Technical Field
Embodiments of the subject matter disclosed herein generally relate to methods and systems and, more particularly, to mechanisms and techniques for controlling internal pressure of a marine vibratory source element for maintaining hydrostatic balance with the ambient pressure.
Discussion of the Background
Reflection seismology is a method of geophysical exploration to determine the properties of a portion of a subsurface layer in the earth, information that is especially helpful in the oil and gas industry. Marine reflection seismology is based on the use of a controlled source that sends energy waves into the earth. By measuring the time it takes for the reflections to come back to plural receivers, it is possible to estimate the depth and/or composition of the features causing such reflections. These features may be associated with subterranean hydrocarbon deposits.
For marine applications, a seismic survey system 100, as illustrated in FIG. 1, includes a vessel 102 that tows plural streamers 110 (only one is visible in the figure) and a seismic source 130. Streamer 110 is attached through a lead-in cable (or other cables) 112 to vessel 102, while source 130 is attached through an umbilical 132 to the vessel. A head float 114, which floats at the water surface 104, is connected through a cable 116 to a head end 110A of streamer 110, while a tail buoy 118 is connected, through a similar cable 116, to a tail end 110B of streamer 110. Head float 114 and tail buoy 118 maintain the streamer's depth and are also provided with GPS (Global Positioning System) or other communication equipment 120 for determining the streamer's position.
In this regard, knowing the exact position of each sensor 122 (only a few are illustrated in FIG. 1 for simplicity) is important when processing the seismic data these sensors record. Thus, vessel 102 is also provided with GPS 124 and a controller 126 that collects the position data associated with streamer head and tail ends and also the position of the vessel and calculates, based on the streamer's known geometry, the absolute position of each sensor.
The same happens for source 130. A GPS system 134 is located on float 137 for determining the position of the source elements 136. Source elements 136 are connected to float 137 to travel at desired depths below the water surface 104. During operation, vessel 102 follows a predetermined path T while source elements (usually air guns) 136 emit seismic waves 140. These waves bounce off the ocean bottom 142 and other layer interfaces below the ocean bottom 142 and propagate as reflected/refracted waves 144 that are recorded by sensors 122. The positions of both the source element 136 and recording sensor 122 are estimated based on GPS systems 120 and 134 and recorded together with the seismic data in a storage device 127 onboard the vessel.
A source element may be impulsive (e.g., an air gun) or vibratory. A vibratory source element is described in U.S. patent application Ser. No. 13/415,216 (herein the '216 application), filed on Mar. 8, 2012, and entitled, “Source for Marine Seismic Acquisition and Method,” assigned to the same assignee as the present application, the entire content of which is incorporated herein by reference.
A vibratory source element experiences increased ambient pressure as its depth increases. The increase in ambient pressure is approximately 1 bar for every 10 m of added depth. For vibratory source elements with a large radiating surface (pistons), the resultant force acting on this surface due to the hydrostatic force can become so great that, in fact, the resultant force exceeds the force capability of the actuator used to drive the piston. If this happens, the seismic source element becomes unable to generate seismic waves. Transient effects, for example sea swells, can also produce localized fluctuations in ambient pressure near the source that can also result in significant forces that act on the piston face. FIG. 6 illustrates an estimate of the variation in ambient pressure for a source located at 25 m depth that might be experienced during a seismic survey.
To make best use of the force that can be developed by the source element's actuator, one approach is to counteract (i.e., balance) the static forces acting on the pistons so the actuator only provides a dynamic force for generating the seismic waves.
Thus, it is desirable to provide systems and methods that balance the hydrostatic force/pressure acting on the source element while being towed underwater.