EP3304136A1 - Calibration method, system and controller for a multi-level source - Google Patents

Calibration method, system and controller for a multi-level source

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Publication number
EP3304136A1
EP3304136A1 EP16736614.5A EP16736614A EP3304136A1 EP 3304136 A1 EP3304136 A1 EP 3304136A1 EP 16736614 A EP16736614 A EP 16736614A EP 3304136 A1 EP3304136 A1 EP 3304136A1
Authority
EP
European Patent Office
Prior art keywords
seismic
source
individual sources
survey
depth
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP16736614.5A
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German (de)
French (fr)
Inventor
Yuan NI
Thierry Payen
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Sercel SAS
Original Assignee
CGG Services SAS
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Filing date
Publication date
Application filed by CGG Services SAS filed Critical CGG Services SAS
Publication of EP3304136A1 publication Critical patent/EP3304136A1/en
Withdrawn legal-status Critical Current

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Classifications

    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
    • G01V13/00—Manufacturing, calibrating, cleaning, or repairing instruments or devices covered by groups G01V1/00 – G01V11/00
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
    • G01V1/00—Seismology; Seismic or acoustic prospecting or detecting
    • G01V1/38—Seismology; Seismic or acoustic prospecting or detecting specially adapted for water-covered areas
    • G01V1/3817—Positioning of seismic devices
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
    • G01V1/00—Seismology; Seismic or acoustic prospecting or detecting
    • G01V1/38—Seismology; Seismic or acoustic prospecting or detecting specially adapted for water-covered areas
    • G01V1/3861—Seismology; Seismic or acoustic prospecting or detecting specially adapted for water-covered areas control of source arrays, e.g. for far field control

Definitions

  • Embodiments of the subject matter disclosed herein generally relate to a calibration procedure of a multi-level seismic source; more particularly, to obtaining actual depths from data recorded when firing a subset of individual sources.
  • Marine seismic surveys have long been and remain an important tool for exploring the structure of formations beneath the floor of a body of water in order, for example, to locate underground oil and gas reservoirs.
  • FIG. 1 illustrates a marine seismic data acquisition system 100.
  • a vessel 1 10 tows seismic sources 1 12a and 1 12b and a streamer 1 14 at predetermined depths under the water surface 1 1 1.
  • Streamer 1 14 which has a tail buoy 1 18 and likely other positioning devices attached, houses receivers/sensors 1 16.
  • the seismic sources 1 12a and 1 12b generate seismic excitations (i.e., an sum of waves) such as 120a and 120b that propagate through the water layer 30 toward the seafloor 32.
  • seismic excitations i.e., an sum of waves
  • layers e.g., water layer 30, first layer 34, and second layer 38
  • the seismic excitations' propagation directions change as they are reflected and/or transmitted/refracted/diffracted.
  • Seismic excitations 120a and 120b are partially reflected as 122a and 122b and partially transmitted as 124a and 124b at seafloor 32.
  • Transmitted excitations 124a and 124b travel through first layer 34 and are then reflected as 126a and 126b, and transmitted as 128a and 128b at interface 36.
  • seismic excitations 128a and 128b are then partially transmitted as 130a and 130b, and partially reflected as 132a and 132b.
  • the seismic excitations traveling upward e.g., 122a and 122b, 126a and 126b, 132a and 132b
  • Maxima and minima in pressure versus time data recorded by the receivers carry information about the underground formation's structure (e.g., the location of the interfaces and wave propagation velocities inside the layers).
  • Figure 1 illustrates two individual sources.
  • a multilevel marine seismic source including plural individual sources towed at two or more distinct depths has been used in marine surveys to alleviate notches in the source signature spectra.
  • the notches (depletion of the spectra at certain frequencies) occur due to destructive interference of direct waves with water-surface reflections thereof.
  • the notch frequencies are determined by the depth at which the seismic excitations are emitted.
  • Using individual sources at different depths means that the notches corresponding to each of the sources occur at different frequencies rendering the resulting spectrum more uniform than when using one or more sources at the same single depth.
  • Seismic data that represents the seismic excitations detected by the receivers is a convolution of the seismic signal (i.e., a combination of the seismic excitations generated by all the individual sources) penetrating the underground formation, and a response function of the explored underground formation.
  • seismic data is subjected to deconvolution using a source signature corresponding to the seismic signal.
  • the source signature may be calculated by combining the contributions (notionals) of each of the individual sources fired substantially simultaneously (i.e., so as to merge in a stable waveform) in a manner determined by their positions when fired.
  • the individual source positions are collectively known as the source geometry.
  • a nominal source geometry is a planned arrangement of the individual sources of a towed marine source. However, in reality, one or more of the individual sources may drift from the nominal source geometry.
  • One or more depth sensors have been attached to the marine seismic source to monitor the depth(s). Yet, the depth sensors' readings are often affected by offsets.
  • a calibration procedure is performed to acquire better knowledge of actual depths of individual sources of a multi-level marine seismic source and the surface reflection coefficient.
  • the calibration procedure is based on analyzing calibration data acquired when one or a few of the individual sources are fired.
  • a method for calibrating a multi- level marine seismic source including individual sources disposed at two or more distinct depths.
  • the method includes firing a subset of the individual sources to emit seismic excitations that combine while propagating away from the multi-level marine seismic source, acquiring calibration data representing the seismic excitations, using near-field receivers placed to distinguish the seismic excitations as emitted by each of the individual sources, and a distant seismic receiver placed to detect an overlap of the seismic excitations and surface reflections thereof, and then processing the calibration data to infer at least one actual depth among the distinct depths, and/or a sea-surface reflection coefficient.
  • a seismic data acquisition system including a multi-level marine seismic source, near-field sensors, a distant seismic receiver, a controller and a data processing unit.
