Method for continuous sweepting and separtion of multiple seismic vibrators

Abstract

A method for simultaneously operating multiple seismic vibrators using continuous sweeps (little or no “listening” time between sweeps) for each vibrator, and recovering the separated seismic responses for each vibrator with the earth signature removed. Each vibrator is given a unique, continuous pilot signal. The earth response to the motion of each vibrator is measured or estimated. The vibrator motion records for each vibrator and the combined seismic data record for all the vibrators are parsed into separate shorter records. The shorter records are then used to form a system of simultaneous linear equations in the Fourier transform domain, following the HFVS method of Sallas and Allen. The equations are then solved for the separated earth responses.

Description

[0001] This application claims the benefit of U.S. Provisional Patent Application No. 60 / 494,194 filed on Aug. 11, 2003.FIELD OF THE INVENTION [0002] This invention relates to the field of seismic data acquisition. Specifically, this invention is a method of acquiring seismic data using multiple vibrators without a multiple sweep listening time requirement. BACKGROUND OF THE INVENTION [0003] Seismic vibrators have long been used in the seismic data acquisition industry to generate the acoustic signals needed in geophysical exploration. The conventional use of vibrators involves several well-understood steps. First, one or more vibrators are located at a source point on the surface of the earth. Second, the vibrators are activated for several seconds, typically ranging from four to sixteen, with a pilot signal. The pilot signal is typically a sweep signal that varies in frequency during the period of time in which the vibrators are activated. Third, seismic receivers are used to rece...

AI Technical Summary

Benefits of technology

[0016] In other embodiments, the vibrator motion records and the seismic data records may be parsed into M shorter records where M>N. In addition, the motions of the vibrators may be approximated by using theoretical waveforms for the source signatures instead of measured motion records. Harmonic noise is further reduced in preferred embodiments by using sweep signals for the multiple vibrators that differ from each other only in the phase rotation of one or more sweep segments.

Problems solved by technology

Several problems are known to exist with conventional vibrator technology.

First, the correlation process is known to result in correlation side lobes, which can Influence the accuracy of the final processed data.

Second, vibrator harmonic distortion results in noise, known as harmonic ghosts, after correlation with the pilot.

In addition, multiple sweeps are often required to inject sufficient energy into the ground.

However, the use of multiple sweeps with each sweep followed by a listening time limits the rate at which energy can be put into the ground and the survey acquired.

Fourth, the recording of high frequencies can be limited by the simultaneous recording of the signals from an array of vibrators, each vibrator at a different position and elevation and having a different coupling with the ground.

The cost of land surveys largely depends on the time it takes to record the survey, and cost is affected by the length of time required to record data at each source station as well as the time it takes to move the vibrators to the next station.

Nevertheless, even with this improved quality data the limitations of the correlation process, problems with harmonics ghosting, and array limitations are present.

Separation of the data is imperfect.

If the sweeps are cascaded without a listening time, then the reflection data from one segment would interfere with data from the subsequent sweep segment.

In addition, there would not be a one-to-one correspondence between the data and the measured vibrator motions which represent the signatures put into the ground, so that harmonics would not be handled properly.

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