1. Field of the Invention
This application relates to noise reduction methods; specifically, it relates to methods and apparatus for reducing unwanted sound waves produced by seismic vibratory sources.
2. Background of the Art
Geophysical surveys to estimate the depth, shape, and composition of subterranean formations commonly use seismic vibrators to induce seismic waves, which may be detected using geophones. Seismic vibrators typically vibrate according to a controlled sweep of frequencies. In the commonly used xe2x80x9cupsweepxe2x80x9d methods, the vibrations start at a very low frequency and ending at a high frequency. In downsweeps, the vibrations start at a high frequency and end at a low frequency. Both compressional waves (xe2x80x9cPxe2x80x9d waves) or shear waves (xe2x80x9cSxe2x80x9d waves) may be used for the purpose.
Geophones, usually placed in an array or grid-like pattern on the surface of the earth or just beneath, are used to detect reflections of vibrations from rock strata. Measurement of the intensity and time delay of an arriving reflected wave at numerous locations allows the mapping of rock strata, and gives information about the thickness and composition of the layers.
Each of the rock layers underneath a seismic vibrator reflects the seismic waves induced at the surface according to its contrast in acoustic impedance. For example, an interface in which a low impedance layer lies above a high impedance layer will reflect a large proportion of the incident wave; therefore, the reflected wave will be of greater amplitude than an interface in which the layers on opposite sides of the interface have a small difference in impedance.
Geophones typically record the amplitude of detected vibrations at a given time for later analysis. When a vibratory source of energy is used, a correlation operator is commonly used to xe2x80x9ccompressxe2x80x9d the record so that the arrival times of various reflection signals can be estimated. This xe2x80x9ccompressionxe2x80x9d of data is not necessary when an impulsive source of energy is used. Determination of the number of layers, and their depths are made through comparison of the amplitude of the wave with the time at which the reflection arrived after the initial induced vibration. The time-delay for a reflected wave to arrive at a geophone is an indication of the depth from which the wave is reflected.
Air-coupled waves are coherent noise trains produced by a surface seismic source, propagating at the speed of sound in air. Air waves may be entirely coupled with the air, or, in the case of low frequency (6-8 Hz) waves, may be partially coupled with the near surface if the phase velocity of the Rayleigh wave and the speed of sound in air are the same.
The latter has been described in Press and Ewing, xe2x80x9cGround Roll Coupling to Atmospheric Compressional Wavesxe2x80x9d, Geophysics 16, pp. 416-30. Seismic vibrators usually operate above ground, with the vibrational energy transmitted into the earth via a baseplate resting on the ground. In field surveys, it is common to make use of a vibrator mounted on a truck. Since the majority of the vibrator is exposed to the air, including the upper surface of the baseplate, some of the vibrational energy during operation is transmitted through the air as sound waves.
These air-coupled sound waves are often of sufficient intensity to disrupt or impair measurements. The reflection seismic signals are small in magnitude and waves propagating through the air may cause slight vibrations of the geophone or of the ground itself, which are of relatively high amplitude, causing air-coupled waves to be recorded. Recordings of the air-coupled waves can be of sufficient intensity to mask underlying moderate depth reflection data. Because air wave noise can cause the ground to vibrate, burial or shielding of geophones fails to alleviate the problem.
Air wave noise is strongest at higher frequencies (i.e., 30 Hz and above). Prior art filtering techniques for removing Rayleigh waves (typically having a frequency less than 15 Hz.) have proven ineffective at such high frequencies because current geophone group spacing is too great, and creates a spatial aliasing problem. The effects of air wave noise may be suppressed through more closely spaced arrays of geophone elements; however, this results by an increase in the cost and complexity of conducting a survey.
U.S. Pat. No. 4,922,473 to Sallas (the ""473 patent) discloses a passive absorption system for attenuation of air waves. In passive absorption, attenuation is achieved using a rigid, non-resonant structure which encloses the baseplate and is isolated from the vibrations of the baseplate and the ground. A cabinet provided with air bags is used as the enclosing structure. The problem with passive methods is that it is very difficult to build lightweight rigid attenuating structures that are well sealed.
U.S. Pat. No. 4,930,113 (the ""113 patent) also to Sallas discloses an active cancellation method. In the ""113 patent, base plate accelerometer signals are used to drive loudspeakers to produce a counterpart wave that is equal in amplitude and opposite in phase to the base plate motion. This method requires the generation of acoustic energy of a power level equal to that produced by the source. This requires a high power speaker and a large power source. In addition, the system must be well matched to produce real-time signals that are opposite in polarity to the base plate signal. This adds to the complexity of the system.
U.S. Pat. No. 4,890,264 to Crews et al discloses the use of microphones to record the airborne noise and using the recorded air noise signal for adaptively filtering the data recorded by the geophones. For successful airwave suppression, this method requires a microphone at each geophone location.
There is a need for a simple system with few components that is able to suppress airborne noise. Such a system should preferably have low power requirements. The present invention satisfies this need.
The present invention includes an acoustic energy source (speaker system) used in conjunction with the seismic source. As described here, the seismic source is a vibratory source, although the present invention is not limited to vibratory seismic sources. The speaker system produces a sound signal that propagates primarily through the air. In one embodiment of the invention, microphones are located near the seismic spread and arranged to be decoupled from ground vibrations. In another embodiment of the invention, the mechanical decoupling is not used: instead, an acceleration canceling microphone is used. A transfer function between the microphone and the geophones is determined from the speaker signal and used to filter out the airborne noise produced by the vibrator. In one embodiment of the invention, the speaker system signal is uncorrelated with the vibrator sweep. In another embodiment of the invention, the speaker signal may be correlated with the vibrator signal. In the alternate method, two vibrator sweeps are executed with opposite speaker polarities so that the sum and difference of the two records provides the speaker and the vibrator data respectively. In either case, the speaker system is made of two speakers located equidistant and on opposite sides of the baseplate so as to be able to simulate a speaker centered at the baseplate location. On vibrators with no drive shaft and a tall stilt structure, a speaker at the center of the baseplate is used. In an alternate embodiment of the invention, a single speaker is used and a duct is run to the center of the baseplate so that the speaker sound emerges at the center of the baseplate.