  • the multi-level marine seismic source includes individual sources disposed at two or more distinct depths, and a controller configured to cause a subset of the individual sources to fire emitting the seismic excitations that combine while propagating away from the multi-level marine seismic source.
  • the new-field receivers are placed to distinguish seismic excitations as emitted by each of the individual sources.
  • the distant seismic receiver is configured and disposed to detect an overlap of the seismic excitations and water- surface reflections thereof.
  • the data processing unit is configured to infer at least one actual depth among the distinct depths, and/or a sea-surface reflection coefficient by processing the calibration data.
  • the calibration seismic data is acquired using the near-field receivers and the distant seismic receiver when firing the subset of the individual sources.
  • a marine source controller of a multi-level marine seismic source including individual sources disposed at two or more distinct depths.
  • the controller includes a communication interface and a central processing unit.
  • the communication interface is configured to receive source-related information and to send commands to the multi-level marine seismic source.
  • the central Processing unit is configured to generate the commands based on the source-related information, the commands causing the multi-level marine seismic source to fire a subset of the individual sources during a calibration procedure, and to control the communication interface to send signals triggering calibration data acquisition.
  • Figure 1 illustrates a marine seismic data acquisition system
  • Figure 2 is a flowchart of a method for calibrating a multi-level marine seismic source according to an embodiment;
  • Figure 3 illustrates a multi-level marine seismic source;
  • Figure 4 illustrates a data acquisition system including a middle field receiver
  • Figures 5, 6, and 7 are spectra of seismic waves recorded with a distant receiver when different sets of individual sources are fired;
  • Figure 8 is a graphical illustration of a calibration procedure
  • Figures 9 and 10 illustrate seismic surveys performed after a calibration procedure
  • Figure 1 1 illustrates a calibration procedure according to an
  • Figure 12 illustrates obtaining a seismic source signature in view of the calibration procedure according to an embodiment
  • Figure 13 is a block diagram of a seismic data acquisition system according to an embodiment.
  • Figure 14 is a schematic diagram of a source controller according to an embodiment.
  • a calibration procedure for a multi-level marine seismic source infers actual depths of individual sources and/or surface reflection coefficients from calibration data acquired when a subset of the individual sources is fired.
  • Figure 2 is a flowchart of a method 200 for calibrating a multi-level marine seismic source including individual sources disposed at two or more distinct depths, according to an embodiment.
  • Figure 3 represents a nominal geometry of a multi-level marine seismic source array in a three-dimensional graph.
  • This multi-level marine seismic source array includes individual sources disposed at three distinct depths.
  • the term "disposed" means deployed and guided to achieve planned positions relative to one another and/or a reference point, but it is understood that the source is towed through water, not stationary.
  • the individual sources may be air-guns or marine vibrators.
  • First groups of individual sources 310, 320 and 330 in Figure 3 are towed at a depth of about 5.5 m, second groups of individual sources 340 and 350 are towed at depths of about 8 m.
  • the groups which include individual sources at the same depth and the same cross-line position, are surrounded by imaginary envelopes drawn with dashed lines to help the viewer identify the groups.
  • in-line direction is parallel to towing direction T, and cross-line direction is perpendicular to the in-line direction and gravity g (which is the direction of depth increase).
  • the individual sources in a group are thus arranged along the in-line direction and may be connected via a same support structure.
  • the individual sources attached to the same support structure are sometimes called subarrays of the marine source. However, whether or not the individual sources at different depths form a subarray has no bearing on using the calibration methods described here. During marine surveys, some of the individual sources may be inoperative and therefore are drawn using black.
  • the multi-level marine seismic source in Figure 3 is merely an illustration, not intended to be limiting in terms of number of individual sources and their particular arrangement (e.g., the individual sources may be arranged at more than two levels).
  • Method 200 includes firing a subset of the individual sources to emit seismic excitations that combine while propagating away from the multi-level marine seismic source at S210.
  • the fact that the seismic excitations combine while propagating away from the multi-level marine seismic source means that the individual sources in the subset are fired substantially simultaneously (i.e., relative time delays are too short to separate the seismic excitations at large distances such as few times the source depth).
  • the subset of individual sources includes fewer than all the individual sources but may include a single one of the individual sources. In one embodiment, the subset may include plural sources at the same depth. For example, method 200 may be performed first for the individual sources in group 310, 320 and 330, and then for the individual sources in groups 340 and 350.
  • the method may be performed when only some of the individual sources of one depth are fired, (e.g., only the individual sources in group 310). Yet, in one embodiment, the method may be performed by firing a single individual source each time.
  • Method 200 further includes acquiring calibration data representing the seismic excitations, using near-field receivers placed to distinguish the seismic excitations as emitted by each of the individual sources, and a distant seismic receiver placed to detect an overlap of the seismic excitations and water-surface reflections thereof, at 220.
  • Near-field receivers may be hydrophones placed above the individual sources.
  • the near-field receivers are illustrated as points above each of the individual sources. For simplicity, a single pair including individual source 31 1 (in group 310) and corresponding near-field receiver 341 is labeled in Figure 3.
  • near-field receiver 341 Due to its proximity to individual source 31 1 (e.g., about 1 m above this source), near-field receiver 341 is able to identify a seismic excitation emitted by individual source 31 1 from seismic excitations emitted simultaneously by the other individual sources.
  • the distant seismic receiver may be a mid-field receiver as illustrated in Figure 4, a lateral receiver or a water bottom receiver.
  • a near-field receiver associated with an individual source in group 340 may be used when individual sources in group 320 are fired. This receiver is a lateral receiver placed on a side of the fired sources.
  • mid-field receiver refers to a receiver placed too far from the seismic source (i.e., at more than about 20 m) to be able to distinguish the seismic excitations emitted by each of the individual sources, but not far enough from the seismic source (i.e., at less than about 200 m) to detect the source signature (which is a stable waveform resulting from merging of the seismic excitations and
  • Figure 4 illustrates a mid-field receiver 410 towed under a multi-level source 420 by a vessel 430.
