MARINE SEISMIC STREAMER SYSTEM CONFIGURATIONS, SYSTEMS, AND METHODS FOR NON-LINEAR SEISMIC SURVEY NAVIGATION

[0001] Background [0002] Marine seismic exploration investigates and maps the structure and character of subsurface geological formations underlying a body of water. One or more streamer cables containing acoustic seismic receivers are deployed into the water behind a vessel, and one or more sources may be towed by the same or different vessel. Less than perfect knowledge of the actual positions of the source at the time of firing and receivers at the time of arrival of reflected seismic waves may result in less than acceptable seismic data.

[0003] Most marine seismic surveys are acquired in straight lines and with parallel streamers that have constant separation. Thus most acoustic distance measuring systems have a fixed acoustic range length expectation. This length is used to set up the transmitter/receiver pairs in a configuration file, sometimes referred to as a "set-up file" of nominal separations, which is used to control the acoustic devices.

[0004] Of all previously known acoustic ranging systems employed in seismic data acquisition, only systems employing intrinsic ranging by modulated acoustics (IRMA) are integrated inline to the streamer. Historically all other acoustic ranging systems are provided by a third party in the sense that they are adapted to various streamers by attaching externally to the coil lines and are not tightly integrated with the software that transforms the acoustic and other positioning measures to coordinates for source and receivers in the seismic acquisition spread. These third party acoustic ranging systems can have difficulty if the relation between acoustic transmitting and receiving units change significantly in relation to the set-up file of nominal separations.

[0005] When any particular transmitter/receiver pair changes their relative position beyond the distance constraints that apply for the acoustic system, the relative position measurement may be lost. This may occur for example during network changes from straight line to curved line, in areas where the current changes feather or when coil shooting with varying radii of curvature. If this continues for a large number of measurements, the network quality decreases, and may lead to operational down time (also called non-productive time) if the relative positioning specifications are not met.

[0006] It is known in some instances to have the user manually update the approximate separations between transmitter and receiver in order to keep the acoustic ranging system functioning (this method may be referred to as manual range tracking). This is critical since many of these systems must separate the measurement times by time sharing transmissions in order for the transmissions not to interfere with each other. This constraint limits the length of the acoustic recording time and thus the range length and optimum transmitter/receiver pairs.

[0007] In periods when the transmitter/receiver pairs become poorly matched due to relative change in separation, acoustic range information may be missing from the network solution. In this case, there is a dependency on compass measures to control the crossline position estimate, while the inline acoustics are not so sensitive to streamer dynamics and will continue to track.

[0008] Another known method for range tracking is use of signal-to-noise ratio ("s/n") as a guide for how the range is changing. This method depends on both a good s/n and no competing signal from reflections. Reflecting surfaces can be the sea bottom, sea surface, or other density interface. A diabolical situation is when there is refraction of most of the direct acoustic signal making it weak compared to a signal reflection that is not different from the direct by more than the record length. In this case the reflection gives the best s/n and can cause a tracking method to lock onto the reflection signal rather than the direct signal.

[0009] Range tracking has been used for acoustic systems such as IRMA to reduce the cpu needed to correlate ranges. However, it is not known to have been applied to systems that operate with set-up files pairing positioning sources and receivers in a timing sequence that avoids interference from reflection signals.

[0010] Summary

[0011] When towing marine streamers non-linearly, for example through a turn, or when feather changes for straight streamers, or during coil shooting, a problem with acoustic networks occurs when streamers move inline relative to each other. This disclosure describes marine seismic streamer system configurations and methods of marine seismic data acquisition for use during any seismic survey navigation, and in particular during non-linear seismic survey navigation, that overcomes this problem. As used herein the term "non-linear" refers to the navigation path of the streamer tow vessel during at least a portion of a marine seismic survey. The term "non-straight" is used interchangeably with the term "non-linear."

