4.Slip-sweep
4.1Overview
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.
4.2General description
4.2.1Principle
A 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.
- 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.
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.
TB
Seismic traces
Auxiliary trace
(Pilot)
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.
Naturally, slip-sweep requires that no stacking be performed.
4.2.3Cycle time
We 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
Flip-flop
Production: 4 records in 96 s 24 s per record.
Theoretical maximum: 21 s per record (listening time + sweep time).
Slip-sweep
Production: 8 records in 101 s 12.6 s per record.
Theoretical maximum: 6 s per record (listening time).
4.3Operation with sn388 and ve432
4.3.1Connections
On 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,
- a2 if the DPG is managing fleet 2,
- a3 if the DPG is managing fleet 3,
- a4 if the DPG is managing fleet 4.
In addition to the Time Breaks the pilots need to be connected.
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.