Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
  • G01V1/00—Seismology; Seismic or acoustic prospecting or detecting
  • G01V1/16—Receiving elements for seismic signals; Arrangements or adaptations of receiving elements
  • G01V1/18—Receiving elements, e.g. seismometer, geophone or torque detectors, for localised single point measurements
  • G01V1/186—Hydrophones
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
  • G01V1/00—Seismology; Seismic or acoustic prospecting or detecting
  • G01V1/16—Receiving elements for seismic signals; Arrangements or adaptations of receiving elements
• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
  • G01V1/00—Seismology; Seismic or acoustic prospecting or detecting
  • G01V1/16—Receiving elements for seismic signals; Arrangements or adaptations of receiving elements
  • G01V1/18—Receiving elements, e.g. seismometer, geophone or torque detectors, for localised single point measurements
• • 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
  • G01V1/3808—Seismic data acquisition, e.g. survey design
• • 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
  • G01V2200/00—Details of seismic or acoustic prospecting or detecting in general
  • G01V2200/10—Miscellaneous details
  • G01V2200/12—Clock synchronization-related issues

Definitions

• Drift is the rate at which an oscillator of the seismic node gains or loose frequency in relation to a specified frequency. All oscillators will experience frequency changes though at different rates. Drift in an oscillator causes changes in the frequency of the oscillator of the seismic node, which will results in timing errors.
• the frequency accuracy of an oscillator is the offset from the specified target frequency.
• the frequency stability of the oscillator is the spread of the measured oscillator frequency relative its operational frequency during a period of time.
• FIG. 10 A comparison among different type of oscillators is shown in Fig. 10.
• Oscillators may also need some time from startup before they reach the necessary stability in their output frequency. According to different accuracy, stability and cost requirements, different types of oscillators are developed. Compensation of the temperature dependence has resulted in oscillators based on different
• CXO which uses a temperature compensation circuit
• Oven Controlled Crystal Oscillator "OCXO” which uses an oven to control the crystal temperature
• US 2005/0246137 illustrates a method and system for acquiring seismic data without the need for wire line telemetry or radio-telemetry components or radio initiation.
• a plurality of individual wireless seismic data acquisition units are used wherein the individual data acquisition units may function as data sensor recorders and/or as source-event recorders.
• Each data acquisition unit records an
• the data acquisition units do not require radio contact with other data acquisition units, nor do they require direct synchronization with other receiver units or with a source start time.
• US 2009/0080290 discloses a nodal seismic data acquisition system that utilizes an external, common distributed time base for synchronization of the system operation.
• the system implements a method to correct the local time clock based on intermittent access to the common remote time reference.
• the method corrects the local time clock via a voltage controlled oscillator to account for
• US 2010/0034053 discloses a method for acquiring seismic data by recording seismic data with a plurality of autonomous seismic data acquisition units wherein each acquisition unit comprises a digitally controlled temperature-compensated crystal oscillator. Oscillator-based timing signals are acquired that are associated with the plurality of digitally controlled temperature compensated crystal oscillators and a time correction is determined using the oscillator-based timing signals from the first and second autonomous seismic data acquisition unit.
• US 7254093 discloses a seismic data collection unit or pod comprising a water tight case. The case houses other components that may include a clock, a power source, a control mechanism and a seismic data recorder.
• Seafloor Seismic Recorders "SSR" units of the Ocean Bottom Seismic “OBS” type generally include one or more geophone and/or hydrophone sensors, a power source, a seismic data recorder, a crystal oscillator clock, a control circuit, and, in instances when gimbaled geophones are used and shear data are recorded, compasses or gimbals.
• US 7558157 illustrates that in order to reliably and accurately accomplish error- free data from a suite of independent sensors/nodes or an array of sensors, each node includes an atomic clock.
• the central data receiver/processor also includes an atomic clock.
• Each node transmits a time-stamped pseudo-random code. The processor compares the time-stamped pseudo-random code
• US 8050140 discloses self-contained ocean bottom pods characterized by low profile casings.
• a pod may include an inertial navigation system to determine ocean bottom location and a rubidium clock for timing.