  • a mid-field receiver may be attached to lead-in cables used to tow streamers.
  • Figure 5 is a spectrum corresponding to individual sources in group 310 being fired
  • Figure 6 is a spectrum corresponding to individual sources in groups 320a and/or 320b being fired
  • Figure 7 is a spectrum corresponding to individual sources in groups 330a and/or 330b being fired. Since the individual sources fired in the three cases are at different depths, the notches in the resulting spectra occur at different frequencies. If a resulting spectrum would still allow identifying notches frequencies, the subset of simultaneously fired individual sources for calibration may include individual sources at different depths.
  • method 200 further includes processing the calibration data to infer at least one actual depth among the distinct depths, and/or a sea-surface reflection coefficient, at 230.
  • the steps of method 200 are repeated for different sets of sources to determine all the relevant depths and reflection coefficients (i.e., for each individual source or per group of individual sources).
  • Calibration data may be used to estimate the source signature (e.g., with joint inversion as described in WO 2015044207A1 , or without joint inversion as in the Ziolkowski's method), to update and/or correct individual sources' depths and/or to update a water-surface reflection coefficient for each part of the source. Calibration data may also be used to determine an offset affecting measurements of a depth sensor, as further discussed. The calibration also enables reconstruction of a more reliable ghost mask for the entire incident angle, the ghost mask being an operator that accounts for interference between groups of individual sources due to the geometry and surface reflections. This ghost mask may then be used to enhance the designature processing, i.e., separating the underground formation's response from survey seismic data.
  • Figure 8 illustrates a calibration procedure for a multi-level marine seismic source including individual sources disposed at two distinct depths.
  • a first calibration phase individual sources at a first nominal depth z1 are fired, yielding a first spectrum with notches frequencies F1.
  • An actual depth z1 ' and a reflection coefficient R1 may be inferred from calibration data recorded after firing these individual sources.
  • individual sources at a second nominal depth z2 are fired, yielding a second spectrum with notches frequencies F2.
  • An actual depth z2' and a reflection coefficient R2 may be inferred from calibration data recorded upon firing these other individual sources.
  • the results of the calibration may then be used for accurate reconstruction of the far-field signature, enabling proper designature of the seismic data acquired, and for survey source deghosting.
  • the individual sources in the calibrated arrangement are fired, to yield a notchless signature.
  • source depths may be adjusted in view of the calibration to match nominal depths z1 and z2. In this case, if only the individual sources at one of the depths were fired, resulting spectra would have notches at frequencies F1 ' and F2', respectively (FT and F2' being different from F1 and F2). After having their depths adjusted, all the individual sources are fired simultaneously to yield again a notchless signature.
  • Figure 1 1 is a graphic illustration of a calibration procedure according to an embodiment. Firing a subset of the individual sources (air-guns) at the same depth at 1 100 triggers recording data using near-field receivers at 1 1 10 and recording data using a mid-field receiver at 1 120. About the same time, at least one depth sensor attached to the multi-level marine seismic source measures depth at 1 130. At 1 140, the data recorded by the near-field receivers and the measured depth are used to calculate individual-source contributions (known as "notionals") to the source signature. Then, at 1 150, the data recorded by the mid-field receiver and the notionals is then used to infer an actual depth and/or reflection coefficient. The source geometry is updated according to the actual depth.
  • a difference between the measured depth and the actual depth is calculated as a calibration factor for the fired individual sources at 1 160 and stored for later use (i.e., when survey seismic data is processed) at 1 170.
  • More than one depth sensor may be attached to the multi-level marine seismic source.
  • One embodiment may have a depth sensor attached to each group of individual sources, while another embodiment may have a depth sensor attached to each individual source.
  • Figure 12 is a graphic illustration of obtaining the source signature for seismic survey data processing following the calibration procedure in Figure 1 1.
  • Firing all operational individual sources at 1200 triggers recording data using near- field receivers at 1210.
  • the depth sensor(s) attached to the multi-level marine seismic source measures (measure) depth(s) at 1220.
  • Previously stored calibration factor(s) is/are retrieved at 1230, to generate together with the measured depth(s), the source's actual geometry at 1240.
  • the data recorded by the near-field receivers and the actual geometry are then used to obtain notionals at 1250.
  • the notionals and the actual geometry are used to calculate the far-field source signature.
  • the calibration procedure may be performed at the beginning of a marine survey.
  • Marine survey plans typically include substantially parallel sail lines at predetermined distances from one another.
  • the calibration procedure may be performed at the beginning of each sail line.
  • the calibration procedure is performed while gradually increasing emitted power to limit disruption of marine life. For example, a single gun may be fired first, followed by firing two guns, then more until all operational guns are fired.
  • This calibration procedure has the advantage of improving knowledge of actual source geometry. Since the calibration procedure can be performed between survey data acquisition periods, it leads to increasing seismic data processing quality without extending the survey time.
  • FIG. 13 is a block diagram of a seismic data acquisition system 1300 able to perform the calibration methods.
  • System 1300 includes a multi-level marine seismic source 1310 having individual sources (not shown) disposed at two or more distinct depths.
  • Source 1310 includes a controller 1315 configured to cause a subset of the individual sources to fire emitting the seismic excitations that combine while propagating away from the multi-level marine seismic source.
  • Source 1310 also has attached a depth sensor 1317.
  • System 1300 further includes near-field receivers 1320 placed to distinguish seismic excitations as emitted by each of the individual sources, and a distant seismic receiver 1330 configured and disposed to detect an overlap of the seismic excitations emitted by the individual sources and surface reflections thereof.
  • Near-field receivers 1320 and distant seismic receiver 1330 collect data when a subset of the individual sources is fired during calibration.
