2 R3 R4 R5 R24 S2

Spread of shallow refraction.

- Layout the spread with tapered group interval.- Take shots at both side of spread.

T Vsw=dX/dT

Tw Vw=dX/dT Or Dw/Tw

Dw X

X= Offset T= First break time at different offset. Dw= Thikness of weathering layer. T = Time of signal in weathering layer.

Vw= Velocity of weathering layer. Vsw= Velocity of sub weathering layer.- Plot offset time curve for both shot at two sides of spread.- Calculate the Dw,Vw & Vsw for shots at both end and average all three.- Prepared contour maps for full area for all three .

Uses of data.- The thickness of weathering layer is used to decide the depth of shot holes during

regular production work.- All three , weathering layer thickness, weathering layer velocity and sub weathering

layer velocity are used for calculation of static corrections also.

Calculation of static correction. The static corrections are calculated for each shot points and receiver points. These corrections are sent to processing center along with data and it is used in processing. In seismic data acquisition our objective is to map subsurface accurately. While acquiring seismic data we record seismic signal on surface along the seismic line. To map subsurface structures from reflection time accurately it is essential to correct reflection times for predictable irregularities which do not associate with the actual structure of subsurface formation. The source of such irregularities are variation of elevation and weathered layer just below the earth surface. The static correction is sum of elevation correction and weathering correction.Elevation correction: Variation of elevation along the seismic line caused a irregularities in reflection time. Practically we are recording the reflection times by planting the geophones on the surface of the earth. For achieving our objective that is for mapping subsurface recording should have been done at the same level or at the same elevation, which is practically not possible. Reflection times can be corrected for the effect of these irregularities by subtraction or addition of time correction.Weathering correction: A layer just below the surface of the earth is weathering layer. This layer is generally consists of unconsolidated material and having low velocities. The variation of density and velocity in this layer is more. Therefore, it is essential to correct the reflection time which are effected by the irregularities of weathering layer. This effect of low velocity can be corrected by replacing the low velocity weathering layer by sub-weathering layer. For applying static correction we choose one datum line and apply the time correction so as to bring all shot points and receiver points at datum line and replace low velocity weathering layer by sub-weathering layer.Static correction for explosive source: In case of explosive source static corrections of shot points & receiver points should be calculated separately, since the elevation of the shot point & receiver point are different because of depth of hole. The total static correction is the sum of static correction of shot point and static correction of receiver point.

Tc = tsp + trp

Static correction of shot point: As stated earlier also the total static correction is the sum of elevation correction and weathering correction. In case of explosive source the weathering layer is already avoided at shot point, since depth of the hole are generally taken below weathering layer. However, practically it is not possible to drill the every shot hole upto or below the base of weathering layer at each point. Hence at these points where shot depth is above the base of weathering layer, weathering corrections are to be applied by replacing the low velocity layer by sub-weathering layer. In such cases first shot point is to be shifted from shot depth to base of weathering layer by subtracting the time taken by seismic signal from shot depth to base of weathering layer, and then shot point is to be shifted from base of weathering layer to datum by subtracting or adding time required by seismic signal from base of weathering layer to datum by sub weathering velocity. Dw D Hence total static correction will be Tcs = - + Vw Vsw

Where Dw is the distance between the shot depth to base of weathering layer. D is the distance between base of weathering layer and datum. Vw is the weathering velocity. Vsw is the sub-weathering velocity.

If shot depth is below the base of weathering layer or at the base of weathering layer then static correction for shot point can be applied by shifting the shot depth to datum by sub-weathering velocity.

Dsd Hence total static correction Tcs = + Vsw

Where Dsd is the distance between shot depth to datum. Vsw is the sub-weathering velocity.

Static correction of receiver points: For calculating the static correction for receiver points weathering layer is essential to take into account, since receivers (geophones) are planted on surface of the earth. By subtracting the travel time by the signal within weathering layer receiver points are to be shifted first from surface of the earth to the base of the weathering layer and then by adding or subtracting the travel time required by the signal from base of the weathering layer to datum line by replacing the weathering layer with sub-weathering layer, receiver points can be shifted from base of the weathering layer to datum. If datum is below the

base of weathering layer then there is no need to replace of weathering layer by sub-weathering layer. Hence static correction of receiver point is.

