seg marine energy sources

7/27/2019 Seg Marine Energy Sources

1/14

1 of 14

Special Report of the SEG Technical Standards Committee 1

SEG standards for specifying marine seismic energy sources 2

R. C. Johnston 3, D. H. Reed 4, and J. F. Desler 5

1 1988 Society of Exploration Geophysicists. All rights reserved.2

These standards were approved for publication by the SEG Executive Committee on March 13, 1987. Weanticipate that revisions and additions will be necessary from time to time. Please address all suchsuggestions to the current Chairman of the Technical Standards Committee, Ben B. Thigpen, 13914 NimberlyLane, Houston, TX 77079, or to his successors.

3 Subcommittee member, Consultant, McNeese State University, Box 976, Lake Charles, LA 70609.4 Chairman, Technical standards subcommittee for marine source and detector standards, Consultant, ARCO

(retired), 10415 Coleridge, Dallas, TX 75218.5 Subcommittee member, TGS Offshore Geophysical, 333 Clay, Ste. 3900, Houston, TX 77002.

PURPOSE AND SCOPE

PurposeThe purpose of these standards is to specify, in

an easily understood standard form,measurable characteristics of marine seismicenergy sources to assist engineers, scientists,and operations managers in choosing,comparing, and/or specifying particular systemsfor their exploration programs.

ScopeThese standards cover equipment used toacquire geophysical exploration seismic data inwater-covered areas.

The attributes of the wavelet that radiates to thefar field (the far-field signature) are the primaryfocus of these standards.

Only impulsive sources (explosive or implosive)are covered by these standards. Single-frequency or sweep-frequency sources may beadded in the future.

DEFINITIONS

Near-field signature A near-field signature is an acoustic waveletwhose direct arrival from the marine source isvery large (>20 dB) compared with reflectionsfrom adjacent boundaries or interfaces. Near-field signatures are usually recorded for a singlesource or a point source. A point source is onewhose dimensions are small compared with theshortest wavelength of interest, L .

Far-field signature A far-field signature is characterized by a directwavelet plus its reflection (ghost) from the air-water interface. The ratio of the direct-to-ghosttravel distances should approach 1.0. Far-fieldsignatures are usually recorded for an array of sources (a directional source). A directionalsource is one whose dimensions are of thesame order as the wavelengths of interest, L .

This document has been converted from the original publication:

Johnston, R. C., Reed, D. H. and Desler, J. F., 1988, Special report on marine seismic energy source standards *: Geophysics, 53, no. 04, 566-575. (* Errata in GEO-53-7-1011)


2/14


3/14

3 of 14

publication, The S. I. Metric System of Unitsand SEG Tentative Metric Standard, SEGMetrication Subcommittee, 1981, should beconsulted.

DESCRIPTIVE PARAMETERS

General Systeme International d'Unites, also known asSI units, are used to quantify physicalparameters and acoustic performance. Other units may be given in parentheses but theequivalent SI unit should be given first. Use of prefixes, such as micro, kilo, mega, etc., shouldbe used for convenience in notation.

Physical dimensions Overall dimensions of an individual sourceelement and the distance between sourceelements in an array should be provided. Thetype, location, and specification for near-fieldsource hydrophone monitors, depth gauges,pressure gauges, or any other ancilliaryequipment should be provided.

Weights The approximate weight of individual sourceelements and the weight of the total arrayshould be given.

Operating conditions Operating conditions that affect the acousticperformance of the source should be given.Examples for an air-gun array are the pressure,volume, and depth of each air gun, the towingconfiguration, boat speed, and method of timingthe air guns.

Input requirements Input power requirements to operate the sourceat a stated repetition rate should be given, e.g.,standard cubic meters per minute of air required. Input signal requirements to initiatethe source should be stated, e.g., signalcharacteristics to operate an air-gun solenoid.

Other parameters There may be other parameters of importancethat should be stated.