  • System 1300 also includes survey receivers 1340 carried by streamers.
  • Near-field receivers 1320 and the survey receivers 1340 collect seismic survey data.
  • Data processing unit 1350 processes the calibration data to obtain actual depths of the individual sources, and processes the seismic survey data in view of the calibration data.
  • Figure 14 illustrates a source controller 1400 according to an
  • the controller which may be installed onboard the vessel towing the multi-level source, includes a communication interface 1406 and a central processing unit (CPU) 1404.
  • the communication interface is configured to receive source- related information and to send commands to the multi-level marine seismic source.
  • the CPU is configured to generate the commands based on the source-related information, the commands causing the multi-level marine seismic source to fire a subset of the individual sources during a calibration procedure.
  • the CPU is also configured to control the communication interface to send signals triggering calibration data acquisition.
  • Controller 1400 is in communication with the marine source, the near- field receivers, the distant receiver and the survey receivers, which are collectively labeled 1412 in this figure.
  • the controller may also include a memory 1408 configured to store calibration factors, calibration data, seismic data and other survey information. Memory 1408 may also store executable codes making CPU 1404 to perform calibration-related calculations.
  • the controller may also include an operator interface 1410. The communication interface, the CPU, the memory and the operator interface may be encapsulated in a single piece of equipment 1402, or may be modular and distributed.
  • the controller may receive an actual depth of at least one of the individual sources, via the communication interface (the actual depth having been inferred from the calibration data) and compare the actual depth with a planned depth for the at least one of the individual sources. Then, if a difference between the actual depth and the planned depth exceeds a predetermined thresholds, the control unit may generate a command to adjust the actual depth (or firing moment in the firing sequence) of the at least one individual source to match the planned depth, and control the communication interface to transmit this command to the multi-level marine seismic source. Marine sources able to adjust depth of individual sources are described, for example, in U.S. Patent Application Publication No. 2014/01 12096.
  • the disclosed exemplary embodiments provide methods, a system and a controller for calibrating a multi-level seismic source to achieve an improved seismic signature. It should be understood that this description is not intended to limit the invention. On the contrary, the exemplary embodiments are intended to cover alternatives, modifications and equivalents, which are included in the spirit and scope of the invention as defined by the appended claims. Further, in the detailed description of the exemplary embodiments, numerous specific details are set forth in order to provide a comprehensive understanding of the claimed invention. However, one skilled in the art would understand that various embodiments may be practiced without such specific details.

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Abstract

A multi-level marine seismic source is calibrated by firing a subset of the individual sources, acquiring calibration data using near-field receivers and a distant seismic receiver, and processing the calibration data to infer at least one actual depth related to the fired sources and/or a sea-surface reflection coefficient.

Description

Calibration Method, System and Controller
for a Multi-Level Source
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority and benefit from U.S. Provisional Patent Application No. 62/169,599 filed on June 2, 2015, for "Use NF and MF hydrophones for multi-layer source signature calibration," the content of which is incorporated in its entirety herein by reference.
BACKGROUND TECHNICAL FIELD
[0002] Embodiments of the subject matter disclosed herein generally relate to a calibration procedure of a multi-level seismic source; more particularly, to obtaining actual depths from data recorded when firing a subset of individual sources.
DISCUSSION OF THE BACKGROUND
[0003] Marine seismic surveys have long been and remain an important tool for exploring the structure of formations beneath the floor of a body of water in order, for example, to locate underground oil and gas reservoirs.
[0004] Figure 1 illustrates a marine seismic data acquisition system 100. In this vertical view, a vessel 1 10 tows seismic sources 1 12a and 1 12b and a streamer 1 14 at predetermined depths under the water surface 1 1 1. Although only one streamer is visible in this vertical view, plural streamers are typically spread in a three-dimensional array. Streamer 1 14, which has a tail buoy 1 18 and likely other positioning devices attached, houses receivers/sensors 1 16.
[0005] The seismic sources 1 12a and 1 12b generate seismic excitations (i.e., an sum of waves) such as 120a and 120b that propagate through the water layer 30 toward the seafloor 32. At interfaces (e.g., 32 and 36) between layers (e.g., water layer 30, first layer 34, and second layer 38) distinguishable due to their different wave propagation velocities, the seismic excitations' propagation directions change as they are reflected and/or transmitted/refracted/diffracted. Seismic excitations 120a and 120b are partially reflected as 122a and 122b and partially transmitted as 124a and 124b at seafloor 32. Transmitted excitations 124a and 124b travel through first layer 34 and are then reflected as 126a and 126b, and transmitted as 128a and 128b at interface 36. At the upper surface of reservoir 40, seismic excitations 128a and 128b are then partially transmitted as 130a and 130b, and partially reflected as 132a and 132b. The seismic excitations traveling upward (e.g., 122a and 122b, 126a and 126b, 132a and 132b) may be detected by receivers 1 16. Maxima and minima in pressure versus time data recorded by the receivers carry information about the underground formation's structure (e.g., the location of the interfaces and wave propagation velocities inside the layers).
[0006] Figure 1 illustrates two individual sources. However, lately, a multilevel marine seismic source including plural individual sources towed at two or more distinct depths has been used in marine surveys to alleviate notches in the source signature spectra. The notches (depletion of the spectra at certain frequencies) occur due to destructive interference of direct waves with water-surface reflections thereof. The notch frequencies are determined by the depth at which the seismic excitations are emitted. Using individual sources at different depths means that the notches corresponding to each of the sources occur at different frequencies rendering the resulting spectrum more uniform than when using one or more sources at the same single depth.
