08 Seismic Methods_slipswp

VE432 Training Course Manual

Chapter 8

VIBROSEIS SOURCE MANAGEMENT

MAIN APPLICATIONS (V8.4)

VIBROSEIS METHODS Issue : June 1999

VIBROSEIS METHODS Issue : February 2000

ii

TABLE OF CONTENTS

1. INTRODUCTION ---------------------------------------------------------------------------------------------------

2. SIMULTANEOUS OPERATION ------------------------------------------------------------------------------

3. FLIP-FLOP MODE ------------------------------------------------------------------------------------------------

3.1 PRINCIPLE--------------------------------------------------------------------------------------------------------

3.2 HCI SETUP FOR FLIP-FLOP OPERATIONS-----------------------------------------------------------

4. SLIP-SWEEP -------------------------------------------------------------------------------------------------------

4.1 OVERVIEW--------------------------------------------------------------------------------------------------------

4.2 GENERAL DESCRIPTION------------------------------------------------------------------------------------

4.3 OPERATION WITH SN388 AND VE432------------------------------------------------------------------

4.4 MORE ABOUT SLIP-SWEEP--------------------------------------------------------------------------------

5. HIGH-LINE NOISE ------------------------------------------------------------------------------------------------

5.1 GENERAL----------------------------------------------------------------------------------------------------------

5.2 IMPLEMENTING HIGH-LINE NOISE ELIMINATION ON THE HCI-------------------------------

6. HARMONIC LINE ELIMINATION -----------------------------------------------------------------------------

6.1 INTRODUCTION-------------------------------------------------------------------------------------------------

6.2 REVIEW OF PHYSICAL RELATIONSHIPS ON SINE WAVES------------------------------------

6.3 GENERAL LAW--------------------------------------------------------------------------------------------------

6.4 EXAMPLE OF PRACTICAL IMPLEMENTATION------------------------------------------------------

6.5 APPLYING THE PRINCIPLE---------------------------------------------------------------------------------

7. DPG MASTER SLAVE OPERATION -----------------------------------------------------------------------

8. NAVIGATION -------------------------------------------------------------------------------------------------------

8.1 IMPLEMENTATION--------------------------------------------------------------------------------------------------

8.2 ONE FLEET OR FLIP-FLOP NAVIGATION SETUP-------------------------------------------------------------

8.3 SLIP SWEEP NAVIGATION SETUP-------------------------------------------------------------------------------


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Error! Use the Home tab to apply titre 1 to the text that you want to appear here.

1. INTRODUCTION

This chapter covers different methods of sweeping, serving two major purposes:

- Saving time: Flip-flop, Slip-sweep, Simultaneous or Master/Slave operation

- Getting better data: High-Line or harmonic elimination.

The description in this chapter is provided as an overview of the flexibility offered by the SN388/VE432 system. Naturally, it is for the user to make the best possible use of that flexibility to extend it to further applications.


0



1


2. SIMULTANEOUS OPERATION

This method uses two groups of vibrators sweeping at the same time on two shotpoints but generating different sweeps. Overleaf is an example of setup for simultaneous operation.

Naturally the sweeps are assumed chosen with careful consideration for the frequencies from the two sweeps not to mix.


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Group 2

Group 1

a1*a1, a2*a2 wherea1: pilot group 1a2: pilot group 2



1



3. FLIP-FLOP MODE

3.1 PRINCIPLE

The Flip-flop mode uses two groups of vibrators sweeping alternately. This allows one group to move while the other is sweeping.

Fleet # 1

Fleet # 2

SeismicAcquisition

System

SN388

VE 432


0



1

2Listen

Pilot

Listenmoving moving

movingmoving

With this mode of operation the aim is to have the shortest possible time between two consecutive sweeps. To achieve this goal the SN388 should be set to Continuous operation mode and acquisitions should be started as the vibrator leader presses the Ready button on his DSD.

3.2 HCI SETUP FOR FLIP-FLOP OPERATIONS

As a matter of fact there are two ways of setting the Setup parameters in the OPERATION environment. The OPERATION environment allows us to use a wide range of setups illustrated in the two methods below.


