What’s new in exploration

 Vol. 232 No. 8

After 50 years of development, seismic vibrators remain firmly entrenched as the industry’s workhorse for acquiring seismic on land. Geophysicists and engineers continue to refine these acquisition systems.

Slip-sweep. In 1996, H. J. Rozemond described slip-sweep acquisition at the 66th annual meeting of the Society of Exploration Geophysicists. In the technique, a particular sweep begins at a source point without waiting for the previous sweep to terminate. This time overlap decreases the cycle time for recording data and increases survey acquisition rates, but can also introduce harmonic noise. 

In the absence of harmonic noise, if the time interval between the start of consecutive sweep sequences (slip time) is greater than the listening time, then the seismic responses to the consecutive sweep sequences do not overlap in the time-frequency domain, which facilitates separation of the measurements.

At the 72nd annual SEG meeting in 2002, Julien Meunier and Thomas Bianchi of CGGVeritas described how decreasing harmonic noise could reduce array size in seismic vibrator operations.

CGGVeritas developed its high-productivity vibroseis acquisition (HPVA) system to reduce harmonic noise while increasing shot density. HPVA is a deterministic noise attenuation technique that uses ground force measurements to remove the harmonic noise created by short slip times. Through Ardiseis, a regional joint venture with Abu Dhabi National Energy Company (Taqa), HPVA was tested and deployed in Oman.

According to PDO’s Paul Matheny, the company has acquired more than 60 slip-sweep 3D seismic surveys during a decade of exploration (1998–2008).

Dithering. Last year, Claudia Bagaini and Ian Moore received a US patent for a dithered slip-sweep seismic vibration acquisition system and technique. Generating the sweeps entails temporarily arranging them into time-overlapping groups. The technique also involves regulating the timing of the groups based on a slip time. The consecutive sweep firings of each group are spaced apart by substantially less than the slip time.

Simultaneous sweeps. At the SEG annual meeting in 1998, WesternGeco’s Craig G. Beasly, Ron Chambers and Zerong Jiang suggested a simultaneous-source technique for marine seismic acquisition but published only an extended abstract. Others have extended the concepts to onshore acquisition systems. A key concept is that, when the source separation is greater than the longest generated wavelengths, effective array response is diminished and simultaneous sources appear distinct.

At the European Association of Geoscientists and Engineers’ vibroseis workshop in October 2008, Jack Bourska discussed BP Oman’s development of distance-separated simultaneous sweeping (DSSS or DS3), calling it the “world’s fastest vibroseis technique,” based on a broad distribution of synchronized vibrating sources. At the Society of Petroleum Engineers’ Abu Dhabi conference in November 2010, Bourska presented results from a recent DS3 survey in Oman. It involved 15 vehicles, spaced 12 km apart, simultaneously generating wave fronts. The subsurface response was recorded by 8,000 receiver channels and resulted in a peak acquisition rate of 1,024 records per hour. The source separation causes the wave fields to overlap below the zone of interest and eliminates the need for special data processing or noise attenuation.

Low frequencies. Shell and CGGVeritas ran a low-frequency seismic vibrator test in 2009. The test objective was to emit and record low-frequency (down to 1-Hz) energy using a seismic vibrator and to assess the propagation characteristics of dedicated low-frequency sweeps using both downhole receivers (Geospace HS-1, 2 Hz at 1-km depth) and surface recordings (3-km-long line, with receivers deployed at 15-m increments).

Using a conventional vibrator on an articulated hydrostatic vehicle (AHV-III) from Ion Geophysical, the team recorded about 200 noise records with different record lengths (10–70 seconds) and, at one location, repeated sweeps 360 times to provide records with a high signal-to-noise ratios. Repeating low-frequency sweeps improved the signal-to-noise ratio, and the team detected 2-Hz signals both downhole and along the surface line.

DS3, DS4, ISS. In April 2011, the Bureau of Geophysical Prospecting (BGP, owned by China National Petroleum Corp.), announced the completion of a high-productivity 3D seismic vibrator survey test for PDO in Oman.

BGP used equipment from Inova Geophysical (a BGP-Ion joint venture), including the Aries II recording system, Vib Pro source controllers, and AHV-IV vehicles. The purpose of the field test was to validate the capabilities of Inova recording technology in acquiring slip-sweep, distance-separated simultaneous sweep (DS3), distance-separated simultaneous slip-sweep (DS4) and independent simultaneous sweep (ISS) data. The ISS technique was published by Dave Howe et al. of BP in 2008, and relies on randomized source start times and continuous recording.

During the test, the Aries II-Vib Pro control system successfully recorded both DS3 and DS4 data with dynamic fleeting, in rough terrain. The peak rate for DS4 surpassed 830 vibration points per hour.

New-generation acquisition equipment is increasingly rugged, land surveys are speedier, more data can be recorded over a broader range of frequencies, and wireless systems are more common. Whether or not explorationists can make better decisions with the plethora of seismic data remains to be seen.