[0007] Seismic data that represents the seismic excitations detected by the receivers is a convolution of the seismic signal (i.e., a combination of the seismic excitations generated by all the individual sources) penetrating the underground formation, and a response function of the explored underground formation. In order to separate the structural information from the seismic signal, seismic data is subjected to deconvolution using a source signature corresponding to the seismic signal. As disclosed in U.S. Patent Application Publication No. 2013/0258808 (the entire content of which is incorporated herein by reference), the source signature may be calculated by combining the contributions (notionals) of each of the individual sources fired substantially simultaneously (i.e., so as to merge in a stable waveform) in a manner determined by their positions when fired. The individual source positions are collectively known as the source geometry.
[0008] A nominal source geometry is a planned arrangement of the individual sources of a towed marine source. However, in reality, one or more of the individual sources may drift from the nominal source geometry. One or more depth sensors have been attached to the marine seismic source to monitor the depth(s). Yet, the depth sensors' readings are often affected by offsets.
[0009] It is desirable to find a technical solution for acquiring better knowledge of the individual sources' actual arrangement and surface reflection coefficients during a seismic survey, in order to improve designature (i.e., extracting the underground formation's response, which is independent of the probing seismic excitations, from the seismic data).
SUMMARY
[0010] A calibration procedure is performed to acquire better knowledge of actual depths of individual sources of a multi-level marine seismic source and the surface reflection coefficient. The calibration procedure is based on analyzing calibration data acquired when one or a few of the individual sources are fired.
[0011] According to an embodiment, there is a method for calibrating a multi- level marine seismic source including individual sources disposed at two or more distinct depths. The method includes firing a subset of the individual sources to emit seismic excitations that combine while propagating away from the multi-level marine seismic source, acquiring calibration data representing the seismic excitations, using near-field receivers placed to distinguish the seismic excitations as emitted by each of the individual sources, and a distant seismic receiver placed to detect an overlap of the seismic excitations and surface reflections thereof, and then processing the calibration data to infer at least one actual depth among the distinct depths, and/or a sea-surface reflection coefficient.
[0012] According to another embodiment, there is a seismic data acquisition system including a multi-level marine seismic source, near-field sensors, a distant seismic receiver, a controller and a data processing unit. The multi-level marine seismic source includes individual sources disposed at two or more distinct depths, and a controller configured to cause a subset of the individual sources to fire emitting the seismic excitations that combine while propagating away from the multi-level marine seismic source. The new-field receivers are placed to distinguish seismic excitations as emitted by each of the individual sources. The distant seismic receiver is configured and disposed to detect an overlap of the seismic excitations and water- surface reflections thereof. The data processing unit is configured to infer at least one actual depth among the distinct depths, and/or a sea-surface reflection coefficient by processing the calibration data. The calibration seismic data is acquired using the near-field receivers and the distant seismic receiver when firing the subset of the individual sources.
[0013] According to yet another embodiment, there is a marine source controller of a multi-level marine seismic source including individual sources disposed at two or more distinct depths. The controller includes a communication interface and a central processing unit. The communication interface is configured to receive source-related information and to send commands to the multi-level marine seismic source. The central Processing unit is configured to generate the commands based on the source-related information, the commands causing the multi-level marine seismic source to fire a subset of the individual sources during a calibration procedure, and to control the communication interface to send signals triggering calibration data acquisition.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate one or more embodiments and, together with the description, explain these embodiments. In the drawings:
[0015] Figure 1 illustrates a marine seismic data acquisition system;
[0016] Figure 2 is a flowchart of a method for calibrating a multi-level marine seismic source according to an embodiment; [0017] Figure 3 illustrates a multi-level marine seismic source;
[0018] Figure 4 illustrates a data acquisition system including a middle field receiver;
[0019] Figures 5, 6, and 7 are spectra of seismic waves recorded with a distant receiver when different sets of individual sources are fired;
[0020] Figure 8 is a graphical illustration of a calibration procedure;
[0021] Figures 9 and 10 illustrate seismic surveys performed after a calibration procedure;
[0022] Figure 1 1 illustrates a calibration procedure according to an
embodiment;
[0023] Figure 12 illustrates obtaining a seismic source signature in view of the calibration procedure according to an embodiment;
[0024] Figure 13 is a block diagram of a seismic data acquisition system according to an embodiment; and
[0025] Figure 14 is a schematic diagram of a source controller according to an embodiment.
DETAILED DESCRIPTION
[0026] The following description of the exemplary embodiments refers to the accompanying drawings. The same reference numbers in different drawings identify the same or similar elements. The following detailed description does not limit the invention. Instead, the scope of the invention is defined by the appended claims. The following embodiments are discussed in the context of a marine seismic survey.
[0027] Reference throughout the specification to "one embodiment" or "an embodiment" means that a particular feature, structure or characteristic described in connection with an embodiment is included in at least one embodiment of the subject matter disclosed. Thus, the appearance of the phrases "in one embodiment" or "in an embodiment" in various places throughout the specification is not necessarily referring to the same embodiment. Further, the particular features, structures or characteristics may be combined in any suitable manner in one or more embodiments.
[0028] A calibration procedure for a multi-level marine seismic source infers actual depths of individual sources and/or surface reflection coefficients from calibration data acquired when a subset of the individual sources is fired.
[0029] Figure 2 is a flowchart of a method 200 for calibrating a multi-level marine seismic source including individual sources disposed at two or more distinct depths, according to an embodiment. For example, Figure 3 represents a nominal geometry of a multi-level marine seismic source array in a three-dimensional graph. This multi-level marine seismic source array includes individual sources disposed at three distinct depths. Here, the term "disposed" means deployed and guided to achieve planned positions relative to one another and/or a reference point, but it is understood that the source is towed through water, not stationary. The individual sources may be air-guns or marine vibrators.