Tcr = - Tw + Twb+sd Dw D = - + Vw Vsw

ds W

Es ----------------------------------------------------------------------------------- D ds + Ed - Es W + Ed - Es

Ed

MSLa. Shot correction. For shot correction shot depth is to be shifted only from shot depth to datum by sub-weathering velocity. Distance between shot depth and datum = Ed + ds – Es, time taken by seismic signal from depth of shot hole to datum = Ed + ds – Es /Vsw. Sign for this time correction will be plus, since it is to be added for shifting shot point from shot depth to datum. Ed + ds - EsHence total shot point static correction Tcs = Vsw b. Receiver correction.

Receiver static correction will be same as in case 4 & 5.

W Ed + W – Es Hence Tcr = - + Vw Vsw

c. Uphole time. W ds – W

In this case uphole time will be Tup = + Vw Vsw

d. Relationship between Tcs, Tcr & Tup. W Ed + W – Es Tcr = - + Vw Vsw

W Ed W Es = - + + - Vw Vsw Vsw Vsw

Add ds /Vsw on both side. Ds W Ed W Es ds Tcr + = - + + - + Vsw Vw Vsw Vsw Vsw Vsw

Ed + W – Es W ds – W Tcr = - + Vsw Vw Vsw

Tcr = Tcs - Tup

STATIC CORRECTION FOR VIBRATOR SOURCE

When vibrator is the source of energy in seismic survey, the source and receiver both are on the surface. Static correction of shot and receiver are calculated by same method. First source is shifted from surface to bottom of weathering layer by weathering velocity and then from bottom of weathering layer to datum by sub-weathering velocity. Procedure for receiver correction is also same. In vibroseis we can not record up hole time like in explosive source, hence up hole shooting and weathering shooting is the only tool for the calculation of static correction. Up hole shooting or weathering shooting should be conducted at close interval in vibroseis. The static correction between up hole/weathering shooting points are generally interpolated with respect to elevation of the shot/receiver station. If variation of elevation and thickness of weathering layer is more then up hole shooting or weathering shooting should be taken at close interval to maintain the accuracy.

Figure 13.

------------------------------------------------------------------------------D

W W + Ed - Es Es Ed

MSL

Static correction.

If elevation of shot point is Es, elevation of datum is Ed, and thickness of weathering layer is W, then distance between shot point and bottom of weathering layer is W. Hence time taken by seismic signal from shot point to bottom of weathering layer = W/Vw. Where Vw is the weathering velocity.

Time taken by seismic signal from bottom of weathering layer to datum = W+Ed-Es/Vsw. W W+Ed-EsHence the total static correction Tcs = - + Vw Vsw Where Vsw is the sub-weathering velocity.

Static correction for receiver will also be same. W W+Ed-Er Tcr = - + Vw Vsw In some area multi-weathering layers are detected by up hole shooting or weathering shooting. In such area static corrections are calculated as below.

NOISE TEST Noise test are conducted in the area to know the characteristics of noise, to optimize the near offset and to decide the geophone pattern. The basic idea is to design geophone array so that waves traveling vertically or nearly vertically are reinforced while those traveling horizontally(ground roll) are reduced.

Low frequency waves with an amplitude level that is higher compared reflections. These events were referred to as ground roll. Because of low frequency low cut filters were introduced in to the amplifier circuits to eliminate interference from this source.

The groups of series connected geophones laid out over distances of one or more wavelengths ( or in such position by which ground roll will be added in opposite phase ) result the suppression of the horizontally travelling ground roll and enhancement of vertically travelling reflections. There are two ways of shooting a noise profile.1. Walk away spread method2. Walk away shot point method. In walk away spread method shot are taken at one place and spread moves successively further away from the shot point. This method has advantage that it has some similarity in shot but it is time consuming. In walk away shot point the shots are progressively moved keeping the spread stationary. This method is generally preferred due to less time taken to shift shot point. Noise tests should be conducted more then one location the area depending upon the type of area. For conducting noise test surface coverage should be of the order of the far offset.Field parametersGeophones/ channels = 6/12 bunchedChannel spacing = 5 mSpread length = approx. 475 mCharge size = approx. 5 kgCharge depth = optimum depth decided by up hole survey.Number of holes = to cover far offset.Recording parametersNumber of channels = approx. 96Low cut filter = outHigh cut filter = to be used according to sampling intervalNotch filter = outGain = IFPSampling interval = 2 msRecord length = 5 /6 sec.

