Single-Sweep Methods Offer Efficiency Gains

?In an increasingly aggressive industry, the seismic acquisition business consistently must look for ways to remain competitive. Only those companies that are able to keep costs down and production high can compete. Seismic crews must become efficient in every aspect of the recording operation. Spurred by the quest for more productivity, a number of Vibroseis techniques have developed over the last 15 years. The most popular of these techniques include:

• Slip sweep – the second vibrator fleet shaking

after the first fleet has finished but while the recorder still is recording. The recording system can separate records based on start time and sweep parameters. There always will be some level of interference from the harmonics of the second fleet of vibrators in the first correlated record. Depending on the level of harmonic interference relative to the signal, some additional noise removal might be necessary;

• Distance-separated simultaneous recording – simultaneous recording of two or more source points with fleets separated by enough distance that the opposing fleet does not interfere with the data at the target horizon;

• Random simultaneous recording – multiple fleets all shaking independently at random times with different sweep parameters and positions. Because several locations can be acquired at the same time, the recording truck is unable to separate them, requiring a complex separation and noise-removal process to be run afterward; and

• High-fidelity vibratory seismic – using phase-rotated sweeps to separate out multiple positions acquired simultaneously.

The common principal in all of these Vibroseis techniques is that more vibrator points (VPs) can be shaken if two or more fleets are working simultaneously. This certainly is true, but additional production usually comes at the cost of some level of interference between the vibrators.

A different methodology

One under-utilized method for getting higher production without causing interference between vibrators is to use a single long-duration sweep. In the 1990s, almost all 3-D seismic data were acquired with four or more short sweeps. This tradition goes back to 2-D acquisition geometries, where multiple sweeps created a large source array.

With newer vibrators and vibrator control electronics, it is possible to shake the same amount of time, putting the same amount of energy into the earth, with a longer single sweep. This in turn necessitates only one listen time, causing no change in data quality. Vibrating with a single long sweep has additional advantages:

• A longer sweep reduces the effects of start-time tapers, thereby adding to the very low-frequency content. Very low frequencies can help to both widen the bandwidth of the data and improve deep imaging;

• The shorter overall time allows less opportunity for environmental noise such as vehicle traffic to intrude on the data;

• If something causes a bad record, it takes less time to reacquire the single sweep than it does for multiple sweeps; and

• The output SEG-D record requires no special processing to remove noise or harmonics from other simultaneous sweeps as is necessary with other simultaneous techniques.

Validation

Sweep tests on multiclient jobs acquired by Global Geophysical Services have allowed the company to verify that the data content is consistent with a single long sweep when compared to multiple shorter sweeps.

For faster production, the long single sweep is the most efficient. Over a one-month period (23 days plus weather days) on a recent project using a 36-second single sweep with a six-second listen time, the single sweep approach cut approximately nine days off the multiple sweep format. The average production was 455 VPs per day, totaling 10,456 VPs. Disregarding line problems or other delays and factoring in only the sweep and listen times, this works out to 5.3 hours of production per day. If the four- by nine-second sweeps had been used with the same six-second listen time after each sweep, the same 5.3 hours per day would provide approximately 318 VPs per day, with the same 10,456 VPs taking more than 32 days to complete. Using the single sweep allowed the crew to finish 28% faster than if it had used four sweeps for the same sweep effort.

Looking more in depth at the numbers, purely on total time to shake a VP, the time savings varies inversely on total sweep time directly with listen time. For long listen times, the single-sweep method can increase productivity by more than 40% with less total sweep time. For the examples shown with a six-second listen time, efficiency can be increased by 25% with a 48-second total sweep effort, up to 37% when sweeping 24 seconds total.

Whatever the parameters for sweep and record length, the single-sweep option will provide the data faster so the vibrators can move to the next point and the crew can move to the next project.

While there is bound to be an exception, so far single sweeps of long duration and multisweep efforts have produced comparable data. In conjunction with client partners, Global has successfully used single-sweep recording in the unconventional resource plays of Eagle Ford, Haynesville, Niobrara, and Bakken, all with excellent data quality results.