[0030] First groups of individual sources 310, 320 and 330 in Figure 3 are towed at a depth of about 5.5 m, second groups of individual sources 340 and 350 are towed at depths of about 8 m. In Figure 3, the groups, which include individual sources at the same depth and the same cross-line position, are surrounded by imaginary envelopes drawn with dashed lines to help the viewer identify the groups. In this figure, in-line direction is parallel to towing direction T, and cross-line direction is perpendicular to the in-line direction and gravity g (which is the direction of depth increase). The individual sources in a group are thus arranged along the in-line direction and may be connected via a same support structure. The individual sources attached to the same support structure are sometimes called subarrays of the marine source. However, whether or not the individual sources at different depths form a subarray has no bearing on using the calibration methods described here. During marine surveys, some of the individual sources may be inoperative and therefore are drawn using black. The multi-level marine seismic source in Figure 3 is merely an illustration, not intended to be limiting in terms of number of individual sources and their particular arrangement (e.g., the individual sources may be arranged at more than two levels).
[0031] Method 200 includes firing a subset of the individual sources to emit seismic excitations that combine while propagating away from the multi-level marine seismic source at S210. The fact that the seismic excitations combine while propagating away from the multi-level marine seismic source means that the individual sources in the subset are fired substantially simultaneously (i.e., relative time delays are too short to separate the seismic excitations at large distances such as few times the source depth). The subset of individual sources includes fewer than all the individual sources but may include a single one of the individual sources. In one embodiment, the subset may include plural sources at the same depth. For example, method 200 may be performed first for the individual sources in group 310, 320 and 330, and then for the individual sources in groups 340 and 350.
Alternatively, the method may be performed when only some of the individual sources of one depth are fired, (e.g., only the individual sources in group 310). Yet, in one embodiment, the method may be performed by firing a single individual source each time.
[0032] Method 200 further includes acquiring calibration data representing the seismic excitations, using near-field receivers placed to distinguish the seismic excitations as emitted by each of the individual sources, and a distant seismic receiver placed to detect an overlap of the seismic excitations and water-surface reflections thereof, at 220. Near-field receivers may be hydrophones placed above the individual sources. Returning now to Figure 3, the near-field receivers are illustrated as points above each of the individual sources. For simplicity, a single pair including individual source 31 1 (in group 310) and corresponding near-field receiver 341 is labeled in Figure 3. Due to its proximity to individual source 31 1 (e.g., about 1 m above this source), near-field receiver 341 is able to identify a seismic excitation emitted by individual source 31 1 from seismic excitations emitted simultaneously by the other individual sources. [0033] The distant seismic receiver may be a mid-field receiver as illustrated in Figure 4, a lateral receiver or a water bottom receiver. For example, a near-field receiver associated with an individual source in group 340 may be used when individual sources in group 320 are fired. This receiver is a lateral receiver placed on a side of the fired sources.
[0034] The term "mid-field receiver" refers to a receiver placed too far from the seismic source (i.e., at more than about 20 m) to be able to distinguish the seismic excitations emitted by each of the individual sources, but not far enough from the seismic source (i.e., at less than about 200 m) to detect the source signature (which is a stable waveform resulting from merging of the seismic excitations and
propagating beyond 100 m). Figure 4 illustrates a mid-field receiver 410 towed under a multi-level source 420 by a vessel 430. In one embodiment, a mid-field receiver may be attached to lead-in cables used to tow streamers.
[0035] Focusing now on the spectra of calibration data recorded by a mid-field receiver, Figure 5 is a spectrum corresponding to individual sources in group 310 being fired, Figure 6 is a spectrum corresponding to individual sources in groups 320a and/or 320b being fired, and Figure 7 is a spectrum corresponding to individual sources in groups 330a and/or 330b being fired. Since the individual sources fired in the three cases are at different depths, the notches in the resulting spectra occur at different frequencies. If a resulting spectrum would still allow identifying notches frequencies, the subset of simultaneously fired individual sources for calibration may include individual sources at different depths.
[0036] Returning again to Figure 2, method 200 further includes processing the calibration data to infer at least one actual depth among the distinct depths, and/or a sea-surface reflection coefficient, at 230. The steps of method 200 are repeated for different sets of sources to determine all the relevant depths and reflection coefficients (i.e., for each individual source or per group of individual sources).
[0037] Calibration data may be used to estimate the source signature (e.g., with joint inversion as described in WO 2015044207A1 , or without joint inversion as in the Ziolkowski's method), to update and/or correct individual sources' depths and/or to update a water-surface reflection coefficient for each part of the source. Calibration data may also be used to determine an offset affecting measurements of a depth sensor, as further discussed. The calibration also enables reconstruction of a more reliable ghost mask for the entire incident angle, the ghost mask being an operator that accounts for interference between groups of individual sources due to the geometry and surface reflections. This ghost mask may then be used to enhance the designature processing, i.e., separating the underground formation's response from survey seismic data.
[0038] Figure 8 illustrates a calibration procedure for a multi-level marine seismic source including individual sources disposed at two distinct depths. In a first calibration phase, individual sources at a first nominal depth z1 are fired, yielding a first spectrum with notches frequencies F1. An actual depth z1 ' and a reflection coefficient R1 may be inferred from calibration data recorded after firing these individual sources. Then, in a second calibration phase, individual sources at a second nominal depth z2 are fired, yielding a second spectrum with notches frequencies F2. An actual depth z2' and a reflection coefficient R2 may be inferred from calibration data recorded upon firing these other individual sources.
[0039] The results of the calibration (z1 , z2, R1 and R2) may then be used for accurate reconstruction of the far-field signature, enabling proper designature of the seismic data acquired, and for survey source deghosting. In one embodiment, as illustrated in Figure 9, the individual sources in the calibrated arrangement are fired, to yield a notchless signature. In another embodiment, as illustrated in Figure 10, source depths may be adjusted in view of the calibration to match nominal depths z1 and z2. In this case, if only the individual sources at one of the depths were fired, resulting spectra would have notches at frequencies F1 ' and F2', respectively (FT and F2' being different from F1 and F2). After having their depths adjusted, all the individual sources are fired simultaneously to yield again a notchless signature.