Analysis:- Generate a section for the entire length of profile by placing the field monitors one

after other. - Mark the trend of all the noise events seen in the generated section- Mark the offset at which the shallowest reflector of interest is out side the noise. This

give the approximate idea of near offset.- Calculate velocities, frequency and wave length of each events.- Calculate t, d & T for every events.Where t = Time period of total noise. d= Depth of the noise. T = Time period of noise. Calculate Vapp = d / t

Frequency f = 1 / T

Wavelength = Vapp / f D/t = -------

1 /T when two elements in the array are spaced a distance equal to /2 , perfect cancellation of the noise will occur because the noise received will be 180 degrees out of phase. However the assumption of a single wavelength to be cancelled os not realistic because actual noise often consists of several types arriving from different directions with each type comprising a range of wavelengths. The nature of noise may also change from point to point along the line. For these reasons many elements are used in the array and in some cases the array may be spread out in two dimensions. Thus wavelengths for each events of noise can be calculated.Choose minimum and maximum wavelength of noise.Plot /D versus attenuation of noise.Now select /D for which attenuation of noise is optimum.

D will be the array length of geophones.

Computer analysis.

The data of noise test should also sent to computer center for necessary processing.The processing request should be made.- Raw plot of noise data.- Filtered output plot with 8 Hz and 12 Hz low cut frequencies and 18 db/oct slope.

Slope varies as per recording instruments.- Pickup traces from noise section at different offsets at an interval of channel spacing

decided for regular work- F-K plot for different space and time windows up to the time for the deepest horizon. Interpretation of processed data.

- From the raw plot of the noise data different noise wave trains are to be picked up and

the velocity, frequency and wavelength for the corresponding wave trains have to be calculated. This will give the necessary geophone array length to be used for ground roll noise.

- Filtered output plot of the noise record with different low cut filters will give the idea of frequency content of the coherent noise ( ground roll of low frequencies).

- A careful study of the amplitude spectrum of traces at different offsets for low frequency events indicate that the amplitudes of events at lower frequency range of the order of 8 to 12 Hz is prominent in case of traces at smaller offsets . these amplitudes go on decaying with increase of offsets. The offset ft which these amplitudes are minimum and do not change appreciably there after may be decided as near trace offset.

- The study of F-K plots will give the idea of different modes of waves travelling with different apparent velocities.

SEISMIC SURVEY WITH VIBRATOR SOURCE The reason for the wide acceptance of dynamite or explosive technique was –- A reasonable sharp signal.- Sufficient energy.- Low level of surface waves.

In spite of the above advantages there are some short comings of explosive technique- In populated areas the explosive technique is risky in view of the possible damages.- Drilling is itself a problem in some areas.- The two parameters amplitude(or penetration) and sharpness (or resolution) are

contradictory. An increase in the charge increase the amplitude but simultaneously decreases the resolution.

In view of the first and second problem the surface sources become attractive. In the surface sources vibroseis is the more popular technology in the world. Off course the third problem can be short out in explosive survey also by shooting many shots simultaneously instead of one. Short comings of vibroseis survey.- low amplitude.- Variation of the signal with variation of surface conditions (which is more variable in

comparison to variation in holes in explosive survey). The low amplitude can be overcome by the use of several sources, and the surface waves can be reduced by the use of well designed source and receiver patterns, but the variation of the spectrum are stable. If the sources can be synchronized that is off course not possible other techniques like weight drops. . in this method several sources can be used simultaneously, with pattern formation. This technique some extent allows to control the spectrum. However, there are technological and economical limitations to the number of simultaneously used sources. Thus the only solution is to distribute the sources not only in space but also in time. The method known under the technical term vertical stacking. Vertical stacking – one stack is a record of one sweep with number of vibrators. Signal should be chosen in such a way which can fulfill our required record or amplitude. Chose an individual signal with the right spectrum and stack as often as necessary to obtain sufficient amplitude in time window of interest. This works well if the required number of stacks is moderate, ie when the amplitude generated by an individual signal is nearly sufficient . it is obvious that this can not always be the case. If the required number of stacks becomes excessive, the time required to make a single stacked observations becomes unwieldy because after every individual transmission one has to wait until the complete seismogram has been observed and the noise has quieted down(ie one has to have a listening period). We need a method that distribute the total signal in such a way that the total energy is sufficient to build up the required amplitude in the seismogram.