1



The first set-up method is also the easiest:

You define two process types and two acquisition types. The two process types are exactly the same but one is defined for fleet 1 (acq.1) and the other for fleet 2 (acq.2).

Then you alternate the two process types in the Operation table.


2

A



The above example of double zigzag Setup will allow flip-flop operations.

The following points are worthy of note:

- The method is very easy to set up.

- Each acquisition is independent.

- The setup, however, needs to be modified if one of the fleets has to sweep any of the shotpoints of the other.


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The second method allows us to avoid the requirement mentioned above.

This time we use a common Process Type for the two fleets.

In that case you have to prepare two groups of shotpoints in the Operation Setup, for example odd shotpoints for fleet 1 and even shotpoints for fleet 2. But there is still no link between shotpoints and any fleet (unlike with method A) as a single Process type is used for both fleets and all shotpoints use the same Process Type.

The fleet is selected by selecting a shotpoint in either pane in the Operation main window: for example choosing a shotpoint in the upper pane will select fleet 1 whereas choosing one in the lower pane will select fleet 2.


4

B



The two panes allow all shotpoints to be displayed. You may select two shotpoints at a time. As a result, the spread will be prepared as the addition of the two spreads selected. When the observer activates the GO button the HCI will take the first shot, then the second. Then the data will be dumped to the tape (two files) and the spread for the next two shots will be formed.


5

Pane for fleet 2

Shot 4

Shot 1

Pane for fleet 1



The advantages of the second method are.

- It is very easy to have a fleet sweep a shotpoint prepared for the other fleet.

- Less time is lost between acquisitions as the spread is formed every two shots.

The only disadvantage lies in that the spread is the addition of the individual spreads for the shotpoints of the two fleets.

NOTE: The above example is for flip-flop operations but naturally it is possible operate with stacking, as in the example below.


6




7




8



4. SLIP-SWEEP

4.1 OVERVIEW

This chapter describes the technique called slip-sweep(*) available when using the VE432 and the SN388.

The principle of this method is to have a group of vibrators sweeping without waiting for the previous group to finish. As the frequencies are distinct, the correlation process separates the different records.

Using this method, the cycle time can be decreased significantly and production increased in the same proportion.

(*) Technique developed by PDO.


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4.2 GENERAL DESCRIPTION

4.2.1PRINCIPLEA vibrator group starts sweeping without waiting for the other group’s sweep to be completed.

A comparison between flip-flop operations and slip-sweep is made below.

- In flip-flop operations, no sweep can start until the current sweep is complete.

F re q u e n c y

L is te n in g

S w e e p in g

T im e

S w e e p # n

F 1

F 2

S w e e p # n +1

- That is not the case in slip-sweep operations. The frequencies are separated by the correlation process. The minimum time required between two consecutive sweep starts is the listening time.

F re q u e n c y

L is te n in g

S w e e p in g

T im e

S w e e p # n

F 1

F 2

S w e e p # n +2S w e e p # n +1


1

Group 2Group 1

Slip time

Slip time Listening time

Group 3Group 2Group 1



4.2.2CORRELATION

In the case of flip-flop operations, the records are correlated one after the other, using the pilot recorded on the auxiliary channel.

S1 S2

Single Sweep

R1 R2

In the case of slip-sweep operations, the record is continuous. It is cut into individual records (which overlap) using the Time Breaks recorded on auxiliary traces. Then each individual record is correlated independently.


2

TB

Seismic traces

Auxiliary trace(Pilot)



S1 S2 S1 S2

R1 R2 R3 R4

Slip-Sweep*

TB 1 (Aux1)

TB 2 (Aux2)

Pilot 1

Pilot 2

S1

S2

Naturally, slip-sweep requires that no stacking be performed.


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4.2.3CYCLE TIMEWe have seen that the slip time should not be shorter than the listening time. Although there is no maximum, the shorter the slip time, the higher the production.

WARNING !

Below is an example that illustrates the gain in production. That is only a theoretical gain, since the

practical implementation in the field is dependent upon logistic requirements.