[0040] Figure 1 1 is a graphic illustration of a calibration procedure according to an embodiment. Firing a subset of the individual sources (air-guns) at the same depth at 1 100 triggers recording data using near-field receivers at 1 1 10 and recording data using a mid-field receiver at 1 120. About the same time, at least one depth sensor attached to the multi-level marine seismic source measures depth at 1 130. At 1 140, the data recorded by the near-field receivers and the measured depth are used to calculate individual-source contributions (known as "notionals") to the source signature. Then, at 1 150, the data recorded by the mid-field receiver and the notionals is then used to infer an actual depth and/or reflection coefficient. The source geometry is updated according to the actual depth. A difference between the measured depth and the actual depth is calculated as a calibration factor for the fired individual sources at 1 160 and stored for later use (i.e., when survey seismic data is processed) at 1 170. More than one depth sensor may be attached to the multi-level marine seismic source. One embodiment may have a depth sensor attached to each group of individual sources, while another embodiment may have a depth sensor attached to each individual source.
[0041] Figure 12 is a graphic illustration of obtaining the source signature for seismic survey data processing following the calibration procedure in Figure 1 1. Firing all operational individual sources at 1200 triggers recording data using near- field receivers at 1210. About the same time, the depth sensor(s) attached to the multi-level marine seismic source measures (measure) depth(s) at 1220. Previously stored calibration factor(s) is/are retrieved at 1230, to generate together with the measured depth(s), the source's actual geometry at 1240. The data recorded by the near-field receivers and the actual geometry are then used to obtain notionals at 1250. Then, at 1260, the notionals and the actual geometry are used to calculate the far-field source signature.
[0042] The calibration procedure may be performed at the beginning of a marine survey. Marine survey plans typically include substantially parallel sail lines at predetermined distances from one another. The calibration procedure may be performed at the beginning of each sail line.
[0043] In one embodiment, the calibration procedure is performed while gradually increasing emitted power to limit disruption of marine life. For example, a single gun may be fired first, followed by firing two guns, then more until all operational guns are fired.
[0044] This calibration procedure has the advantage of improving knowledge of actual source geometry. Since the calibration procedure can be performed between survey data acquisition periods, it leads to increasing seismic data processing quality without extending the survey time.
[0045] Figure 13 is a block diagram of a seismic data acquisition system 1300 able to perform the calibration methods. System 1300 includes a multi-level marine seismic source 1310 having individual sources (not shown) disposed at two or more distinct depths. Source 1310 includes a controller 1315 configured to cause a subset of the individual sources to fire emitting the seismic excitations that combine while propagating away from the multi-level marine seismic source. Source 1310 also has attached a depth sensor 1317.
[0046] System 1300 further includes near-field receivers 1320 placed to distinguish seismic excitations as emitted by each of the individual sources, and a distant seismic receiver 1330 configured and disposed to detect an overlap of the seismic excitations emitted by the individual sources and surface reflections thereof.
Near-field receivers 1320 and distant seismic receiver 1330 collect data when a subset of the individual sources is fired during calibration.
[0047] System 1300 also includes survey receivers 1340 carried by streamers.
Near-field receivers 1320 and the survey receivers 1340 collect seismic survey data.
Data processing unit 1350 processes the calibration data to obtain actual depths of the individual sources, and processes the seismic survey data in view of the calibration data.
[0048] Figure 14 illustrates a source controller 1400 according to an
embodiment. The controller, which may be installed onboard the vessel towing the multi-level source, includes a communication interface 1406 and a central processing unit (CPU) 1404. The communication interface is configured to receive source- related information and to send commands to the multi-level marine seismic source. The CPU is configured to generate the commands based on the source-related information, the commands causing the multi-level marine seismic source to fire a subset of the individual sources during a calibration procedure. The CPU is also configured to control the communication interface to send signals triggering calibration data acquisition.
[0049] Controller 1400 is in communication with the marine source, the near- field receivers, the distant receiver and the survey receivers, which are collectively labeled 1412 in this figure. The controller may also include a memory 1408 configured to store calibration factors, calibration data, seismic data and other survey information. Memory 1408 may also store executable codes making CPU 1404 to perform calibration-related calculations. The controller may also include an operator interface 1410. The communication interface, the CPU, the memory and the operator interface may be encapsulated in a single piece of equipment 1402, or may be modular and distributed.
[0050] The controller may receive an actual depth of at least one of the individual sources, via the communication interface (the actual depth having been inferred from the calibration data) and compare the actual depth with a planned depth for the at least one of the individual sources. Then, if a difference between the actual depth and the planned depth exceeds a predetermined thresholds, the control unit may generate a command to adjust the actual depth (or firing moment in the firing sequence) of the at least one individual source to match the planned depth, and control the communication interface to transmit this command to the multi-level marine seismic source. Marine sources able to adjust depth of individual sources are described, for example, in U.S. Patent Application Publication No. 2014/01 12096.
[0051] The disclosed exemplary embodiments provide methods, a system and a controller for calibrating a multi-level seismic source to achieve an improved seismic signature. It should be understood that this description is not intended to limit the invention. On the contrary, the exemplary embodiments are intended to cover alternatives, modifications and equivalents, which are included in the spirit and scope of the invention as defined by the appended claims. Further, in the detailed description of the exemplary embodiments, numerous specific details are set forth in order to provide a comprehensive understanding of the claimed invention. However, one skilled in the art would understand that various embodiments may be practiced without such specific details.
[0052] Although the features and elements of the present exemplary embodiments are described in the embodiments in particular combinations, each feature or element can be used alone without the other features and elements of the embodiments or in various combinations with or without other features and elements disclosed herein.
[0053] This written description uses examples of the subject matter disclosed to enable any person skilled in the art to practice the same, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the subject matter is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims.