• a clock that is affected by gravitational and temperature effects can cause a frequency shift in the oscillator frequency, thereby resulting in errors in the seismic data.
• the use of a rubidium clock which is less susceptible to temperature or gravitational effects or orientation of the unit on the ocean bottom, will result in accurate seismic data recording.
• the seismic nodes can remain underwater for a longer duration with less maintenance. This will provide more flexibility to operations and also reduce expenses.
• seismic nodes have been proven to be difficult to operate due to the operational difficulties mentioned above.
• the invention has been conceived to remedy or at least alleviate the above stated problems of the prior art.
• the invention provides a seismic node device comprising an primary oscillator and a reference oscillator, a processor for controlling the frequency calibration of the primary oscillator using a frequency calibration value based on a frequency provided by the reference oscillator, and wherein the processor repeatedly calculates the frequency calibration value.
• the TCXO used as primary oscillator in the present invention, may only use about 8-12 mW or less, but it will provide an inaccurate frequency leading to inaccurate sampling of the seismic data. In general, the more accurate an oscillator is, the more power it will use.
• an RF unit can be chosen to work with a TCXO unit in a seismic node to provide high precision data recordings from the ocean bottom with optimal power efficiency at an average power consumption of 20 - 50 mW or less.
• the method comprises calculating a frequency calibration value for the frequency of the primary oscillator based on a frequency of a reference oscillator, calibrating the frequency of the primary oscillator using the frequency calibration value, wherein the frequency calibration value is repeatedly calculated at predetermined time intervals, and wherein start and stop signals are sent to the reference oscillator in longer or shorter cycles of predetermined time intervals depending upon the requirement of overall system time accuracy.
• a seismic data acquisition system comprises a plurality of autonomous seismic nodes, at least one of the autonomous seismic nodes comprising a primary oscillator and a reference oscillator generating a reference frequency, a temperature sensor for detecting ambient temperature, an inertial sensor for detecting movements of an oscillator, a memory storing digital recorded seismic data, generated from hydrophone(s) and/or geophone(s), and a power source supplying electrical power to the system, wherein a processor calculates a calibration factor based on input from the temperature sensor, the inertial sensor, and the reference frequency, and wherein the processor repeatedly calculates and calibrates the primary oscillator using the calibration factor at predetermined time intervals, providing high precision data readings from the ocean bottom with optimal power efficiency.
• a computer device in another embodiment, includes a recording device and a calibration module.
• the calibration module has instructions that are executed by the computer device.
• the instructions include the following logic: receiving environmental data from the environmental sensors and decide upon a predetermined program, comparing a frequency of a primary oscillator and a frequency of a reference oscillator, calculating a calibration factor based on the comparison of the received data, wherein the computing device is configured to repeatedly calibrate the frequency of the primary oscillator using the calibration factor at predetermined time intervals, and this is used to achieve high precision data readings with optimal power efficiency.
• a reference oscillator is operating continuously during deployment of the seismic nodes and during retrieval.
• the reference oscillator is stopped first after the node at the ocean bottom has become stationary and the temperature sensor indicates that the ambient and/or the internal temperature have stabilized.
• the reference oscillator will be calibrated and synchronized against an onboard vessel master clock before being deployed, and will be calibrated, if necessary, against said vessel master clock after the seismic nodes have been recovered onboard.
• the reference oscillator can be an atomic or any other high quality oscillator.
• a primary oscillator that consumes much less power is placed outside a reference oscillator. After a predetermined time the reference oscillator is started and regulation/correction or calibration of the frequency of the primary oscillator is performed using the frequency of the reference oscillator.
• the primary oscillator may have a relatively substantial drift in frequency over time, but as long as it is frequently corrected or calibrated, the primary oscillator frequency will be stabilized over time and provide accurate readings.
• the reference oscillator may receive power from a power source using a power switch.
• the power source can be an internal battery or an external power source.
• the oscillator device as per the invention operates at high power efficiency because of intelligent starting and stopping of the reference oscillator.