[0012] In one aspect, the present disclosure describes a method for determining by acoustic ranging relative positions of marine seismic streamers in a network of streamers, the network comprising a plurality of acoustic positioning transceiver pairs, the method comprising first implementing a network solution-based reconfiguration of the acoustic transceiver pairs (for example continuously or intermittently recomputing the acoustic configuration file automatically based on the latest network shape detected), and then, when the network of streamers changes more than a critical amount, acoustically reconfiguring the network, the critical amount being when the network solution-based reconfiguration is no longer adequate to provide enough acoustic signals to give reasonable relative positions of the acoustic transceiver pairs in the network due to their spatial relation.

[0013] As used herein the term "acoustically reconfiguring" means physically changing the network, whereas the term "solution-based reconfiguring" means changing, assisting, or guiding the software algorithm (and/or data input into the algorithm) used to maintain an acceptable amount of acoustic network positioning data. In certain embodiments, "assisting or guiding" may comprise range tracking. The definition of "range tracking" for this disclosure is to use the computed solution, thought to be correct by virtue of QC factors available, to determine the separation between all transmitter and receiver pairs. Range tracking may be employed to update the set-up or job file, exclude reflected ranges from the record containing the signal traveling along the direct path, and/or find an intermittent acoustic signal (one that is weak for a period but then becomes detectable and useful again).

[0014] In most acoustic navigation systems currently employed in seismic data acquisition, there may be periods when signal to noise may be marginal. This includes pulse and coded/cross-correlation signal types. In certain embodiments described herein, in order to focus in the most likely part of the record to identify the signal, it is useful to guide the signal detection algorithm. This can be done by using the most recently computed distance between the positioning acoustic transmitters and receivers through the acoustic network. This method of range tracking uses the power all the acoustical information available at one epoch to guide the search for the signal at the next epoch. The method is also extremely valuable when the transmitter to receiver distances change by more than the record length as when shooting along non-straight trajectories, like a turn.

[0015] This disclosure describes methods and systems employing a tight integration between acoustic network solver and acoustic range tracking system to follow the range change through periods of high dynamics. Thus all ranges used in the network solution contribute to defining the separation between any transmitter and receiver pair. This information is used to update the set-up files described above for systems using such files or information. It is also valuable in acquisition methods such as coil shooting, where streamers are in addition to experiencing varying curvature, are also experiencing additional dynamics by being exposed to current coming from different directions as they progress through a coil. [0016] Another problem addressed by relating the range length to a recent solution is reflections. Tracking a range reduces the probability of detecting a reflection. Unless the reflection is very similar in length to the direct signal, it will not appear in the search region of the record. For example, if the difference between direct and reflection is greater than 5 meters, the detection region can be as small as 5 meters. As long as the transmitter/receiver separation does not change by more than 5 meters in one measurement cycle, the direct range can be found in the narrow 5 meter band of the record, given an adequate s/n.

[0017] In those cases where there is a temporary drop in the s/n, for some number of measurement cycles for example, in certain embodiments described herein the relation between transmitter and receiver can still be known based on other acoustic ranges that have an adequate s/n. When the weak range again strengthens, the detection algorithm will be searching in the correct narrow record area to find it based on other acoustic ranges

that contributed to establishing the separation between the intermittent transmitter/receiver pair.

[0018] In certain embodiments, acoustically reconfiguring the network comprises deploying longer streamers on the outside of the turn, i.e., designing a marine seismic survey knowing in the planning stage what the outer streamer radius of curvature will be, and deploying streamers having a length so that in the tightest turn planned the inner streamers during a turn will still maintain a predetermined minimum of acoustic connections to some or all of the outer streamers during a turn.

[0019] In other embodiments, acoustically reconfiguring the network comprises designing a marine seismic survey in which some or all of the streamers are longer than the geophysical requirement, the "extra" length being used only for acoustic positioning. Thus traces at the end of the outer streamers during a turn may not be well positioned but are not critical for the geophysical objective.

[0020] In certain embodiments acoustically reconfiguring the network comprises use of multi-vessel networks. In multi-vessel marine seismic data acquisition, acoustic ranges from other vessels involved in the survey are used to augment weak parts of the acoustic network that have changed shape.