Conditions in the example below:

- Sweep: 15 s

- Listening time: 6 s.

- Move-up: 23 s


4



Flip-flop

96 s

1

2Listen

Pilot

Listen

15 25 48 63 71 86

Production: 4 records in 96 s 24 s per record.

Theoretical maximum: 21 s per record (listening time + sweep time).

Slip-sweep

Listen

Pilots

Slip time= 11s

11 22 33 44 55 66 77

101s

881234

Sliptime

Production: 8 records in 101 s 12.6 s per record.

Theoretical maximum: 6 s per record (listening time).


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4.3 OPERATION WITH SN388 AND VE432

4.3.1CONNECTIONSOn the next page is an example of possible configuration. For other configurations, refer to the DPG Installation and Reference Manual, Section 2.

This setup is similar to standard multi-module connections using MMI4 or MMI8 and MMCI4 connection boxes.

The only difference is the connection between the DPG and the APM:

- An ethernet link is used that, in addition to its usual purpose, transfers the Firing Order.

- There is no link with the BLASTER plug of the APM, but the TB needs to be connected to the auxiliary line. The TB should be connected to:

- a1 if the DPG is managing fleet 1,



- a4 if the DPG is managing fleet 4.

In addition to the Time Breaks the pilots need to be connected.


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R E A D C A M E R A L A N

A P M 1 (m as te r)

M / SA U X

C A M E R AS C S I

A P M 2 (s lav e )

M / S A U X

M M C I4IN

IN

IN

O UT

S U 6

AC

CB

AC

CB

P O W E R

(1 1 0 /2 20 V )

SQ

C-P

ro

P O W E R (+ 12 V )

C A M E R A

D P G 1R E C O R D E R

A N A LO GP ilots R A D IO

L A N



L A N



L A N

(6 )M /S 8

(6 )M /S 7

(6 )M /S 6

(6 )M /S 5

(6 )M /S 4

(1 )

(6 )M /S 3

B L A S T E R

M /S 2

M M I-8

M /S 1

B L A S T E R

B L A S T E R

(1 )(3 )

L A NS C S I

H C I

5 0

A U X 1 A U X 2 A U X 3 D P G 1 D P G 2 D P G 3 D P G 4

(4 ) (4 ) (4 )

5 0

AC

CB

(2 ) (2 ) (2 )

S U 6

to R A D IO 1 to R A D IO 2 to R A D IO 3

P art N o .(1 ) M /S cab le 1A 130 7193 0B (2 ) M M I-8 A C C B cab le 1A 130 7760 0A(3 ) A P M B LA S T E R M M I-8 ca b le 1A 130 7759 9A(4 ) D P G R E C O R D E R M M I-8 cab le 1A 130 7759 8A(5 ) A N A LO G P ILO T S cab le see V E 4 32 D P G k it(6 ) 14 -19 p lugs , M /S te rm ina to r 1A 130 7256 0 (w ith A w ired to S , J w ired to K )

(1 ), (2 ), (3 ), (4 ), an d (6 ) co n ta ined inS lip -S w ee p op tion 171 7077 328 B

(5 ) (5 ) (5 )

The above example is a slip-sweep configuration with two APMs and three DPGs. Two pilots per DPG are connected to the AUX line via ACCB interfaces. Additional ACCBs can be connected if more auxiliaries need to be recorded. Up to eight APMs and four VE432 DPGs can be connected together via the MMI-8 box.


7



4.3.2HARDWARE MANAGEMENTBecause the sweeps overlap, continuous recording is required. Also, the spread recorded is the sum of each sweep’s spread. Therefore, the resulting spread is very large.

As a result:

- A multi-module configuration is often required for the spread to be large enough.

- The continuous acquisition is divided into salvos. For the APM, each salvo is regarded as an acquisition, but it contains several overlapping sweeps. A new salvo is automatically started if:

- A new spread is selected in the OPERATION environment.

- The memory is full. The APM computes the predicted moment when the memory is going to be full and it starts a new salvo.

