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Advances in geophysical sensor data acquisition
- 1. Advances in geophysicalsensor data acquisition Robert GIRAULT, Philippe ALAIN 12 th ASF Congress, Rennes FRANCE
- 2. Contents Introduction ECHOESOverview Transducer Design Power Calculation Resolution Comparison Data Acquisition Data Processing
- 3. Introduction IXSEA manufactures a full range of product including: Motion Sensors OCTANS – FOG Motion Sensor PHINS – High Grade INS ROVINS – Construction Grade INS Underwater Positioning GAPS – Pre-calibrated USBL with embedded INS Moorings OCEANO – Light-Weight to Heavy Duty Acoustic Releases Geophysics ELICS – Side-Scan Sonar SHADOWS – Synthetic Aperture Mapping Sonar GRADIO/MAGIS – Gradiometer & Magnetometer ECHOES – Sub-Bottom Profilers DELPH – Geophysical Acquisition & Processing Software
- 4. Introduction What arewe trying to achieve with Sub Bottom Profilers ? High resolution Deep penetration Efficiency – Time between acquisition and deliverable chart. ECHOES 10000 ECHOES 5000
- 5. ECHOES Overview ECHOESis a Range of four Chirp based Sub-Bottom Profilers From low frequency and very high power model “deep tow boomer replacement” To very high resolution 10kHz systems. ECHOES use either Janus-Helmholtz or Tonpilz transducers. Acoustic output is flat (within 3dB) over the entire operating range of the transducers, thus allowing fully tuneable pulse models ECHOES 1500 Towable High Power 650 Hz to 2500 Hz ECHOES 3500 Hull mount High Power 1800 Hz to 5200 Hz ECHOES 5000 Towable High Power 2000 Hz to 7500 Hz ECHOES 10000 Configurable High Frequency 5000 Hz to 15000 Hz
- 6. Transducer Design –Tonpilz Transducer Tonpilz Transducer: Stack of ceramic rings Heavy tail mass Lighter head mass The ceramic rings are compressed between the tail and head ECHOES 3500 ECHOES 10000
- 7. Transducer Design -Janus Helmholtz Janus Helmholtz: Two Tonpilz transducers mounted back to back. Cylindrical housing creates Helmholtz cavity. Helmholtz resonance caused by fluid inertia. Resonance extends the operating frequency of the transducer at the low end of the spectrum Typical Signal Level response of a transducer. ECHOES 1500 ECHOES 5000
- 8. Advantage of chirptechnology Correlation process improves the signal to noise ratio Additional gain, referred as processing gain , is defined as: PG = 10 log (B * T) where : B = bandwidth of the pulse T = pulse length ECHOES 1500 : B = 2850 Hz T=100ms PG=24dB ECHOES 5000 : B = 5500 Hz T=50ms PG=24dB Measured Emitted Acoustic power: 201dB re 1 μPa at 1m for the 1500 203dB re 1 μPa at 1m for the 5000 ECHOES 1500 has an apparent acoustic power of 225dB ECHOES 5000 has an apparent acoustic power of 227dB
- 9. Resolution of conventionalsystems For a monochromatic Sub-Bottom Profiler system such as a boomer or a sparker, the resolution may be calculated by: R=C*L where: R = Resolution (m) C = sound speed (1470 m/s) L = Pulse Length (s) Typical boomer: At low energy, 120 µs pulse Resolution = 17 cm More often resolution in the range of 25 cm.
- 10. Resolution of Chirpsystems Resolution function of the bandwidth (maximum frequency less the minimum frequency of the sweep): R=C/2*B where B= bandwidth of the pulse (Hz) C= Sound speed (1470 m/s) ECHOES 1500 650-2500Hz 1.7kHz Bandwidth theoretical resolution = 25cm ECHOES 3500 1,800-5,200Hz 3.4kHz Bandwidth theoretical resolution = 21cm ECHOES 5000 2,000-7,500Hz 5.5kHz Bandwidth theoretical resolution = 13cm ECHOES 10000 5,000 -15,000Hz 10kHz Bandwidth theoretical resolution = 7cm
- 11. How to enlargethe bandwidth ... One Janus-Helmholtz transducer has 3 kHz bandwidth around 3000 Hz. Mounting three tuned transducer increase the bandwidth up to 5000 Hz
- 12. Power versus ResolutionBoomer 200J= 212dB 26cm resolution. Increasing the Power of a boomer adversely impacts resolution. ECHOES 1500 223dB 25cm resolution ECHOES 5000 227dB 13 cm resolution Increasing ECHOES power has no effect on resolution.
