Method of using cascaded sweeps for source coding and harmonic cancellation

#This invention relates to the field of geophysical prospecting and, more particularly, to acquiring seismic vibrator data.

#The seismic exploration industry uses acoustic impulses to impart sonic vibrations into the earth to delineate subsurface structure for mineral exploration and development. These acoustic impulses may be from an explosive, implosive, swept-frequency (chirp) or random source. A recording of the acoustic reflection and refraction wavefronts that travel from the source to a receiver is used to produce a seismic field record. Variations in the travel times of reflection and refraction events in these field records indicate the position of reflection surfaces within the earth. The analysis and correlation of events in one or more field records in seismic data processing produces an acoustic image that demonstrates subsurface structure. The acoustic images are used to find valuable mineral deposits.

#The swept-frequency or chirp type seismic source may utilize a relatively long pilot signal such as 2 to 15 seconds to assure sufficient energy is imparted to the earth. The swept-frequency or chirp type source method relies on signal compression to compress the signal and ensure sufficient vertical resolution to resolve the position of subsurface reflectors. Signal compression generally is called deconvolution, with many techniques well known in the art of seismic data processing. Deconvolution of sweep or chirp signals compresses the source signal into a much shorter signal representative of a subsurface reflective boundary. The accuracy and effectiveness of any deconvolution technique is directly related to how well the source signal is known or understood. Most deconvolution operators are derived from statistical estimates of the actual source waveform.

#The present invention and its advantages will be better understood by referring to the following detailed description and the attached drawings in which:

#FIG. 1 illustrates the overall structure of the invention and its objectives.

#FIG. 2a illustrates a cascaded sweep with phase rotation.

#While the invention will be described in connection with its preferred embodiments, it will be understood that the invention is not limited thereto. On the contrary, it is intended to cover all alternatives, modifications, and equivalents that may be included within the spirit and scope of the invention, as defined by the appended claims.

#The overall structure of the invention is best understood with reference to FIG. 1. This will aid in understanding the detailed description that follows. The method of the invention 1 as shown in FIG. 1, uses cascaded sweeps for acquiring seismic data. The purpose of the invention 3 is to be able to separate data from a plurality of sources as well as attenuate harmonics. In one embodiment of the invention, this is achieved by correlating a reflected seismic signal with a theoretical, predetermined signal related to the pilot sweep used to drive the vibrators 5. This may be done for any number of sources and any desired level of harmonic cancellation 9. Broadly speaking, when the correlation signal is related to the pilot sweep, each vibrator is swept with N sweep segments and the correlation signal comprises N+1 segments 13.

#In an alternate embodiment of the invention, ground force signals are measured at the vibrators and the correlation signal is related to the ground force signals 7. As with the first method, any number of sources may be used and any degree of harmonic cancellation is possible 11. As with the first embodiment, vibrators are swept with N sweep segments; however, the correlation signal comprises N+2 segments of recorded ground force data. This is explained below.