Mechanical vibrators In principle, one can built completely mechanical vibrators though they are not practical for our purposes because the modification of the phase function is rather difficult. In mechanical vibrators the suspension is rigid. The conceptually simplest design moves the

reaction mass linearly up and down along a vertical shaft resting on the support. The vertical motion is produced either by a rack and pinion system or by a can and gear system.Electromechanical vibrators In principle, all systems that are used in electro acoustical transducers can be adopted to supply the oscillating force that drives a vibration. piezoelectric capacitive electromagnetic and electrodynamics systems are used in this.Servo hydraulic vibrators. The servo hydraulic vibrator is currently the standard type. In this design, the actuator contains a vertical cylindrical chamber in which a piston integrated into the vertical support shaft travels the piston divides the chamber into two sub=chambers that can be separately charged with hydraulic oil. A pressure difference in the two sub-chambers produces a net force on the actuator. In this vibrator suspension is not necessary as a pressure bias balances the weight of the actuator at any desired level. Vibrator is the most popular surface source possibly due to signal enhancement through cross-correlation techniques. Dynamite and other surface sources generate a high amplitude short time duration signal the vibrator on the other hand produces a low amplitude long time duration signal. Vibrator transmit the signal in sweeps. The sweep signal is generally sinusoidal varying in apparent frequency over the duration of the sweep signal.

Figure shows the reflected vibrator signal detected at a geophone station. Two or more vibrator sweeps may be summed to build up the energy level and attenuate random noise. The signal is further processed in the field or in the data processing center by cross-correlation. The cross correlation process is illustrated in the figure. The sweep signal (1) is correlated with the recorded signal (2). The cross-correlation signal (6) displayed as a function of time shows high amplitude events that look exactly like reflections of each trace in between correlation peaks being more or less quiescent depending on the noise level. The sweep frequency of the vibrator is usually selected to best match the local conditions to obtain optimum signal to noise and high resolution data. The frequency content of the signal is controlled so that the total energy of the vibrator system results in useful reflection data. The frequency content of a dynamite signal is not subject to control and may be influenced primarily by the material in which the charge is located in many places the best signal to noise ratio is obtained over a limited range of frequencies which can be specifically programmed into the vibroseis source signal. Shallow reflection ma call for a sweep over a range of frequencies at the high end of the usual seismic spectrum while deep reflections would call for a lower sweep frequency range.Vibrator sweep signals Characteristics of the sweep signal play a critical role in the effectiveness of the vibrator as a seismic source. Important parameters are as under.- Sweep frequency range.- Length of sweep and sweep taper.- Up sweep and down sweep.- Linear vs exponential vs pseudocode.- Multiple sweeps.

Linear sweep: in the linear sweep frequencies transmitted constant or increase/decrease with time.

Non-linear sweep: in non linear sweep frequency can vary irrespective of time.

Upsweep: in the upsweep first lower frequency and then increasingly higher frequency transmit by vibrator.Down sweep: in the down sweep first higher frequencies and the lower frequencies are transmitted by the vibrator.Sweep frequency range: It is range of minimum and maximum frequency in the sweep. The sweep frequency range is usually established on the basis of the desired resolution, but is also limited by noise conditions in the local area. The propagating wavelet of a vibrator system is a zero phase wavelet equivalent to the autocorrelation of the sweep signal. Therefore the greatest resolution wavelet is obtained with the broadest sweep range. The overall goal is to achieve a high S/N level as well as resolution and the vibrator energy needs to be concentrated in the frequency band of acceptable noise level. Also, it dose little good sweep up to very high frequencies which will be lost through normal absorption. Physical limitations of the vibrator also need to be considered. Large displacement of the reaction mass at low frequency may cause it to reach its mechanical limits. At high frequency derive signals (above 100 Hz) the vibrator is very difficult to synchronize. These factors usually result in the sweep signal ranging between 10 and 100 Hz. As per communication theory the greatest amount of information is transmitted when the product of amplitude, band width and signal duration is large. For a vibrator signal, this means that the vibrator force should be large, the sweep frequency range should be wide, and the sweep length should be long.

S/N (improvement) = FTW Where F is the hydraulic force T is the sweep length W is the sweep bandwidth

It shows that doubling the sweep length theoretically has the same effect as doubling the bandwidth. Bandwidth is usually limited by unmanageable problems of noise and absorption and it may be desirable to make up for this with longer sweeps or a greater number of sweeps at the same VP. Two 8 second sweeps should be equivalent to one 16 second sweep in terms of S/N improvement, but the move-up time between two short sweeps would make the one long sweep more preferable.Length of sweep: length of sweep is the time period of a sweep T.Tapering of sweep: decreasing of amplitude in the beginning and end of sweep is called tapering of sweep.There are two advantage by tapering of sweep.