- The COG is red (out of bounds) in the POSITIONING environment.


8



The number of sweeps that can fit in the memory depends on the Sample Rate, number of channels, sweep length, listening time and slip time.

Example with two sources

The APM starts the acquisition as GO is activated in the OPERATION main window and it starts recording. It sends the Firing Order to the different DPGs, depending on the slip time, slip delay and on the ready signal it receives from the DSDs The previous record is cut in accordance with the TBs and processed as usual.

In the line controller part of the 388 there is a memory: MPM lc board (192 megabytes if MMI-3 or 64 megabytes if MMI-1 ). This memory is divided in two parts in operation : two buffers. A salvo is a group of VPs with the corresponding data transferred in memory without changing of buffer. On the last VP of the salvo the system goes to the end of the acquisition length before to start a new salvo. The maximum length of the salvo is 32VPs and 256 seconds (software limits in version 8.4).

Salvo stops when: salvo of 32 VPs; 256 seconds of recording; memory buffer full with one more record, the system take care of the complete timing including TB window; Positioning error: radial error; Ready signal not received from vibrator before the end of acquisition length; Stop or abort; Line formation

After a salvo stop, when the two buffers are in use, the line controller display becomes red in the activity window, waiting for one empty buffer to start a new salvo.


9

Salvo 1 Salvo 2

S1

S2

Acquisitionin the APM

FO

Prediction of memory full or spread change

FO

Aux1 TB

Aux2 TB



4.3.3HCI OPERATION ENVIRONMENTOnly two HCI environments are different when using the slip-sweep technique:

- OPERATION environment

- VE432 environment

First of all, you need to input the appropriate password into the GO388 environment. This will add an option (slip-sweep) in the OPERATION environment. As a result, three options are available: Source and Signal, as usual, plus Slip-Sweep. When Slip-Sweep is selected, the only difference in the OPERATION main window is an extra button, labeled DELAY.

PROCESS TYPE SETUP

The Stack number must be 1. You must define one Process Type and one Acquisition Type per vibrator fleet. The auxiliary channels used for correlation can be defined as desired but the following channels are reserved for the TBs:- a1 if you use fleet 1- a2 if you use fleet 2- a3 if you use fleet 3- a4 if you use fleet 4

In the above example four fleets are used, therefore four Process Types. Auxiliary channels


10



a1 to a4 are used for the TBs, a5 and a6 are used for the pilots.


11



OPERATION SETUPYou have to define the Line for each sweep and each vibrator fleet. The spread should be the same for several successive shots. If a different spread is used on each shot, no slip-sweep can be achieved as a new salvo is initiated on each spread change, therefore on each shot. Also, you have to choose the Continuous mode.

DELAY SETUP

The Slip Window is the time before an automatic Firing Order is generated for the fleet if no Ready signal is received during the Slip Time (or before).


12

Fleet 1Fleet 2Fleet 3Fleet 4 Salvo 2

Salvo 1




13



The Slip Time is the minimum time until the next fleet is allowed to start sweeping. This setup is accessed by selecting: Setup, Slip Time.

READY (or DOWN) starts the sweep at t = Slip Time or t = Ready receive time if Ready is received during the Slip Window.

Otherwise, Start time = Slip Time + Slip window.


14

Fleet 1

Fleet 2

Automatic start if no Ready received

Slip Time

DSD READY (or DOWN) button pressed

Slip Window



DPG ENVIRONMENT SETUP

Fleet # 1

Fleet # 2

Fleet # 3

Fleet # 4

SeismicAcquisition

System

SN388VE 432

In slip-sweep you can use up to four fleets (only two in flip-flop) and you need to define one Acquisition Type for each fleet. The TBs need to be defined in accordance with the definition of the fleets


15



4.4 MORE ABOUT SLIP-SWEEP

Slip-sweep dramatically increases production. Tests have been conducted that showed production increasing from 1000 VPs/day (125 VPs/hour) to 1700 VPs/day (212 VPs/hour).

