4.4More 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.

• Slip-sweep ghosts:

1 - In flip-flop mode, acquisitions are totally independent. Therefore, harmonics from a sweep cannot mix with those from another sweep.

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

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.

5.High-line noise

5.1General

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.

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.

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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).