1. It minimize the strain on vibrator.2. It minimize the correlation noise.

Upsweep: In the upsweep initial frequencies are less and it increases with time at constant acceleration.

Down sweep: In down sweep initial frequencies are high and it decreases with time at constant acceleration.

The advantage of down sweep is that it remove ghost noise, but it enhance the wear and tear of mechanical parts of the vibrator.

Generally upsweep is preferred because of less strain on vibrator.Convolution theorem:

Convolution in the time domain corresponds to multiplication if the frequency domain. And multiplication in the time domain corresponds to convolution in the frequency domain.Correlation: Cross-correlation is a quantitative measure of similarity. Such measure of similarity must give the 1 for two identical series.Autocorrelation: Autocorrelation is the cross-correlation of a signal or with itself.

In explosive technique the signal transmitted into the earth are usually considered unit pulse and short length signal, where as in vibroseis the transmitted signal is sweep signal which is long period signal.

The recoded vibragram can not be considered as seismogram as it will not be readable. By cross-correlating the vibrogram with transmitted signal vibrogram can be converted into seismogram and it become readable.

- Transmit a signal of low power density (of low amplitude of particle velocity) that has a narrow autocorrelation. The record obtain in this way is called the vibrogram,

- Cross-correlate the vibrogram with the transmitted signal to obtain the correlogram. The correlogram can be seen as the seismogram that would have been obtained. Had the autocorrelation of the wavelet been transmitted.

SEISMIC DATA ACQUISITION IN MARINE In marine seismic data acquisition some of the techniques are same as in land data acquisition. However some aspects are uniquely associated with marine data acquisition like energy source, receivers and cables are of different quality which are being used in marine survey. Because of the more amount of data acquired in digital recording system was put to use for nearly all marine shooting within a few years after its introduction. ie. In the middle of 60s. the multifold coverage were introduce by 1967. The explosive were used as source of seismic energy till 1980. After that air gun is the main energy source used for marine seismic data acquisition. Marine acquisition can be 2D with seismic lines spaced 1000 ft or more apart and 3D with line spaced at 500 ft apart.Source energy in water. The seismic energy source in water introduce a sudden positive (or some time negative) pressure impulse into the water. This impulse involves a compression of the water particles, creating a shock wave that spreads out spherically into the water and then into the earth, a delayed effect of the shock wave is an oscillatory flow of water in the area around the explosion. Which gives rise to subsequent pressure pulses designated as bubble oscillations.

Air gun The most widely energy source used in marine survey are air gun. The air guns are manufactures in number of models with capacity range of 1 to 2000 in cube. The mostly air guns operates at a pressure of about 2000 lb/in square. Figure shows the working of air gun. High pressure air which passes through a hose from the compressor to the towed submerged unit enters through the connection at the upper left. It flows into the upper chamber across which is fitted the top piston of a shuttle consisting of a shaft with a triggering piston at the upper end and a firing piston at the lower. There is a hole in the shaft through which the air from the upper chambers enters the lower one. Although the same pressure is developed in each chamber, the area of the triggering piston above is somewhat greater than that of the firing piston below and the net downward force on the shuttle causes it to move down until it is stopped by the base of the upper chamber. At the instant the gun is to be fired, a solenoid opens a valve that injects high pressure air between the triggering piston and the base of the upper chamber through the opening on the right side of this chamber. The sudden introduction of the air through the solenoid controlled valve upsets the equilibrium of the system, and the shuttle moves upward at a high velocity. As the firing piston passes the four large ports most of the high pressure air bubble quite pulses at a rate determined by the oscillation period of the air mass thus generated. The larger the volume of the air the longer the period. Figure shows the waveform obtained from as air gun at different depths. The decrease in oscillation period with depth conforms the Rayleigh Willis prediction that the period should fall off as the five sixth power of source depth. He amplitude of the initial impulse is necessarily greater than that of any produced by subsequent collapses of the bubble because energy is lost each time the bubble expands and collapses. However the seismic record will show the first bubble pulse to be substantially stronger than the initial impulse, because the initial signal produced by abrupt mechanical release of compressor and produced sharp and high frequency signal. The collapsing bubbles on the other hand produce signals with a much more gradual onset, the energy of the bubble within the seismic band. Since seismic recording instruments are generally set to reject energy outside the restricted band in which deeper seismic reflections are observed. They can reject a substantial part of the energy of the initial impulse and pass all of the bubble energy. This effect is even greater when the initial impulse is produced by an explosive charge. The effect of the bubble pulse repetition is to give an oscillatory and, hence unsatisfactory reflection record, special measures are therefore taken in the shooting and in the processing center to eliminate the bubble oscillations. The most effective way of doing this in the field is to use an array of guns having a variety of air chamber capacities and all fired in synchronism. The intervals between the initial pulse and the first bubble pulse will be different for each gun having a different air capacity. The pressure signal actually recorded from the array will consist of an impulse representing the sum of the initial pulses from all the funs followed by a train of much weaker bubble pulses spread out over a period of time and partially canceling one another provided that the guns could be synchronized to emit their initial pulses nearly simultaneously, and provided that the gums are far enough apart that they do not interact substantially, the initial pulse sound pressure produced at a great distance below the array is equal to the sum of the sound pressures of the individual guns. Because perfect synchronization is not attainable in practice, the best primary to bubble ratio attainable in practice for signals in the seismic