[0021] In some embodiments, advancements in satellite positioning technology may supplement the disclosed systems and methods, as further described herein. [0022] The described methods may be used in 3-D and 4-D marine seismic data acquisition, wherein the data may be selected from seismic data, electromagnetic ("EM") data, and both seismic and EM data. Apparatus for carrying out the methods are also described and are another aspect of the present disclosure.

[0023] Brief Description of the Drawings [0024] The manner in which the objectives of the methods and systems of this disclosure and other desirable characteristics may be obtained is explained in the following description and attached drawings in which: [0025] FIGS. 1-3 illustrate prior art streamer configurations during a turn; [0026] FIGS. 4-6 illustrate marine seismic network configurations within the disclosure; [0027] FIG. 7 illustrates a method and system wherein a solution-based reconfiguration is followed by an acoustic reconfiguration; [0028] FIG. 8 is an actual record of a prior art range tracking, illustrating a record of range data that is not centered on the signal, and as the range travel time increases (y-axis increases down), the signal migrates out of the recording period; and [0029] FIG. 9 illustrates a range data record measured in accordance with the present disclosure, illustrating a range data record centered on the signal.

[0030] Detailed Description [0031] In the following description, numerous details are set forth to provide an understanding of the present disclosure; however, it will be understood by those skilled in the art that the methods and systems may be practiced without these details and that numerous variations or modifications from the described embodiments may be possible. [0032] In many towed streamer marine seismic data acquisition methods, the model of fixed (within some limits) relations between acoustic positioning transmitter/receiver pairs is inadequate. The methods and systems of the present disclosure allows for the acoustic configuration file to be recomputed based on an updated network shape; in certain embodiments the file is automatically recomputed based on the latest network shape. (To avoid unnecessary repetition, the term "network" means acoustic network, unless otherwise explicitly defined otherwise.) This solution-based reconfiguration is most valuable when the acoustic network shape is changing such as during transition from a straight line to a curved line, in areas where the current changes streamer feather, during coil shooting with varying radii of curvature, and similar situations. [0033] While solution-based reconfiguration is useful for maintaining a good network during periods of network shape variation, the inner streamers of a conventionally deployed spread can shift so significantly in a curved shape that solution-based re- configuration does not help. The problem must be solved by providing acoustic ranges of other positioning units to these isolated acoustic points used to position seismic traces of interest. Methods of providing acoustic ranges include deploying streamers of longer length at least on the inner streamers and optionally to deploy streamers longer than are necessary to achieve the geophysical objective, or to augment the missing ranges with additional vessels either dedicated to this task or present for geophysical purposes as part of a multi-vessel survey. [0034] FIGS. 1-3 illustrate prior art streamer configurations during a turn. It is advantageous to acquire seismic data during turns, but if the position of the traces is not adequately determined the traces may not be useful. FIG. 1 illustrates four streamers 2a, 2b, 2c and 2d (more or less could be provided) having a plurality of acoustic positioning transmitters, receivers, or transceivers 4 as is known in the art. Streamers may have streamer

