This invention disclosure relates to the processing of marine seismic data, and in particular, to the mitigation of noise in the processing of pressure and particle motion signals recorded in a multi-component marine seismic survey.
This section of this document is intended to introduce various aspects of art that may be related to various aspects of the present invention described and/or claimed below. This section provides background information to facilitate a better understanding of the various aspects of the present invention. It should be understood that the statements in this section of this document are to be read in this light, and not as admissions of prior art.
Seismic exploration involves surveying subterranean geological formations for hydrocarbon deposits. A survey typically involves deploying acoustic source(s) and acoustic sensors at predetermined locations. The sources impart acoustic waves into the geological formations. The acoustic waves are sometime also referred to as “pressure waves” because of the way they propagate. Features of the geological formation reflect the pressure waves to the sensors. The sensors receive the reflected waves, which are detected, conditioned, and processed to generate seismic data. Analysis of the seismic data can then indicate the presence or absence of probable locations of hydrocarbon deposits.
Some surveys are known as “marine” surveys because they are conducted in marine environments. Note that marine surveys may be conducted not only in saltwater environments, but also in fresh and brackish waters. Marine surveys come in at least two types. In a first type, an array of streamers and sources is towed behind a survey vessel. In a second type, an array of seismic cables, each of which includes multiple sensors, is laid on the ocean floor, or seabed, and a source is towed from a survey vessel.
Historically, towed array, marine seismic surveys only employed pressure waves and the receivers detected any passing wavefront. This includes two types of wavefronts. The first are those reflected upward to the receivers from the geological formation. The second are those that are reflected from the surface of the water.
The art has recently begun moving to “multicomponent” surveys in which, for example, not only is the passing of a wavefront detected, but also the direction in which it is propagating. Knowledge of the direction of travel permits determination, for instance, of which wavefronts are traveling upward and will yield useful information and which are traveling downwards and will yield undesirable information if confused with upwards traveling waves. Multicomponent towed-array surveys include a plurality of receivers that detect not only the pressure wave, but also the velocity, or time derivatives (e.g., acceleration) thereof, of the passing wavefront. These receivers will hereafter be referred to as “particle motion sensors” because they measure the velocity or acceleration of displaced particles. The pressure sensor is typically a hydrophone, and the particle motion sensors are typically geophones or accelerometers.
More particularly, the water-air interface is a near perfect reflector of acoustic waves (in the art, the sea surface is known as a “free” surface): for every subterranean reflection, receivers in the array record a corresponding free surface reflection (known as the ghost), which, because of the limited tow-depth, may destructively interfere with the upgoing waves at certain frequencies, creating notches in the spectrum where little or no energy is recorded. This reduces the bandwidth of the recorded data and complicates the waveforms.
It has long been known, and well established for seabed seismic surveying (i.e., by positioning an array or individual receivers on the seabed), that by recording the vertical component of particle motion in addition to the pressure, and by combining the pressure and particle motion data after suitable (spatial) filtering and scaling step, an output signal may be produced that is essentially “ghost” free, consisting of upgoing waves only. This procedure, since it may also produce an output consisting of downgoing free-surface reflected waves only, is known as wavefield decomposition.
Note that there are other methods commonly employed, besides wavefield decomposition, or PZ combination, to arrive at ghost free data. In particular, knowledge of the height of the sea surface may be used to estimate and remove the ghost. The present disclosure distinguishes between deghosting methods that only make use of knowledge about the sea-surface (so-called single-streamer deghosting) and methods that combine pressure and particle motion data (so-called wavefield decomposition or PZ-combination).
However, for marine seismic surveying, the construction of an antenna that accurately records the (vertical component of) particle motion accompanying the pressure, remains a challenge. Since it is difficult to decouple the particle motion measurement altogether from the cable, waves propagating along the cable (induced by small towing vibrations) are recorded and mask the underlying reflections from subterranean formations. This is especially a problem at the lower end of the seismic frequency band (i.e., below say 20-25 Hz), where the wavelength of the cable waves is close to the apparent wavelength of the subterranean reflections and the two cannot be separated using spatial filtering techniques.
Methods have been proposed which aim to circumvent this problem by calculating a “pseudo” particle motion signal involving the steps of single-streamer deghosting the relatively cable noise-free pressure data at low frequencies, re-applying a particle motion ghost and combining or “blending” the “pseudo” particle motion data with the actual recorded particle motion data such that the influence of the cable noise is minimized.
However, since such methods do not estimate the cable noise itself they implicitly disregard the good signal underlying the cable noise and effectively rely on hydrophone data only at low frequencies.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.