reflection band is about 10. the large guns generate a signal richer in low frequencies than do the small guns whose signals are relatively richer in high frequencies. The choice of large, medium, and small guns can achieve an approximately balanced frequency spectrum.Marine receivers (hydrophones) The marine hydrophone is an electro acoustical transducer that converts a pressure pulse into an electrical signal. The heart of the unit is a piezoelectric crystal. In such crystals, if mechanical stresses are applied on two opposite faces, electrical charges appear on some other pair of faces. If the stresses are changed from compression to tension, or vice versa, the charge polarity on the crystal faces is reversed. This generates a voltage proportional to the instantaneous water pressure associated with the seismic signal. Since the output is a function of the velocity of the water particles set in motion, the voltage will vary with the frequency of the seismic wave. Like the velocity geophone this is an undesirable feature and is to be compensated for high frequencies. The compensation is achieved with a matching transformer and a damping resistor similar to the geophone. In normal practice a group of hydrophones, connected in series, are coupled to the streamer cable through the transformer. The output of the station flat over a frequency range from 2 Hz to 400Hz, 0.5 dB. The voltage sensitivity of a single hydrophone may be 24 to 48 microvolts per microbar. The sensitivity of a group of hydrophone is increased to 5 microvolt per microbar. The noise comes from mechanical vibration on the vessel, wave action against the vessel, strumming of the taut cable and vortex action of water around the cable effect the desired signal. The suppression of these noise achieved by combining two crystal elements in one housing and physically mounting them to produce opposite polarity signals for acceleration motion along the axis of the cable while at the same time producing in phase signals for pressure changes. The hydrophone should be effectively sealed fo prevent liquids from reaching the crystal elements. Leakage can result in gradual deterioration of sensitivity.

Cables used in marine survey ( streamers) The streamer cable, originally developed for naval military use. Its design features eliminated the noise and depth control problems associated with surface referenced cables. The continuous tow operation made feasible with the streamer greatly reduced the cost of marine seismic operations. The cables are plastic tube approximately 3 inch in diameter is nearly neutrally buoyant and filled with oil. If the density of the water changes because of temperature or salinity variations, the overall density of the cable can be changed to maintain desired buoyancy by adding or removing thin lead sheets wrapped around the cable. The hydrophone elements, wires, and transformers are inside the plastic tube, which is acoustically transparent and generally also optically transparent. Inside the tube are steel cables, the strain members, that provide the mechanical strength to tow the entire length of the cable. The seismic waves pass through both the plastic and the oil to reach the hydrophones without noticeable interference. Most marine recording makes use of cables 3000 or more meters long, which contain 96 to 240 recording segments, each feeding a separate channel. Each segment are 12 to 30 m long and each may contain 6 to 15 hydrophones.Reflection procedures at sea.

Most of the marine reflection surveys today are carries out as single ship operations. The same ship tows both the energy source and the recording cable. Such arrangement has obvious economic advantages. The streamer is towed by the ship at regular and required speed, to keep shot interval constant and shots are taken at regular time. Recording instrument mounted on ship records all the seismic signal reflects from the subsurface of the earth. For conducting 3D seismic survey multi-streamer are used in the single ship. These streamers are towed at a constant space and multi-source generally dual source are also towed by the same ship and flip flop shots are taken at regular time interval. In such a way 3 dimensional recording can be done.