positioning devices 6 therealong, such as those known under the trade designation Q-FIN from WesternGeco LLC. Other streamer positioning devices may be used. A tow vessel (not illustrated in FIGS. 1-3) pulls streamers 2a-2d generally to the upper left in each figure, making a turn. One or more seismic source units may be towed as well by the same or different tow vessel, and are not shown. Operation of seismic source units is well known and requires no further description herein. An initial configuration file for acoustic positioning of the streamers is loaded into a computer sub-system on the vessel, and defines pairs of receivers/transmitters that allow positioning by acoustic ranging, as indicated by the dotted lines 8 between the streamers 2a-2d. As seen in the upper right portion of FIG. 1, due to tail end inline shift of the streamers during a turn, a portion of streamer 2a is not adequately positioned by the configuration file. FIG. 2 illustrates that ranges 8a are adequate, but ranges 8b are elongated due to inline shift of streamers 2a and 2b. FIG. 3 illustrates that a portion 10 of streamer 2a is not positioned at all, due to extreme inline shift. [0035] FIGS. 4-6 illustrate acoustic reconfigurations. In FIGS. 4A a vessel 12 is illustrated towing four submerged streamers 2a-2d to the left. Streamers 2a, 2b, 2c and 2d are generally pulled away from center line by deflectors units 13a, 13b, 13c and 13d, as known in the art. Tail buoys 14a, 14b, 14c and 14d are illustrated at the tail ends of steamers 2a, 2b, 2c and 2d, respectively, and are shown forming a line 15 having an angle φ with the heading 17. Acoustic ranges are still acceptable in this configuration, but near the tail buoys the positioning may not be ideal. During a port turn of radius r, as illustrated in FIG. 4B, tail buoys 14a-14d are shown forming a line which is more conducive to acoustic ranging, and therefore the positions of the streamers are much better determined during the turn. As the angle φ becomes smaller, a smaller radius turn may be accommodated, with the tradeoff of poorer acoustic ranges near the tail ends of the streamers when they are traveling linearly. As the angle φ increase toward 90°, a larger radius of curvature turn may be adequately ranged. It should of course be noted that the turn need not be circular; it could be coil- shaped (like a spiral) where the radius changes, or elliptical, or some other shape. [0036] FIG. 5 illustrates an embodiment wherein streamers 2a-2d are longer than required to meet the geophysical requirement. A first length "A" is indicated between lines 1 and 3, which in prior art would have been long enough to meet the design geophysical

77 requirement; distance "B" between lines 1 and 5 indicates that longer streamers may be

78 employed, so that during a turn seismic data can be obtained with confidence that the

79 positions of the streamers will be known with sufficient confidence to avoid a second or

80 further survey to gather data missed on a first survey. The relationship of lengths A and B

81 will vary depending on the survey design, number of streamers, and other parameters, but in

82 general the smaller the turning radius r (FIG. 4B), the longer the streamers should be, in

83 other words the ratio of B/A should be greater as turning radius decreases. Since the extra

84 streamer sections between lines 1 and 3 are used for positioning purpose and not used for

85 geophysical survey purpose, they may be structurally identical to the regular streamer

86 sections that are used for geophysical survey or they may be different. In the former case,

87 the signals from the hydrophones in the extra streamers are used for acoustical ranging, not

88 for recording seismic data In the later case, the extra streamer sections are compatible for

89 navigation and handling purpose, but internally are structurally different. They only contain

90 acoustic transceivers for the positioning network.

91 [0037] FIG. 6 illustrates another embodiment, wherein multiple vessels are

92 employed to supplement acoustic ranges that are either too weak or not possible with a

93 solution-based reconfiguration. Vessels 20 and 22 provide extra acoustic transmitters (or

94 receivers, or transducers as desired) to provide supplemental ranges 28 and 30, respectfully.

95 Vessels 20 and 22 may be surface vessels or underwater vessels. One or more vessel may be

96 sufficient. Suitable vessels include seismic vessels, work vessels, AUVs, ROVs, and the

97 like. Vessels 20 and 22 which create a minimum of acoustic noise would be used in certain

98 embodiments. Vessels 20 and 22 may or may not communicate and share information with

99 the tow vessel.

100 [0038] FIG. 7 illustrates a method and system wherein a solution-based

101 reconfiguration followed by an acoustic reconfiguration may be beneficial. In FIG. 7A vessel

102 12 tows four streamers to the right through a crosscurrent Cl which cause the streamers to

103 feather at an angle α. As vessel 12 and its towed streamers travels further to the right, as

104 depicted in FIG. 7B, a crosscurrent C2 having a magnitude stranger than Cl is encountered

105 by vessel 12 and the portions of the streamers closest to vessel 12, causing a feather angle of

106 β, wherein β>α. In FIGS. 7 A and 7B, solution-based reconfigurations, computed

107 continuously or at some regular or irregular time period are able to supply adequate positions

108 of the streamers. However, once the vessel and steamers reach the position illustrated in FIG.