Claims

WHAT IS CLAIMED IS:
1. A method (200) for calibrating a multi-level marine seismic source including individual sources disposed at two or more distinct depths, the method comprising: firing (210) a subset of the individual sources to emit seismic excitations that combine while propagating away from the multi-level marine seismic source;
acquiring (220) calibration data representing the seismic excitations, using near-field receivers placed to distinguish the seismic excitations as emitted by each of the individual sources, and a distant seismic receiver disposed to detect an overlap of the seismic excitations and water-surface reflections thereof; and
processing (230) the calibration data to infer at least one actual depth among the distinct depths, and/or a sea-surface reflection coefficient.
2. The method of claim 1 , wherein the individual sources in the subset are disposed at a same one among the distinct depths.
3. The method of claim 1 , wherein the distant seismic receiver is a mid-field sensor located at 20 to 200 m distance from the multi-level marine seismic source.
4. The method of claim 1 , wherein the distant seismic receiver is place on the water bottom.
5. The method of claim 1 , wherein the distant seismic receiver is a receiver located on a side of the subset of the individual sources.
6. The method of claim 1 , further comprising:
measuring a reference depth for the multi-level marine seismic source using a depth sensor; and
calculating a depth correction of the measured reference depth based on the at least one actual depth.
7. The method of claim 1 , wherein the processing of the seismic data includes: obtaining notional signatures corresponding to each of the fired individual sources based on near-field data in the calibration data, the near-field data having been acquired using the near-field receivers;
calculating a predicted source signature based on the notional signatures and a nominal source geometry; and
determining an updated geometry by comparing a source signature detected by the distant seismic receiver with the predicted source signature,
wherein the nominal source geometry and the updated source geometry include relative positions of the individual sources.
8. The method of claim 7, further comprising:
firing the individual sources substantially simultaneously to emit survey seismic excitations;
acquiring survey seismic data representing the survey seismic excitations, using the near-field receivers, and survey seismic receivers disposed to detect reflections of the survey seismic excitations in a surveyed underground structure; calculating a survey source signature based on the notional signatures and the updated source geometry; and
processing the survey seismic data using the survey source signature to generate an image of the surveyed underground structure.
9. The method of claim 1 , further comprising:
firing another subset of the individual sources to emit other seismic
excitations;
acquiring other calibration data representing the other seismic excitations, using the near-field receivers, and the distant seismic receiver; and
processing the other calibration seismic data to infer at least another actual depth among the distinct depths, and/or another sea surface reflection coefficient.
10. The method of claim 1 , wherein the firing and the acquiring are performed at predetermined locations during a marine seismic survey.
1 1. The method of claim 1 , further comprising:
adjusting depths of the individual sources based on the at least one actual depth.
12. A seismic data acquisition system (1300), comprising:
a multi-level marine seismic source (1310) including individual sources disposed at two or more distinct depths, and a controller (1315) configured to cause a subset of the individual sources to fire emitting the seismic excitations that combine while propagating away from the multi-level marine seismic source;
near-field receivers (1320) placed to distinguish seismic excitations as emitted by each of the individual sources;
a distant seismic receiver (1340) configured and disposed to detect an overlap of the seismic excitations and water-surface reflections thereof; and
a data processing unit (1350) configured to infer at least one actual depth among the distinct depths, and/or a sea-surface reflection coefficient by processing the calibration data,
wherein the calibration seismic data is acquired using the near-field receivers and the distant seismic receiver when firing the subset of the individual sources.
13. The seismic data acquisition system of claim 12, further comprising:
a depth sensor associated with the multi-level marine seismic source and configured to measure a reference depth,
wherein the data processing unit calculates a depth correction of the measured reference depth based on the at least one actual depth.
14. The seismic data acquisition system of claim 12, wherein the individual sources in the subset are disposed at a same one among the distinct depths.
15. The seismic data acquisition system of claim 12, wherein the data processing unit
obtains notional signatures of each of the fired individual sources based on near-field data among the seismic data, the near-field data being acquired using the near-field receivers,
calculates a predicted source signature based on the notional signatures and a nominal source geometry, and
determines an updated geometry by comparing a source signature detected by the distant seismic receiver with the predicted source signature,
wherein the nominal source geometry and the updated source geometry include relative positions of the individual sources.
16. The seismic data acquisition system of claim 15, further comprising:
survey seismic receivers configured and placed to detect reflections in a surveyed underground structure of survey seismic excitations emitted by the multilevel marine seismic source, and
the data processing unit is further configured to process survey seismic data related to the survey seismic excitations and acquired using the near-field receivers, and the survey seismic receivers by
obtaining survey notional signatures of each of the fired individual sources,
calculating a survey source signature based on the survey notional signatures and the updated source geometry; and
processing the survey seismic data using the survey source signature.
17. The seismic data acquisition system of claim 12, wherein the source controller controls the multi-level marine seismic source to adjust a depth of one of the individual sources to match a planned value.
18. A marine source controller (1315, 1400) of a multi-level marine seismic source including individual sources disposed at two or more distinct depths, the controller comprising:
a communication interface (1406) configured to receive source-related information and to send commands to the multi-level marine seismic source; and a central processing unit (1404) configured
to generate the commands based on the source-related information, the commands causing the multi-level marine seismic source to fire a subset of the individual sources during a calibration procedure, and
to control the communication interface to send signals triggering calibration data acquisition.
19. The controller of claim 18, wherein the central processing unit receives an actual depth of at least one of the individual sources, via the communication interface, and
compares the actual depth with a planned depth for the at least one of the individual sources.
20. The controller of claim 19, wherein, if a difference between the actual depth and the planned depth exceeds a predetermined thresholds, the central processing unit generates a command to adjust the actual depth of the at least one individual source to match the planned depth, and the communication interface transmits the command to the multi-level marine seismic source.
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