- 13. Towing Surface TowAdvantages Easy deployment No sea surface reflection Disadvantages Motion Vessel Noise “ Deep Tow” Advantages No Vessel Noise Reduced Motion Disadvantages Winch and Cable Sea Surface Reflections
- 14. DELPH Seismic AcquisitionSoftware A New Philosophy Acquisition systems are generally under utilised. Only need basic signal processing to monitor the quality of the gathered data. Extreme processing during acquisition can hide poor quality data, that may only come to light when it is too late to do anything about it. Acquisition systems must be reliable. The more features a software product has the larger the code base needs to be and inevitably there is more potential for bugs. Bugs are really bad in acquisition. Vessel time is expensive! Therefore keep the acquisition simple. Keeping the acquisition simple involves a separate package dedicated to processing and interpretation. It no more needs a moving waterfall display … ONSHORE OFFSHORE Acquisition Storage QC Processing Interpretation
- 15. DELPH Seismic Processing& Interpretation Software ONSHORE OFFSHORE Acquisition Storage QC Processing Interpretation XTF or SEGY XTF or SEGY
- 16. DELPH Seismic AcquisitionSoftware
- 17. DELPH Seismic Processing& Interpretation Software Whole line approach for processing Processing is no more linked to data replay. Escape from the Change-Replay- Change-Replay- Change-Replay- cycle … Processing changes can be done on the displayed area or the whole line without scrolling Whole line approach for interpretation Zoom from sample resolution to large geological features on-screen Speed-Corrected and Geo-Referenced profile to perform thickness, slope measurements Direct access to geo-referenced data at any scale The processing and interpretation software is no more based on an acquisition package Video
- 18. DELPH Seismic Processing& Interpretation Software
- 19. ECHOES 1500 Datain DELPH Seismic Interpretation
- 20. ECHOES 3500 Datain DELPH Seismic Interpretation
- 21. ECHOES 5000 Datain DELPH Seismic Interpretation
- 22. ECHOES 10000 Datain DELPH Seismic Interpretation
- 23. The Future? ECHOES1500 as a source for a conventional multichannel ultra high resolution seismic system. ECHOES with “INS Inside” allowing extremely accurate shot-point distance control and positioning leading to true 2D stacking of multichannel SBP data. Multiple streamer 3D seismic acquisition and processing using conventional streamer technology combined with ECHOES and INS.
Editor's Notes
- #4 Quick intro abaout IXSEA product and then focus on ECHOES and DELPH as associated products This presentation will explain what is different and new about the Echoes products and introduce a new approach to SBP data processing which has the potential to make a real difference to workflow.
- #5 First lets look at what we are trying to achieve with Sub bottom profiling. These two images are shot over the same area with two different SBP systems, the image on the left is a high resolution system, whilst the image on the right is lower resolution but has better penetration. The images are an example of the typical trade off required when picking which SBP system to use. In an ideal world we would have the resolution of the image on the left with the penetration of the image on the right. In the real world we have to make a choice, if we are lucky we can use multiple systems.
- #6 The Echoes products cover the whole range of frequencies and powers typically used offshore. The product of most interest to me when I found out about it was the Echoes 1500, This is a low frequency wide band chirp system operating at up to 9kW This is extremely high power and covers a frequency band ranging from 650Hz to 2500Hz, a massive bandwidth at these frequencies. The Echoes 3500 is a hull mount pinger alternative with high power and good beam width, the wide band capability improving on the resolution however. The 5000 has at least the resolution and penetration of the best surface tow boomer The 10000 is an extremely high frequency system more suited to shallow water ports and harbours type work where absolute maximum resolution in soft sediments is required.
- #7 OK, so what makes the Echoes products special, The key point is with the transducer technology, Echoes transducers are specially designed for very wide band operations. They start with a basic Tonpliz design, but specially modified to produce a wide band response. As described on the slide, a Tonpliz transducer consists of a stack of ceramic rings squeezed between a heavy tail mass and a head mass, when the ceramics are electrically excited they expand and contract to cause movement of the head mass and therefore inducing pressure waves in the surrounding fluid.