109 7C, it may be that the solution-based reconfiguration may no longer supply adequate

110 positions toward the ends of the streamers, and an acoustic reconfiguration may be employed

111 for the streamer ends, as indicated by the extra vessel 20 having supplemental acoustic

112 transmitters, or receivers, or transducers (as in FIG. 6). Thus the solution-based

113 reconfiguration may be employed for most of the streamers, while acoustic reconfiguration

114 may be employed near the tail ends of the streamers.

115 [0039] FIG. 8 is an actual record of a prior art range tracking, illustrating a record

116 of range data that is not centered on the signal, and as the range travel time increases (y-axis

117 increases down), the signal migrates out of the recording period; and

118 [0040] FIG. 9 illustrates a range data record measured in accordance with the

119 present disclosure, illustrating a range data record centered on the signal. Each shot cycle the

120 computed range between any two transceiver pair, based on the strength of all ranges in the

121 acoustic network combined, is compared to the record center. If the difference between these

122 two is greater than the gating threshold, the record is re-centered on the computed value. This

123 allows the range to be tracked, and insures adequate acoustic distance measurements to keep

124 the network solving even in highly dynamic environments.

125 [0041] Streamers useful in the practicing the various embodiments described

126 herein may comprise any type of marine seismic streamer, and if more than one streamer is

127 present, the streamers may be the same or different, as long as they comprise sufficient

128 acoustic positioning devices to carry out the methods described herein. Suitable streamers

129 include traditional streamers comprising only hydrophones or groups or hydrophones along

130 the length the streamers; multicomponent streamers (streamers having more than one type of

131 seismic sensor, for example having both hydrophones and geophones), and the like. The

132 streamers are typically built up into lengths using sections of streamers. The streamers may

133 also comprise streamer steering devices, which may be attached inline or attached to the

134 outside of the streamers. Streamers will typically include internal strength members, and may

135 include buoyancy means, such as solids, liquids, and even gases. All of these features are

136 explained in detailed elsewhere and are known in the art.

137 [0042] Methods and systems described herein that include either solution-based

138 reconfiguration, acoustic reconfiguration, or both may also take advantage of systems

139 employing measurements that relate a GPS antenna position to one or more acoustic devices

140 that make up part of the seismic spread acoustic network, such as the techniques described in

141 assignee's U.S. patent application no. 12/049,923, filed on 3/17/2008, Docket No. 14.0373-

142 US, and incorporated by reference herein in its entirety. The mentioned patent application

143 describes motion measurement devices that will improve the accuracy of the relation between

144 the GPS or other satellite antenna and one or more acoustic distance measuring devices that

145 make up part of the acoustic network. To avoid unnecessary repetition herein we use the term

146 GPS, it being understood that other satellite positioning systems may be used. One such

147 method comprises determining relative positions of seismic positioning components of a

148 towed underwater seismic network by acoustic ranging; relating the relative positions to a

149 coordinate reference frame provided by one or more satellite positioning antennae attached to

150 a rigid body floating on a surface of a body of water above the seismic network; determining,

151 via acoustic signals, a distance from an acoustic device fixed to the rigid body to an acoustic

152 device which is one of the seismic positioning components of the underwater seismic

153 network; measuring a sufficient number of orientation parameters of the rigid body to

154 determine 3D offset in the coordinate reference frame of the acoustic device fixed to the rigid

155 body; and correcting the distance using the 3D offset on a shipboard sub-system. In certain

156 embodiments the orientation parameters are selected from the group consisting of pitch, roll,

157 yaw, heading, and combinations thereof. In other embodiments the rigid body is selected

158 from the group consisting of a buoy and a seismic source float. In other embodiments the

159 buoy is selected from the group consisting of a steerable buoy and a non-steerable buoy. In

160 certain embodiments the 3D offset is determined sufficiently to provide sub-meter accuracy

161 in the determination of the 3D offset. As used herein the term "sub-meter" means the

162 accuracy is within plus or minus Im or less, for example within plus or minus 0.9m, or plus

163 or minus 0.8m, plus or minus 0.5m, or plus or minus 0.3m, or even plus or minus 0.1m. In

164 certain embodiments the rigid body may be a buoy, for example one or more streamer tail