ACOUSTICAL PARAMETERS

Far-field signature The far-field signature is a representation of theacoustic pressure as a function of time. It ischaracterized as a wavelet that propagates withits surface ghost and whose amplitude isinversely proportional to its distance from thesource. An example air-gun array signature isshown in Figure 1. The signature is describedby the peak-to-peak primary pulse amplitude,measured in megapascals and referenced toone meter (MPa-m), and the ratio of theprimary-to-secondary pulse. The recordingfrequency passband should be stated, since thepeak-to-peak primary pulse amplitude and theprimary-to-secondary pulse ratio will beaffected.

Amplitude and energy spectrum The far-field signature should be transformed tothe frequency domain and given as anamplitude and an energy flux spectral densityplot. The total energy flux integrated to theNyquist frequency shall also be given. Anexample spectral representation is shown inFigure 2; it is the transform of the signature of Figure 1. The units are micropascals per Hertzreferenced to 1 m (Pa-m/Hz) for the amplitudescale and joules per square meter per Hertzreferenced to 1 m from the source [J-m/(m 2Hz)],for the energy flux spectral density scale. Thespectral plot shall be scaled in decibelsreferenced to 1 Pa-m/Hz and 1 J-m/(m 2-Hz).

These scales differ by about 182 dB whennominal values for the acoustic impedance ( c )of seawater are used, namely,

= 1026 kg/m 3 (the density of seawater)

and

c = 1500 m/s (the velocity of sound inseawater).

The spectral plot is characterized by thenominal value and its variation in the passbandbetween the minus 3 dB points (see Figure 2).

An additional parameter may be calculatedeasily from the energy flux spectral density: the


4/14

4 of 14

cumulative energy flux as a function of frequency. This parameter is plotted ascumulative energy flux as a percentage of thetotal energy flux, and may be plotted on thegraph with the spectral plot.

MEASUREMENT TECHNIQUE

Philosophy The measurement technique outlined in

Appendix A is intended to serve as a generalguideline for acquiring calibrated acousticcharacteristics of a marine source. Much of theinformation is taken from Fricke et al. (1985) "Astandard quantitative calibration procedure for marine seismic sources." The intent of thisacquisition step is to measure the signaturewith as much fidelity as possible. This requires

measurement specifications which include largedynamic range, low harmonic distortion, lowsystem noise levels, and a linear system re-sponse which is known in both amplitude andphase. If these requirements are satisfied in themeasurement environment, then the recordedresult will be a faithful representation of theoriginal pressure signal.

Requirements Four items should be considered to meet therequirements of the measurement system:

(1) recording electronics,

(2) system calibration,

(3) proper positioning of the calibratedhydrophone, and

(4) proper source operation duringmeasurement.

Figure 3 illustrates an example acquisitionsystem and the environmental conditions thatneed to be considered. Other acquisitionsystems utilizing towed fish and sea floor-basedsystems are routinely used. See Appendix A for

a more detailed discussion of the measurementtechnique.

PROCESSING TECHNIQUE

Philosophy Temporal analysis of a source's acousticperformance has traditionally beenemphasized. Power spectra have been used,but this method makes quantitativecomparisons among sources impossiblebecause power in a transient signal dependsupon the window length used in the analysis.The same signal will have different amounts of power depending upon the analysis window.This is also why "power" spectra have alwaysbeen normalized; if they were not, the samesignal analyzed with two different windowswould produce different results.

Requirements A calibrated energy flux spectral density isrequired to compare performance of sources ona one-to-one basis. The processing techniqueis given in detail in Appendix B.

FIG. 3. Sonobuoy acquisition system.


5/14

5 of 14

FIG. 5. Test conditions.


6/14

6 of 14

FIG. 6. Results for 12 330 cm 3 (7524 inches 3) array.


7/14

7 of 14

PRESENTATION OF RESULTSMinimum requirements

Measurement and processing techniques mayvary from those outlined but the results shouldbe presented in a consistent manner. The

following data should be considered minimum:source description,recording instrument specifications,test conditions,far-field signature, phase spectrum(optional), and energy flux spectraldensity.

Other data may be shown in a similar format.

Example results Figures 4, 5, and 6 show the minimum

information described above for a particular air-gun array.

REFERENCES Fricke, J. R., Davis, J. M., and Reed, D. H.,1985, A standard quantitative calibrationprocedure for marine seismic sources:Geophysics, 50, 1525-1532.