The test conditions were the following:

- 1710 active channels (9 lines 190 channels),

- 15 second sweep, 6 second Listening Time, 11 second Slip Time, 8 sweep salvo.

The cross-section shown below, beginning with Flip-Flop type acquisition and ending with Slip-Sweep does not exhibit any discontinuity in terms of data quality, but it is worth bearing in mind that the harmonics of a sweep are mixed with those of the previous sweep.


16

(Flip-Flop)



Slip-sweep ghosts:1 - In flip-flop mode, acquisitions are totally independent. Therefore, harmonics from

a sweep cannot mix with those from another sweep.

Frequency

Frequency

Time

Time

Sweep # n

Correlated Sweep # n

Sweep # n+1

F1

F1

F2

F2

H2

H2

H3

H3H4

2 - In the case of slip-sweep, sweeps do not mix but harmonics will, giving rise to ghosts (noise) in the record.

Frequency

Frequency

Time

Time

Sweep # n

Correlated Sweep # n

F1

F1

F2

F2

H2

H2

H3

H3H4

Sweep # n+2Sweep # n+1


17

No overlap

Listening Time

Harmonics mixed with the sweep

Ghosts

n+2 n+1



3 - The harmonics in question are mostly encountered in the low-frequency part of the sweep. The VE432, renowned as generating a fundamental ground force with a very small amount of harmonics, is especially well suited for Slip-Sweep operations in optimal conditions.


18




19



5. HIGH-LINE NOISE

5.1 GENERAL

The seismic signal on each channel may be corrupted by the 50 Hz or 60 Hz energy, known as High-Line noise, radiated by any nearby power line. As a result each trace will exhibit a spurious 50-Hz or 60-Hz sine wave added to the acquired seismic data. To get rid of the undesired sine wave, we can take advantage of the process involved in stacking (before or after correlation).

To this end, the background High-Line noise is picked up and fed to a bandpass filter through the High-Line Pickup circuitry. The sweep is first triggered, say, on the positive-going transition of the sine wave. The seismic signal resulting from the corresponding acquisition is correlated, then stored as "correlated seismic data + correlated positive-going noise". The subsequent sweep is triggered on the negative-going transition of the noise sine wave and, again, the acquired seismic signal is correlated against the same pilot as with the previous acquisition. The result is stored as "correlated seismic data + correlated negative-going noise".

By simply adding the results from these two correlated acquisitions, the stacking process will theoretically yield a result equal to twice the correlated seismic data, with no high-line noise left.

The number of sweeps for each Vibrated Point should be even so that the best possible rejection can be achieved. Also, the operator should make sure that any high-line noise is actually properly detected.


0



E 0

H I L I N E 1

C o m p a r a t o rS e c o n dn e t w o r k

F i r s tn e t w o r k

E i

D P G

S A P G b o a r d

1 0 m

H I L I N 1

H I L I N 2

H I L I N Eb a n a n a p l u g s

L 3 0 m

Hi-Line pick-up implementation

The length (L) of the noise sensor wires should be sufficient ( 30 m) not to pick up any 50/60 Hz noise radiated by the recording truck's petrol generator.

The Ei signal at the 50-60 Hz detector input should not be less than

16 mV peak to peak into 1.1 M.

If the input signal is too low, then an error message (no Hiline sync) is generated.

As shown on the filter response curve below, the filter circuitry makes it possible to detect either 50 Hz or 60 Hz noise signals without the need for any operator intervention to select one frequency or the other. (The High-Line noise pickup process is an analog function).

74 Hz

1

56.2 Hz

100 Hz60 Hz50 Hz32 Hz

Ei

2.0

0.25

0.4

2.15

Eo

0 10 Hz

Frequency

Sum of filter networks


1



5.2 IMPLEMENTING HIGH-LINE NOISE ELIMINATION ON THE HCI

High-Line noise elimination is implemented on the HCI by stacking an even number of sweeps (half with the Up option, half with the Down option as shown below).


2



With the above setups, the first two acquisitions will start on a positive going (up) transition of the power line signal and the other two on a negative going (down) transition. Stacking will eliminate the power line signal and keep only the data.