165 buoys or streamer front end buoys. The buoys may be simply towed by a vessel or streamer

166 (having no power or steering mechanism integral therewith, i.e. only passively steerable), or

167 may be actively steerable. "Actively steerable" means a device comprising its own

168 mechanism for changing its position, such as a rudder, one or more wings, hydrofoils,

169 ailerons, and the like, and does not include passively steerable devices. An actively steerable

170 device may or may not receive signals from a remote device, either by wire or wireless

171 transmission, indicating what changes in position are desired. "Actively steerable" does not

172 include devices able to be steered only by virtue of being connected to another device which

173 is actively steerable, such as a marine tow vessel, work vessel, ROV, or similar vessel. In

174 certain embodiments the component of the underwater network is a single streamer and the

175 acoustic ranging is performed between sections of the streamer. In other embodiments the

176 components are more than one streamer, and the relative positions determined are relative

177 positions between two or more streamers. The steamers may comprise acoustic seismic

178 sensors, electromagnetic (EM) sensors, or both. In certain embodiments measuring motion of

179 the rigid body comprises using one or more inertial measuring units, such as accelerometers,

180 gyroscopes, and the like. In certain embodiments measuring motion of the rigid body

181 comprises measuring orientation of at least three satellite antennae fixed to the rigid body. In

182 other embodiments measuring motion of the rigid body comprises measuring heading,

183 inclination to vertical in cross line and inline tow directions.

184 [0043] Another method comprises determining relative positions of components

185 of a towed underwater seismic network by acoustic ranging, the underwater seismic network

186 comprising a streamer tail buoy connected to a streamer, the buoy having one or more

187 satellite antennae fixed thereto and projecting above water; and relating coordinates of one of

188 the satellite antennae to coordinates of an acoustic node in a reference frame, the acoustic

189 node being on or in a non-horizontal portion of the streamer at a point shipward from the

190 buoy and at a known distance from the buoy, comprising measuring inclination, depth, and

191 crossline angle of the non-horizontal portion at the point.

192 [0044] In certain embodiments, seismic streamers are positioned relative to each

193 other by acoustic ranging. These relative positions are then related to an earth fixed

194 coordinate reference frame typically provided by satellite (for example, GPS, GLONASS, or

195 other satellite positioning system, or combination thereof) control points on towed buoys (tail

196 or streamer front buoys) on the sea surface above the submerged streamers. An acoustic

197 device is employed that determines a distance from the towed buoy to one or more of the

198 submerged components (whose relative positions are known). The physical connection or tie

199 between the satellite antenna and the acoustic device is a key component of positioning

200 accuracy. The physical connection must not changed in length detrimentally (more than a

201 few centimeters) over the course of the positioning methods. In certain embodiment,

202 inclinometers, a pressure sensor to determine depth, and a compass may be employed,

203 wherein these instruments are integrated in or attached to a section between the rigid body on

204 which one or more GPS antennae (one antenna is sufficient in certain embodiments) are

205 mounted and an acoustic node in the seismic network. One advantage of this method is that it

206 relates the GPS antenna point and acoustic points without having an acoustic node rigidly

207 attached to the rigid body. A rigid attachment of an acoustic device to a rigid body has

208 caused, in certain instances, acoustic performance problems for transmission, as the deeper

209 the transmitter the better the acoustic signal. Acoustic receivers near the ocean surface are

210 contaminated by sea surface noise, and apparatus such as a rigid pole attached to a floating

211 surface device such as a tail buoy poses a towing risk as the pole may encounter debris in the

212 ocean during tow and be damaged. Further a deployment and retrieval solution is needed to

213 protect the pole and acoustic unit during these operations.

214 [0045] Although only a few exemplary embodiments have been described in

215 detail above, those skilled in the art will readily appreciate that many modifications are

216 possible in the exemplary embodiments without materially departing from the novel

217 teachings and advantages of the methods and systems disclosed herein. Accordingly, all such

218 modifications are intended to be included within the scope of this disclosure as defined in the

219 following claims.