Thigpen, B. B., Dalby, A. E., and Landrum, R.,1975, Report by the subcommittee on polaritystandards: Geophysics, 40, 694-699.


8/14

8 of 14

APPENDIX A

MEASUREMENT TECHNIQUE

Requirements The four items that should be considered for themeasurement technique are: recordingelectronics; system calibration; calibratedhydrophone location; and source operation. Theuse of a digital sonobuoy illustratesrequirements of the system but is not the onlyacceptable technique.

Recording electronics The recording electronics specifications are

satisfied by using, for example, a digitalsonobuoy interfaced to conventional seismicdigital recording equipment SEG standards for polarity (Thigpen et al., 1975) should beobserved, i.e., a pressure increase shouldresult in a negative number on digital tape.

System calibration The recording system must be calibrated. Alow-distortion signal oscillator, a calibrateddigital volt-meter, and a wide-band voltageattenuator are used for the calibration. Thedigital sonobuoy should be interfaced such thatdata from the buoy may be received andrecorded by the recording electronics. Asinusoidal signal with a measured amplitudeand a frequency within the nominal passband isinput into the sonobuoy. This signal is recordedand successive records are taken with the inputsignal attenuated by a specified amount (e.g.,10 dB). This process is continued until arepresentative range of signal amplitudes hasbeen recorded. The data that result on taperepresent sets of counts or values that

correspond to input voltages from the oscillator. A calibration constant may be computed fromthese data, together with the hydrophonecalibration. Sensitivity of the hydrophone mustbe suitable to the anticipated pressure range for the particular source, to avoid clipping of thepeak signal The time delay and the impulseresponse of the recording system should bedetermined during calibration of the system.

Calibrated hydrophone location The proper positioning of the hydrophone in thewater column is an important consideration.There are three constraints on correctplacement of the far-field hydrophone:

(1) the distance between the hydrophone andthe water bottom,

(2) the distance between the hydrophone andthe source, and

(3) the depth of the hydrophone.

Each of these constraints is discussed below.

(1) Hydrophone water-bottom distance The distance between the hydrophone and thebottom must be such that the source signal iscompletely received before the water bottomreflection arrives Figure A-1 illustrates thegeometry required to derive an equationrelating distance between the source and thebottom to wavelet duration. Equation (A-1) isthe resulting equation, represented graphically

in Figure A-2 for c = 1500 m/s. LetcT

D , (A-1)2

where

T = wavelet duration (s),c = speed of sound in water (m/s), andD = minimum distance between the hydrophoneand the bottom (m).

If the criterion established by equation (A-1) isobserved, a clean signature will be receivedbefore onset of a water-bottom reflection.

(2) Hydrophone-source separation distance

The hydrophone-source separation constraintmust be considered only in the case of sourcearrays. Ideally a signal generated by the mostdistant source element in an array must arriveat the receiver simultaneously with the signal


9/14

9 of 14

generated by the closest source element. Indigital recording systems, "simultaneously'' maybe interpreted as occurring within one samplinginterval or less. If the receiver is too close to thearray, this requirement will not be satisfied anddistortion will occur. Figure A-3 illustrates thegeometry of a symmetrical situation andincludes an abbreviated derivation of equation(A-2), which defines the minimum separationbetween the source array and the hydrophone:

d2/4 (c t)2Z (A-2)

2c twhereZ = minimum separation between the sourceand the hydrophone (m),d = maximum lateral crossline or inlinedimension of the array (m),c = speed of sound in water (m/s), andt = sampling interval (s).Equation (A-2) is illustrated graphically in Figure

A-4 for t = 0.001 s.For equation (A-2) to be valid, it is important

FIG. A-1. Minimum distance between the far-

field hydrophone and the ocean bottom.

FIG. A-2. Minimum distance between the far-field hydrophone and the ocean bottom as afunction of the seismic wavelet's duration.

FIG. A-3. Minimum distance between thesource array and the far-field hydrophone.

FIG. A-4. Minimum distance between thesource array and the far-field hydrophone asa function of the length of the source array.


