4.3.2Hardware management

Because 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.

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.

4.3.3Hci operation environment

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

• OPERATION SETUP

You 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).

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.

• DPG ENVIRONMENT SETUP

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