N o is e S ig n a l ( c o r re la t io n )

+

+

+

+

+

+

S ta c k in g

0


3



6. HARMONIC LINE ELIMINATION

6.1 INTRODUCTION

This chapter describes how harmonic lines contained in the Ground Force (GF) signal are removed by the additions performed in the stacking process, through successive, different phase shifts applied to a succession of sweeps — taken on the same point — if the same phase shifts are applied to the reference signal used for the correlation.

6.2 REVIEW OF PHYSICAL RELATIONSHIPS ON SINE WAVES

Prior to establishing a general law on the phase shifts to be applied, let us recall the necessary basics on the physical relationships involved in the process.

A phase shift between the reference and the Pilot causes an Nphase shift on the order-N harmonic in the Ground Force (GF).


0



This can be illustrated by the diagram below, showing an example with the second harmonic line (N = 2) and = 180° :

amplitude

2f1 GF harmonic

f1 Reference

time

without any phase shift

time

amplitude

2f1 GF harmonic + 360°

f1 GF Reference + 180°

with a 180° phase shift

2nd harmonic phase

The relative position of the vibrator's harmonic line 2f1 with respect to the reference f1 is

unchanged, but the phase shift is 180° () on the time scale of f1 and 360° (2) on the

time scale of 2f1.


1



The autocorrelation function of a sine wave (with f as frequency) is a sine wave with the same frequency (f) and a positive first peak located at the zero time.

The cross-correlation of two sine waves with the same frequency (f) but in phase opposition results in a sine wave with the same frequency (f) and a negative first peak located at the zero time.

The above two notions can be illustrated by the diagram below :

A

t

A

t

+

+ + +

A

t* =

A

t

A

t* =

A

t

- -

Autocorrelation

Crosscorrelation

It should also be noted that :

The harmonic line contained in the Ground Force correlates with the reference signal at the same frequency ;

The initial phase shift () applied to the sweep reference is also applied to the reference signal used in the correlation process.


2



6.3 GENERAL LAW

We wish the data to be in phase opposition before stacking, after correlation. The initial phase shift required between the two reference signals used in the crosscorrelation process is :

180 2 180

1

k

N

where N = is the order of the harmonic line

k {0, 1, 2, ... }

This generalized law can be illustrated by two examples, with the graphs of harmonic lines 2 and 3.

The graph of the 2nd harmonic (see next page) allows us to write that the 2nd harmonic in the Ground Force is itself a sine wave at twice the start frequency, varying at twice the sweep rate. As a result, those frequencies which are common to the reference will correlate. Considering a reference frequency from f to 2f in the graph, the 2nd harmonic frequency will be 2f to 4f. Since the reference contains no frequencies higher than 2f and the Ground Force harmonic contains no frequencies lower than 2f, the harmonic correlated result will contain only the 2f frequency. The harmonic correlated result can be thought of as a sine wave correlated with a sine wave giving a sine wave at the same frequency but shifted by 90°.

In fact the 2nd harmonic will produce an "added Ground Force" at twice the sweep rate of the reference signal and containing only the frequencies from the lowest 2nd harmonic frequency to the highest reference frequency.

If we are capable of obtaining phase oppositions on the harmonic correlated results, from one sweep to another, then the harmonic correlated results will vanish through the addition performed in the stacking process. Now, as seen earlier, a phase shift on the reference causes an N phase shift on the order-N harmonic in the Ground Force. It is therefore easy to identify the phase shifts required of the harmonic correlated results.

The same can be applied to the 3rd harmonic.


