TF1.7 How to Guides_Flip-Flop Shooting

How to Guide –

Flip-flop shooting

User Manual - TF1.7 How to Guide – Flip-flop shooting Page 2 of 10

User Manual - TF1.7 How to Guide- Flip-flop shooting Page 2 of 10

Version History

Date Revision TF Version Author(s)

Oct 2012 Draft A 1.7.12 T. Sanerkin

Dec 2012 Rev 1.1 1.7.18 T. Sanerkin



Contents

1. Shot prediction methods ................................................................................................................ 4

1.1 Discrete array prediction (DAP) .............................................................................................. 4

1.2 Gun array mean prediction (GAM) ......................................................................................... 5

2. Configuration .................................................................................................................................. 6

3. Gun controller ID and Gun Array Code ........................................................................................... 6

4. Trigger Options ............................................................................................................................... 7

5. GAM (Gun Array Mean) .................................................................................................................. 8

5.1 To add a GAM node ................................................................................................................ 8

5.2 Shooting Tasks (flip-flop) ...................................................................................................... 10

5.3 Single sail line ........................................................................................................................ 10

5.4 Discrete source lines ............................................................................................................. 10



Flip-flop shooting

1. Shot prediction methods

TriggerFish supports two methods for shot prediction in flip-flop (dual array) mode; discrete array

prediction and gun array mean prediction.

1.1 Discrete array prediction (DAP) In this mode, each shot is fired according to a prediction made for the next array to fire based on the

solved vectors for that array’s centre (i.e. not based on a vessel layback). This mode will produce the

most accurate shot position for each array with respect to preplot but is subject to inconsistent shot

time interval if the in-line array separation varies significantly from the nominal configuration. In-line

separation may change during the line due to currents and steering effects. The graph below shows

how the minimum inter-shot time will vary with increasing deviation from the nominal geometry. If

seismic record length is long and shooting speed is high, the tolerance to variation in in-line

geometry will be reduced as a seismic record may be “talked-over” by the following shot.



1.2 Gun array mean prediction (GAM) In this mode, the prediction for both arrays is made using the vectors of the solved mid-point

between array centres. The result is a very consistent shot time interval at the expense of shot

accuracy versus preplot, there is no risk of records being talked over, unless the shooting speed is

too high. 4D Source position repeatability may be compromised in this mode as in-line array

geometry may change from survey to survey. In-line error is half the in-line array geometry deviation

as per the graph below.



2. Configuration

A second array may be added to the vessel in System Editor by selecting the vessel in the map

window and dragging an array onto it from the vessel editor tools palate. Strings, guns depth and

pressure sensors and GPS nodes may then be dragged onto the array. A quick method of adding a

second array to the vessel is to copy and paste the first array. To copy the array, select it and then

choose “copy” from the System Editor “Edit” menu; then from the “edit” menu select past.

In the “Paste Object Changes” dialogue, enter the positional offsets for the new item; these dictate

where the new item will be created and the offsets are relative to the item being copied.

Name change “‘n’ Addon:” will increment the item number. In the Gun Array section the Name field

may be updated to provide the array prefix, “nn” will be replaced by the “‘n’ Addon:” value. “Gun

controller ID Addon” provides the increment for the gun controller ID. Addon values for the guns,

depth sensors and pressure sensors will provide the item numbers for the new array; for instance if

each array has 24 guns, then the Addon value should be set to 24, this will create a gun number of

25 for the first gun on the new array.

3. Gun controller ID and Gun Array Code

The gun controller ID value will be used in the array selection message sent to the gun controller in

the arm message; this should match the gun controller internal setting for array number.



The Gun array code value is used within Triggerfish, for instance, the SPS array code logged will use

this value. Normally these values will be the same.

4. Trigger Options

Four options are available for firing in flip-flop mode. These are selected in the shooter controller

dialogue. These are selected from the “Trigger position” drop-down menu.

Source

The shot event is predicted based on the solved array centre. For dual-source (flip-flop) each

array is predicted for individually (if the GAM node box is not ticked).

CMP (only applicable to streamer vessels).

The computed midpoint between the array to fire and the CNG of the streamer is used as

the trigger position.

Vessel Layback

The trigger position is calculated on the layback value entered by the user. The layback is

calculated from the vessel reference position as is based on the vessel track.

Source layback

For each array, the trigger position is based on the defined source layback distance in the

System Editor.



5. GAM (Gun Array Mean)

This mode works in combination with the trigger position; it is only applicable to the dual source

(flip-flop) operation. For this option to function a “GAM” node must be defined on one (only) of the

gun arrays.

5.1 To add a GAM node

In system Editor, select a gun array and from the Vessel Editor tools palate drag and drop a “Gun

Array Mean” item onto the array. The GAM node needs no offset. The name can be changed but the

default should be OK.



With the GAM node defined, and the “Trigger Position” in the shooter servlet options set to

“Source” and “Use gun array mean” is ticked, the trigger position will be calculated as the mean

array centre position; this can have advantages if the relative in-line positions of the two arrays

changes due to tow geometry or crab-angle; it ensures that time between shots is consistent,

preventing over-lapping of shots in the seismic records. The positional relationship between preplot

and the fired position is, however, compromised as a result and the in-line fired position may not fall

exactly on the preplot.

The GAM node is displayed on the map mid-way between the arrays



5.2 Shooting Tasks (flip-flop)

Shot tasks may be configured for flip-flop lines using a single sail-line preplot or by defining different

lines for each array.

5.3 Single sail line

If only sail line preplots are available, it is possible to set up the task

using a single line.

In the “Location” section select the swath and in the “Steer” field select

“Mid-line”.

Below the Location section there are two sections for the defined gun

arrays. As only one line is being used, select the same line for each

array. Then using the “Begin Shot”, “End Shot” and increment fields

define the shots that will be fired for each array.

Tip: - Select the shot increment first, and then select the “Begin Shot”

value; the “End Shot” drop-down will then be automatically filtered to

show only valid values for the last shot.

Tip:- The line name field may be edited

5.4 Discrete source lines

Source SPS files may be used directly in the task set-up, saving the need to produce specific sail-line

preplots. TriggerFish will automatically generate the vessel sail-line as the mid-way line between the

source lines.

In this instance, select the swath and mid-line for the steer field. For

each array, select the appropriate preplot line and shot increment.

The mid-line (sail-line) is automatically generated in TriggerFish.

Documents

TF1.7 How to Guides_Flip-Flop Shooting