10/14

10 of 14

that the hydrophone be under the center of thesource array; this requirement is particularlyimportant for super-wide configurations.Frequently, in practice, this ideal is not attained.

Additional calculations should be made for thecase of non-symmetric geometry.(3) Hydrophone depth

The third and final constraint on the position of the hydrophone is its relationship to the surfaceof the water. A source ghost signature isproduced; when observed from a great distance(i.e., infinity), the ghost appears to have thesame amplitude as the emitted pulse but withopposite polarity and is delayed with respect tothe original signature. This source ghost is partof the total source signature, which is what the

earth "observes" as the source; hence, theghost is a desirable part of the signature andshould be recorded faithfully. If the receiver isnot far enough from the surface of the water,the source ghost will not be fully developed.The receiver will see the direct arrival from thesource as being larger than the ghost arrival.The difference in amplitude is due to adifference in the travel paths of the two signals.The ghosted arrival has traveled farther and,due to spherical spreading loss, has a smaller amplitude than does the direct arrival. Equation

(A-3) expresses the ghosted amplitude as apercentage of the direct arrival amplitude for agiven source and hydrophone depth:

dh d s G = x 100. (A-3a)

dh + d s Equation (A-3a) may be rearranged as

d s(100 + G)dh = (A-3b)

(100 G)

where

G = ghost amplitude as a percent of the directarrival amplitude,

dh = hydrophone depth (m), and

d s - source depth (m).

For example, unless the array being measuredis very long (greater than 70 m), the depthcomputed from equation (A-2), with t - 0.001 s,will result in a ghost of less than 95 percent.Thus, hydrophone depth normally dependsupon how close one desires to approach theidealized ghost of 100 percent.

Source operation A crucial factor in proper measurement of thesource signature is that the sources beoperated during the test in their standard

operating configuration. This means that theyshould be fired at their standard pressure,depth, fill time, firing interval, etc. Proper towingspeed is a very important parameter. Proper towing speed ensures that the sources arecooled properly and are in proper geometricrelationship with the survey vessel. An accuratemeasurement of the source cannot, in most

FIG. A-5. Configuration of the dynamicsource test.

FIG. A-6. Results of the dynamic source test.


11/14


12/14

12 of 14

APPENDIX B

PROCESSING TECHNIQUES

Requirements A calibrated energy flux spectral density plot asa function of frequency is required to comparethe acoustic performances of various sources.

Energy flux spectral density Transient source signatures actually have zeropower, since power in the strict definitioninvolves a time average integral from minus toplus infinity. What these source signatures dohave is energy. They are, in fact, zero-power,

finite-energy signals. This is the key to proper signature spectral analysis. Instead of computing a true quantified power flux spectraldensity (which would be zero for allfrequencies), a quantified energy flux spectraldensity is computed.

1 E(m) = X(m)2, m = 0, 1, N1, (B-1a)

cwhere

N-1

X(m) = t x(n) exp [ j (2mn/N)],n=0m = 0, 1, , N1 (B-1b)

and X(m) = Fourier coefficients,E = energy flux spectral density,

x(n) - digital samples of the time series,N = number of samples in the analysis window, = density of water,c = speed of sound in water, andt = sampling interval.Equation (B-1a) produces consistent results for a transient signal no matter what the analysiswindow length may be, as long as there is nosignificant noise in the window. Cumulativeenergy flux and total energy flux are computedfrom the energy flux spectral density. Thecumulative energy flux is interpreted as theamount of energy flux contained by the signal ina frequency band from zero Hz to any particular

frequency of interest. Total energy flux is thecumulative energy that is contained by theentire signal over all frequencies. Equation (B-2) gives the cumulative energy flux as afunction of the energy flux spectral density:

k

U(k) = f E(m), k =0, 1, , N1, (B-2)m=0

whereU(k) = cumulative energy flux from zero Hz to afrequency of k f Hz,

f = frequency sampling interval, andE(m) = energy flux spectral density fromequation (B-1a).

Note that U(N 1) is the total energy flux and isassigned the symbol U r .