3



H2+NH2

Pilot signal Shifted Pilot signal

-

+

2f1 4f1 4f1+360°2f1+360°

Reference Harmonic 2f1 Shifted Reference Harmonic 2f1+2

+ +

Fund. Fund.+

f1+180°

2f1f12f1+180°

Reference signal Shifted Reference signal

Time

Time-

+

Pilot Harmonic 2f1 Shifted Pilot Harmonic2f1+2

+ +

+

Fu

nd

am

enta

l

First sweep Second sweep

Har

mo

nic

+

Cor

rela

tion

Stacking

Stacking

Cor

rela

tion

Cor

rela

tion

Cor

rela

tion


4



Ref. Ref.+

H3+NH3

f1+90°

3f1f1

3f1+90°

Reference signal Shifted Reference signal

Time

3f1 9f1 9f1+270°3f1+270°

Reference Harmonic 3f1 Shifted Reference Harmonic 3f1+3

+

+

-

+ Time

+

Pilot signal

++

Co

rrel

atio

n

Co

rrel

atio

n

Pilot Harmonic 3f1

+

-

+

Co

rrel

atio

n

Shifted Pilot signal

+Stacking

Shifted Pilot Harmonic3f1+3

+Stacking

Fu

nd

am

enta

lH

arm

on

ic

First sweep Second sweep

Co

rrel

atio

n


5



6.4 EXAMPLE OF PRACTICAL IMPLEMENTATION

Assuming 8 sweeps are to be taken for each Vibrated Point, and harmonic lines 2, 3 and 5 are to be removed :

For harmonic lines 2:

Nk

k

2180 2 180

2 1180 360

As a result, = 180° removes harmonic line2.

For harmonic line 3 :

Nk

k

3180 2 180

3 190 180

= 90° removes harmonic line 3.

For harmonic line 5 :

Nk

k

5180 2 180

445 90

= 45° removes harmonic line 5.


6



180°

180°

180°

180°

90° 90°

90°90°

45°45°45°45°

1 2 3 4 5 6 7 8

+0 +45 +90 +135 +180 +225 +270 +315

Sweep

Reference

H5OFF

H5OFF

H5OFF

H5OFF

H3OFF

H3OFF

H3OFF

H3OFF

H2OFF

H2OFF

H2OFF

H2OFF

Harmonic 5

Harmonic 2

Harmonic 3

Combinations of 8 successive sweeps in pairs,

resulting in the removal of harmonics through the stacking process.


7



6.5 APPLYING THE PRINCIPLE

The harmonics can be removed by adding two signals with opposite phase, but also by adding N signals shifted by 360°/N.

Example: 6 sweeps shifted by 60°

Phase of harmonic before correlation

Phase of harmonic after correlationVector

HarmonicSweep Sweep sum

N 1 2 3 4 5 6 1 2 3 4 5 6

Fundamental 0 60 120 180 240 300 0 0 0 0 0 0 6

2 0 120 240 0 120 240 0 60 120 -180 -120 -60 0

3 0 180 0 180 0 180 0 120 -120 0 -240 -120 0

4 0 240 120 0 240 120 0 180 0 -180 0 -180 0

5 0 300 240 180 120 60 0 240 120 0 -120 -240 0

6 0 0 0 0 0 0 0 -60 -120 -180 -240 -300 0

7 0 60 120 180 240 300 0 0 0 0 0 0 6

8 0 120 240 0 120 240 0 60 120 -180 -120 -60 0

9 0 180 0 180 0 180 0 120 -120 0 -240 -120 0

10 0 240 120 0 240 120 0 180 0 -180 0 -180 0

11 0 300 240 180 120 60 0 240 120 0 -120 -240 0

12 0 0 0 0 0 0 0 -60 -120 -180 -240 -300 0

13 0 60 120 180 240 300 0 0 0 0 0 0 6

We can see that after correlation the primary (first harmonic) is in phase.

The terms in the second harmonic cancel since there are three sets of wavelets each 180° out of phase.

The terms in the 3rd harmonic cancel since the first, second and third sweeps are 120 degrees out of phase and have vector sum of zero.

Fourth harmonic: three sets 180° out of phase.

Fifth harmonic: same as third.

Sixth harmonic: six wavelets each out of phase by 60 degrees.

You can see that all terms up to and including the sixth harmonic are zero, while the seventh harmonic does not cancel.


8




9


7. DPG MASTER SLAVE OPERATION

See note (*)

A convenient way of reducing the cost of field operations consists of using two recording trucks in parallel, recording a greater number of traces without increasing the number of vibrators. One recording truck is used as "Master" and the other as "Slave".