- #8 In the Echoes 1500 product we have a single large Janus Hemholtz transducer, the Echoes 5000 uses three different Janus Hemholtz transducers to get even wider bandwidth. A Hemholtz transducer consists of two Tonpliz transducers stacked back to back within a hollow tube. The hollow tube has a gap in it allowing fluid to enter. When the head masses expand and contract the volume inside the tube also changes forcing water to flow in and out of the tube. Water has mass, and therefore inertia so when the tube stops expanding the water continues to flow in to the cavity for some time. The same happens when the cavity stops contracting. The overall effect here is to induce Hemholtz resonance in the cavity which typically peaks at a lower frequency than the Tonpliz transducers could. The overall effect is a widening of the bandwidth of the transducer.
- #10 A typical boomer system running at very low energy settings is able to achieve a pulse width as low as 120 µs. If we assume a velocity of 1450m/s this equates to a resolution of 17cm. In actual fact power is rarely set this low, and increasing the power increases the pulse width so typical operational resolutions are more likely to be in the order of 25cm
- #12 Bandwidth is the key of good SBP, Increasing the bandwidth is a goal to improve resolution. Three tuned transducers are used to increase the bandwidth,
- #14 Resolution and power considerations are only one part of the story. Stability of the system in relation to the seabed, and ambient noise levels also have a huge impact on the quality of the final data. If the platform containing the SBP system is moving around a great deal the produced data may be unusable. Typical examples here are surface towed systems. These systems can be excellent in the right conditions, and even a large swell does not rule out their operation as we can use a sell or heave filter to eliminate the effects. What really impacts the data quality is quick unpredictable motion such as caused by “chop” or small waves which swell filters tend to struggle with. By towing a system “Deep” and here I don't mean Full ocean depth, just deep enough to get away from the sea surface effects and noise from the towing vessel we can maximise our chances of obtaining the best possible data. Of course given enough resources we would like to have our SBP systems very close to the seabed in order to minimise beam spreading and to get the absolutely most stable and quiet environment. However here we hit another of the trade-offs we tend to get used to with SBP systems. When towing close to the seabed it is difficult to get the power we need to the SBP system, cables become large and expensive and handling systems have to grow in proportion. This is why the Echoes 1500 is configured for towing at up to 300m even though the transducer is capable of much much deeper operation.
- #15 On the left we see the typical processing flow for SBP or SSS data. Data is gathered by the sonar operators, passed to a geophysicist to confirm the quality is OK, then either offshore, but more normally onshore it enters a completely different processing and interpretation phase. The Delph approach to software is to provide the right job. The acquisition phase is all about getting the data recorded whilst checking the basic quality of the data. For this we provide a simple small acquisition package that is tuned specifically for this operation. If a function is not absolutely necessary for ensuring the data is of high quality and stored safely then it is not included in the acquisition packaged. Typically in the offshore environment, SBP and Sonar acquisition systems are vastly under utilised. Operators of acquisition systems are there to ensure the acquired data is complete and of good quality. There is a great temptation to an operator of an acquisition system to take a profiling system that is producing dubious quality data and apply a huge array of processing tools to the data during acquisition In this way, data that has obvious flaws is made to look acceptable, with the assumption that further processing will improve it even more. In reality there is no possible improvement because all the processing tools have already been used to their maximum. By limiting the tools available in acquisition to gain control and band pass filtering sub optimal data is quickly identified changes to the data quality during a job can be noted easily
- #16 Here we see the range of DEPH SBP tools and how they correlate with the typical data flow. Explain.
- #18 Typical SBP or SSS processing software takes a scrolling waterfall display approach to processing. Changes are made on a ping by ping approach and the data played back in the same manner as in acquisition Changes to the processing parameters are only applied from the time the change is made rather than for the line as a whole. If the operator makes a change to the gain settings for example he / she has to rewind the data to the beginning of the file and start replaying the data again. Because the delph processing software does not use a waterfall display, but instead treats the data as an image (as far ad the user is concerned) it means processing can be applied to the whole line or section of line as the user wishes, without having to go back to the start and ”replay” the data. Modern advances in computing technology mean it is no problem to keep the entire line in memory and the old view of replaying the data from tape is now obsolete Processing applied to data in memory is vastly faster than reading in data from disk and applying processing on a ping by ping basis.