1. Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and devices for determining a fault (such as a delay or an air leak in an air gun) of individual pressure wave sources of a marine source array based on near-field data acquired by pressure sensors placed near each of the individual pressure wave sources.
2. Discussion of the Background
Since offshore drilling is an expensive process, those undertaking it need to know where to drill in order to avoid a dry well. Marine seismic surveys acquire and process data to generate a profile (image) of the geophysical structure under the seafloor. While this profile does not provide an accurate location for oil and gas, it suggests, to those trained in the field, the presence or absence of oil and/or gas.
During a seismic survey, a vessel tows a seismic wave source and detectors (wave receivers) located on streamers. Reflections of the source-calculated waves are recorded by detectors. The waves are reflected from interfaces between layers, when the density and the wave velocity change (e.g., at an interface between water and air, water to rock, shale to sand, etc).
A popular seismic wave source is the air gun. An air gun stores compressed air and releases it suddenly underwater when fired. The released air forms a bubble (which may be considered spherical), with air pressure inside the bubble initially greatly exceeding the hydrostatic pressure in the surrounding water. The bubble expands, displacing the water and causing a pressure disturbance that travels through the water. As the bubble expands, the pressure decreases, eventually becoming lower than the hydrostatic pressure. When the pressure becomes lower than the hydrostatic pressure, the bubble begins to contract until the pressure inside again becomes larger than the hydrostatic pressure. The process of expansion and contraction may continue through many cycles, thereby generating a pressure (i.e., seismic) wave. The pressure variation generated in the water by a single source (which can be measured using a hydrophone or geophone located near the air gun) as a function of time is called the air gun signature and is illustrated in FIG. 1. A first pressure increase due to the released air is called primary pulse and it is followed by a pressure drop known as a ghost. Between highest primary pressure and lowest ghost pressure there is a peak pressure variation (P-P). The pulses following the primary and the ghost are known as a bubble pulse train. The pressure difference between the second pair of high and low pressures is a bubble pressure variation Pb-Pb. The time T between pulses is the bubble period. A parameter evaluated based on the signature is the peak-to-bubble ratio, which is P-P/Pb-Pb.
Single air guns are not practical because they do not produce enough energy to penetrate at desired depths under the seafloor, and plural weak oscillations (i.e., the bubble pulse train) following the primary (first) pulse complicates seismic data processing. These problems are overcome by using arrays of air guns, generating a larger amplitude primary pulse and canceling secondary individual pulses by destructive interference.
FIG. 1 represents a situation in which the bubble generated by a single air gun drifts slowly toward the surface, surrounded by water having the hydrostatic pressure constant or slowly varying as the bubble slowly drifts upward. However, when another air gun is fired simultaneously in proximity to the first air gun, the hydrostatic pressure is no longer constant or slowly varying. The bubbles of neighboring guns affect each other.
A source array includes plural individual wave sources. An individual wave source may be an air gun or a cluster of air guns. Since the dimensions of the source array, including plural individual sources, are comparable with the wavelengths of generated wave, the wave generated by the source array is directional, i.e., the shape of the wave, or the signature varies with the direction until, at a great enough distance, the wave starts having a stable shape. After the shape become stable, the amplitude of the wave decreases inversely proportional to the distance. The region where the signature shape no longer changes significantly with distance is known as the “far-field,” in contrast to the “near-field” region where the shape varies. Knowledge of the wave source's far-field signature is desirable in order to extract information about the geological structure generating the detected wave upon receiving the far-field input wave.
In order to estimate the source array's far-field signature, an equivalent notional signature for each individual source may be calculated for each of the guns using near-field measurements (see e.g., U.S. Pat. No. 4,476,553 incorporated herewith by reference). The equivalent notional signature is a representation of amplitude due to an individual wave source as a function of time, the source array's far-field signature being a superposition of the notional signatures corresponding to each of the individual sources. In other words, the equivalent notional signature is a tool for representing the contribution of an individual source to the far-field signature, such that the individual source contribution is decoupled from contributions of other individual wave sources in the source array.
However, the stability and reliability of the far-field signature depends on the stability of each of the individual wave sources and of the source array's geometry. During a seismic survey, the individual wave sources' behavior may change (e.g., firing later or earlier than expected, than desirable, or at a smaller amplitude than nominally designed) and thus affect the far-field source signature.
It would be desirable to have methods and apparatuses capable of identifying faults of individual wave sources of a marine source array in order to enable the operator to make an informed decision or implement corrective actions during a marine seismic survey.