• the processor is programmed to perform an additional calibration. Based on a formula determined by survey parameters, the system can also run both reference and primary oscillators simultaneously, this to check frequency variations between normal check points. If the system registers a frequency difference between the reference oscillator and the primary oscillator that is close to being unacceptable, and the same or larger variation is registered at the next calibration point, then both oscillators will be run in parallel until the recording accuracy is acceptable. When the calibration check of the primary oscillator indicates that it is not necessary to run the calibration check as often, then the calibration check interval can be increased by the predetermined program. If the opposite happens and the time or frequency drift is larger than acceptably, the calibration interval can be made longer.
• the reference oscillator runs continuously during deployment and is stopped first after the node at the ocean bottom has become stationary and the temperature sensor indicates that the ambient and/or the internal temperature have stabilized.
• the node receives instructions using a wireless access point and can communicate with a central master clock.
• the reference oscillator may runs continuously during the process of recovering the nodes from the ocean bottom. During this activity the primary oscillator may be influenced by the movements of the nodes and/or temperature changes.
• the calculation and calibration of the frequency calibration value is programmed on board the control vessel, and computed based on water depth, the sound velocity in water, ocean bottom conditions and depends on expected possible delay in acquiring the seismic, or any other operational requirements.
• the present invention will overcome many of the limitations that are inherent in existing systems, as the described use of a reference oscillator together with a primary oscillator enables more precise mapping and that the seismic nodes can stay longer underwater once they have been deployed.
• the new underwater seismic sensor node provides precise timing based upon an adaptive calibration method, where previous data will be used in the calculation.
• the proposed new apparatus will be well suited for conducting OBS surveys at any water depth and the use of such an apparatus will significantly lower the costs compared to the existing systems for acquisition of seismic data.
• Figure 1 illustrates a block diagram of a seismic node apparatus according to an embodiment of the invention.
• Figure 2 illustrates a block diagram of a data acquisition system including multiple seismic nodes according to an embodiment of the invention.
• Figure 3 illustrates data acquisition with multiple seismic nodes deployed at the ocean bottom and a vessel towing a seismic source for generating the seismic signals, according to an embodiment of the invention.
• Figure 4, 5 and 6 illustrate a method of saving power by a programmed sequence to alternate between which nodes that at a certain time uses the reference oscillator.
• Figure 7 Illustrates a method of data acquisition where one or more low power seismic nodes that only contain a primary oscillator are located in between seismic nodes that have both a primary oscillator and a reference oscillator.
• Figure 8a and b illustrate how a correction may be applied.
• the seismic node 100 can have one or a plurality of sensors, such as geophones, hydrophones or accelerometers with associated electronic such as processors, hardware for filtering, digitalizing the recorded data, storing of data and power supplies. Here this is covered by the block marked as 1 18.
• the autonomous seismic node 100 may be a standalone device, a distributed device, or may operate as a standalone computing/data acquisition device.
• the reference oscillator 104 generates a signal repeatedly after a fixed or an adaptively calculated time period based on a set of pre-determined parameters 120 and transmits the signal to a frequency controller 108 using a system bus.
• the primary oscillator 106 also transmits the frequency which it is currently generating to the frequency controller 108.
• the signals from the reference oscillator 104 and the processor 1 12 are compared with the frequency received from the primary oscillator 106 and a digital signal for correction/calibration is sent to the dedicated digital to analog converter 1 10.
• the converter 1 10 sends the analog control signal to the primary oscillator 106 for calibrating the frequency of the primary oscillator 106.
• the central master clock 202 can have a central data receiver and a central processor which receives a data stream from a seismic node 100 and organizes the data streams according to their respective corresponding samples.
• data correction and/or calibration is improved in comparison with conventional seismic nodes not having an AT.
• the AT as the reference oscillator 104 is used with the primary oscillator 106 in the seismic node 100.
• Using the AT at each of the seismic nodes 100 allows simple correlation of data among different seismic nodes 100 as well as with the central master clock 202.
• FIG 3 illustrates how seismic nodes are deployed under water and how the seismic signals are generated from the vessel and received by the seismic nodes.
• a grid of geophysical sensors or the seismic nodes 100 are placed on the ocean bottom 306 to help in determining the likely places where hydrocarbons can be located.