The cumulative energy flux is convenientlyrepresented as a percentage of the total energyflux; it is the energy flux contained by the signalin a frequency band from zero Hz to anyparticular frequency of interest. Equation (B-3)is used to compute this percentage. Thisrepresentation is quite useful in the analysis of individual source characteristics.

K

f E(m), m=0

U P (k ) = x 100,UT

k = 0, 1, , N1 (B-3)

where U P (k) is the cumulative energy fluxexpressed as a percentage of the total energyflux.

Equation (B-2) is used for transient signals thathave zero power and finite energy. The natureof source signatures lends them to this class of analysis. If one uses pressure signal values thatare quantified to some standard, the resultingenergy flux spectral density will also bequantified to the same standard.


13/14

13 of 14

To comply with established standards,quantified acoustic source characteristics arerepresented in SI units referred to a unitdistance from the source. The SI units of pressure are newtons per square meter whichhas been given the name pascal. Scientificunderwater acoustics work has adopted themicropascal (10 -6 pascal) as a referencepressure for all quantitative work. (1micropascal = 10 -5 dyne/cm 2 = 10 -11 bar.)

In addition to the quantification of the acousticpressure referred to as a micropascal, thesource levels are all referred back to a unitdistance 1 m from the source. This is done sothat sources measured at the variousseparations between the source and thereceiver may all be compared on the samebasis. In referring the signal level back to 1 mfrom the source, a simple spherical spreadingloss (1/r) is assumed and removed; r is thedistance between the receiver and the source inmeters.

Processing the dataThe calibrated energy flux spectral density maynow be computed in a few simple processingsteps. A calibration constant relating counts ontape to pressure at the hydrophone can easilybe calculated, given the data obtained

previously (see the section on systemcalibration in Appendix A). Check the calibrationconstant derived from each level of inputoscillator signal. If the recording system islinear, each derived calibration constant will bethe same. This is the total acquisition systemcalibration constant.

The next obvious piece of information needed isthe distance (in meters) between the sourceand the hydrophone. As previously discussed in

Appendix A, determination of true distance isnot trivial, considering the fact that surface

currents may move the visible sonobuoy awayfrom a position vertically above the hydrophone.While it is beyond the scope of this document,those skilled in the art will use appropriatetechniques to determine a distance (in meters)to be used in referring the pressure of thehydrophone back to a 1 m hypotheticaldistance. This simply requires multiplying thepressure by the distance (in meters).

Summarizing, the raw data are multiplied by thecalibration constant and the source-receiver separation in meters. This results in a calibratedsource signature. Application of equation (B-1a)generates the energy flux spectral density.Equation (B-1b) is most conveniently computedusing a Fourier tranform. The Fourier coefficients are scaled by the value of thesampling interval. If the resulting values aredisplayed on a decibel scale, the display will bean amplitude spectrum. If the values aresquared and then scaled by c, the acousticimpedance, and displayed on a decibel scale,the display will be an energy flux spectraldensity. The shape of the curve does notchange, but the interpretation of the data isseen in a different sense. The amplitudespectrum has units of pressure per unitfrequency. In SI units, this would bemicropascals per Hz. The energy flux spectraldensity has units of energy flux per unitfrequency. Again in SI units, this would be

joules per square meter per Hz. An example of a calibrated spectrum is shown in Figure 2. Thetwo scales on the left show both the amplitudespectrum and the energy flux spectral density.

The total energy flux is calculated usingequation (B-2), and the cumulative energy fluxis calculated as a percentage using equation(B-3). A final check of the frequency-domaincalculations is recommended to obviate anyscaling errors. This is simply done byintegrating the time-domain signature with thepressure values squared and dividing the resultby the acoustic impedance of sea water. Theresult is the total energy flux and should equalthe U T from the spectral-domain calculations.

ACKNOWLEDGMENTSMembers of the Technical StandardsSubcommittee for Marine Source and Detector

Standards who contributed to this documentare:

C. O. Berglund, Teledyne Exploration Co.

J. Blue, Naval Research Lab., Orlando, FL.

W. A. Knox, Patent Practice, WesternGeophysical (retired) Bruce Nelson, Digicon

Chris Walker, while with GECO USA


14/14

seg marine energy sources

Documents