Each recording truck is equipped complete with an acquisition system and a DPG.

One of the recording trucks is used as "Master" unit and the other as "Slave".

The DPG in the Master unit does not only perform the standard function (controlling all DSDs and generating a reference signal), but it also controls the DPG in the Slave unit.

The DPG in the Slave unit needs to be configured with the "DPG-Slave" software (PCMCIA card). It is then seen as a DSD from the Master DPG. Once the "DPG-Slave" software is loaded into a DPG, the function of the Slave DPG consists of generating a reference pilot signal synchronous with the Time Break. A Slave DPG does not control any DSD.

An example of Master/Slave configuration is shown on next page, involving two recording trucks.

The Slave DPG receives a DSD-like address (#29 to 32) from the Master DPG. Once addressed by the Master DPG, the Slave DPG acts as a DSD. It receives the t0 parameter and t0 Sync from the master DPG. On the first correct reception of a t0 parameter, the DPG generates a relay contact closure (EXTGO signal). This relay contact closure is used in the recording truck to start the recording.

The time denoted T on the timing diagram may vary from one recording system to another.

In order to keep the time between the Firing Order and the Time Break to a minimum, and to cause the Slave recording truck to initiate the 'Waiting for TB' sequence just before the Sync is decoded, giving rise to the TB, you may need to adjust the number of T0 parameters to be transmitted, by modifying the 'T0 Repeat time' parameter. Try different values until the best possible cycle is achieved.

(*) SN388 software version 8.4 or higher


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8. NAVIGATION

The navigation is an option that can be implemented on the VE432. The two new operation modes are shown in the options setup of the operation environment:

- Source navigation

- Slip Sweep navigation

These two options require a stack of one, the input of the SPS S file, and a DGPS in each vibrator.

8.1 Implementation

A license free microwave radio (2.4 GHz, 200m range) needs to be installed in each vibrator that allows communications within a fleet.

The DSDs need to be slightly modified:

- A modified NAV plug need to be installed and connected to the DVC board's Ethernet socket inside the DSD.

- A cable connects the microwave radio and the DGPS to the NAV plug.

In each fleet, one microwave radio needs to act as a master (an external switch allows this), usually the one of the leader.

With this option, as soon as the leader is ready (pad down), it queries each of the other DSDs in the fleet to see if they are ready using the micro wave radio. When they are ready,


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they send their GPS position to the leader. As soon as all the vibrators are ready, the leader computes the COG of the fleet and sends this location with the ready message to the DPG.

The DPG relays the message to the SN388 HCI that displays the location of the fleet in the Positioning environment (see the example below). This allows the system to identify this location and to shoot automatically the corresponding VP. VPs can be shot in any order by any fleet.

In the operation environment, there is no delay to setup, and the GO button remains gray until the ready is received. Everything is started at maximum speed.


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Example of fleets display before acquisition is launched in the positioning environment in navigation mode.

With no DSD network, the fleet location displayed correspond to the location of the leader vibrator.



8.2 One fleet or Flip-Flop navigation setup

When the navigation option is chosen, one aux. line process descriptor will correspond to each fleet.

There is no modification in the acquisition table only one process type is needed.

In it, a stack of one must be used. In the Aux. Corr. Descr., the button by fleet needs to be selected.

Then the auxiliary description has to be entered for each source. In the example below, the description is done for flip-flop operation; therefore two fleets are described, using the same DPG and the same pilot.


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Process description for flip-flop operation with the navigation option.



8.3 Slip sweep navigation setup

The setup is similar to the one of single fleet and flip-flop one.

When the navigation option is chosen, one aux. line process descriptor will correspond to each fleet; there is no modification in the acquisition table except only one process type is needed.

In the Aux. Corr. Descr., the button by fleet needs to be selected. Then the auxiliary description has to be entered for each fleet.

Using this feature, an acquisition is started as soon as any fleet is ready at any VP.


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Process description for slip sweep operation with the navigation option for 4 fleets.

Date post:	12-Nov-2014
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