• the seismic nodes 100 can be dropped over the side of a vessel or laid down by a remotely operated vehicle.
• the seismic nodes 100 can either be independent deployed or deployed as autonomous nodes in an ocean bottom cable (OBS).
• OBS ocean bottom cable
• Each of the seismic nodes 100 typically includes a hydrophone, one or more geophones, recording units and memory unit to store recorded data.
• a standard seismic source vessel 302 can be used for generating a seismic signal by for example, a traditional air gun array.
• the source vessel 302 moves in a pattern that allows the energy source to be fired from multiple different positions relative to the grid of the seismic nodes 100.
• Some of the seismic energy reflects off the sea bed 306 and back to the sea level 304, the remaining seismic energy penetrates the sea bed 306, travels through geological layers 308, to for example potential reservoir rocks 310.
• the seismic energy reflects back to the seismic nodes 100 on the ocean bottom 306 where it is detected.
• the seismic nodes 100 are retrieved.
• the seismic nodes 100 may form a specialized array that rests on the ocean bottom and is used for data acquisition. Due to the complexity associated with establishing survey lines or laying the seismic nodes 100, different field equipment is used depending on the depth of water, temperature levels and other
• a variety of seismic sources are available for marine applications, including water guns, air guns, sparkers and boomers.
• environmental noise may be recorded. Since the present invention records seismic data in an autonomous mode, this noise may be recorded by a separate data acquisition system with real time data transfer to one of the vessels for quality control purposes.
• Several options for environmental noise recording are feasible.
• One option is to record the data with a short seismic streamer towed behind the source vessel 302.
• the streamer is typically equipped with hydrophones which sense the environmental noise.
• the data is transferred to the recording system on the vessel 302 through the streamer. Analysis of the data can then be performed on the vessel 302.
• the data is transferred through a lead-in cable to a recording buoy and then transferred by radio to one of the vessels for analysis.
• hydrophones can be mounted on the lead-in cable and the geophones left out.
• Figures 4, 5 and 6 illustrate another method that may be preprogrammed to alternate between the nodes in a sequence that will be run with the reference oscillator turned on (in Fig 4 the nodes 100a and 10Of), while the other nodes are controlled by the primary oscillators.
• the sample shows that every 5 th node in a line is turned on.
• the nodes controlled by the reference oscillator have been shifted to the next in line.
• Still every 5 th oscillator is turned on, but now it is another set (marked 100b and 100g) of nodes that have the reference oscillators turned on.
• Fig. 6 illustrates yet another sequence of reference nodes (100c and 10Oh) that are turned on.
• Figure 7 illustrates a method of data acquisition of an ocean bottom seismic survey where one or more low power seismic nodes (i) that only contain a primary oscillator are located on the seafloor (k) in between seismic nodes (j) that have both a primary oscillator and a reference oscillator.
• Improved timing of the seismic data recorded by the seismic nodes (i) may be achieved in post survey data processing, this by using the different arrival times of the direct or reflected seismic arrivals (T1 and T2) at the seismic nodes (j) for adjusting the seismic data recorded at intermediate nodes (i).
• Figure 8a shows a primary oscillator with drift as a function of time, and where a correction is applied down to the specified frequency fo.
• Figure 8b shows a correction where drift is compensated for by making a frequency adjustment to below the specified frequency, this to achieve a reduced deviation from specified frequency fo.
• Figure 10 shows comparison among oscillators.
• Figure 1 1 illustrates an alternative to the block diagram in Fig. 1 .
• a frequency controller 130 with a built-in phase lock loop replaces the frequency controller 108 and Digital to Analog Converter 1 10 shown on Fig. 1 .
• the other components shown on Figure 1 1 are explained in connection with Fig. 1 .
• independent protection e.g. by divisional applications
• this relates to calibrating an oscillator to save power.
• the frequency of an oscillator as defined, without any restriction to any field of application, and/or the use of the basic calibration method for drift calibration, wherein start and stop signals are sent to the clock of the reference oscillator in longer cycles of predetermined time intervals for system time accuracy.

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