1RDI's measuring sticks Performance Quality Reliability Service / Support
2Welcome! Web site: www.adcp.com Welcome to this Training Session from the TRDI University … it's one of most popular whenever it is scheduled at conferences & on InternetTopic: Optimizing ADCP PerformanceOne reason I'm sure that this session is well attended is that the only person I know who enjoys reading a manual is its author.Before we get into the details … there a couple of related items on TRDI Web site that you might find usefulCompanion document with the same title … Tech Tips page … describes more details on this topicPLAN program … Home Page … interactive way to examine more exactly the choices and consequences we talk about here.Web site:
3ADCPs on ShipsThis screen should be up while everyone gets logged in and up to speed. This is a good time for brief conversations with people as they log in and get everything running. If there is time, try and get a feel for the level of experience of the audience by asking direct questions, e.g. have you worked with ADCPs before? How many deployments? Why did you sign up for this class? etc.Once everyone is up and running:This is the first of three courses we have prepared for those who intend to use an Acoustic Doppler Current Profiler from a moving boat. This series of courses is intended to be application-oriented, that is, we will focus on using the ADCP from the perspective of understanding the instrument’s capabilities and the measurements it can make.In this first course we will present a basic description of how the ADCP works from a moving vessel, present information on how to choose the most appropriate ADCP, and give a few examples of successful installations.In the second course we will go into considerably more detail on the operational characteristics of the instruments, talk a bit about the choices to be made prior to and during installation, and introduce the software packages used for data acquisition and playback.In the third course we will focus on data quality control and assurance. We will present additional software packages we use to investigate data, and present some of the operational limitations of the instrument – wherever possible using real data. We will also demonstrate how some users have taken advantage of our ancillary measurements for research other than current measurements.We are presenting this through WebEx, and there are some things that remain out of my control – your connections speed for instance. If your browser is loading slow compared to what I am saying, please let me know and I will try to adjust my presentation. Also, please feel free to ask questions at any time. Let’s get started.
4Your instructor Pete Spain Title: Research Scientist, Technical MarketingTime at RD Instruments:Education: Ph.D., Ocean Physics, Velocity ProfilersRelevant Writings: See TRDI Web site -- Tech Tips pageContact: Cell:Greetings Everyone, thanks for joining me.I am Pete Spain and I will be your host for this session.You can see I've been at Teledyne RDI for a whileBefore that I worked in Ocean Research at the Applied Physics Lab in Seattle and then at Scripps Institution of Oceanography.My mentors were the scientists who invented the very first velocity profilers, almost 40 years ago.You will find a lot of additional technical materials I've put together on the TRDI Web site -- Tech Tips pageAnd you can contact me using the details here.Pete Spain
5Course objectives Current Surveys Ocean Surveyor / WH Mariner Data By completing this course, you will understand clearlyCurrent SurveysOcean Surveyor / WH MarinerDataInstallation ConcernsSoftware SetupData QA/QCOperational Limitations
6Measuring Water in Motion ADCPs Acoustic Doppler Current Profilers So, let’s talk a little bit about RDI. Our slogan is Measuring Water in Motion and Motion in Water. The Water IN motion part is about our current measurement technology.
7Why Make Spatial Current Surveys? Applied ScienceOceanographyCoastal DynamicsEnvironmental InformationImpact statementsAdvection – Dispersion modelingFeeding behaviorForcing on structuresSo, why would measuring the currents over some spatial area be important? Two reasons, really: basic science or monitoring for some other reason. A substantial fraction of ADCPs operating from vessels have been installed and are being operated by oceanographers and coastal researchers who are interested in learning about the currents themselves. There are many interesting questions about ocean flows that remain unanswered, and a large number of people who are working on them. There is a second group, that is becoming larger every day, that are really interested in the currents only as a parameter that is affecting the process they wish to study. For example, a detailed understanding of the currents is required to place an outfall or to construct a coastal structure. Increasingly the ADCP current measurements are being used as inputs to models designed to predict the advection / dispersion of contaminants. Some researchers are using site surveys of currents in order to better understand the feeding and spawning behavior of fisheries. And some understanding of the currents is required to anticipate the possible forcing on offshore structures.
8Who/What is Teledyne RDI? Part of Teledyne TechnologiesHigh technology business in San Diego, California, since 1981Develop advanced technology systemsMake them useful to a broad range of marine/river/navigation applications190 staff and over $36M in 2005 salesMore than 7500 ADCPs sold worldwidePresently 60 units per month24/7 Customer SupportISO certifiedWe gave a lot of background on RDI last time, but I wanted to just put up this summary slide about who we are.In a nutshell, we were first, and we remain the largest ADCP manufacturer. The numbers up there that I find eye popping are that we are now shipping over 50 units a month.Also, we show our dedication to quality by this year obtaining our ISO9001:2000 certification. And we long established the first, and I believe still the only, 24/7 support line in the industry.
9History of Innovation 1981: First commercial ADCP in the world 1985: First river discharge ADCP1991: Broadband ADCP patent1994: Phased array ADCP patent1995: Workhorse ADCP1999: ADCP waves array patent2001: Horizontal ADCP2002: ZedHed: Shallow-depth ADCP2003: StreamPro, Channel MasterHere is an abbreviated list of RDI firsts. We continue this history of new developments today and into the future.
10Coastal Dynamics Chesapeake Bay Plume Offshore of North Carolina Another pole mounted system, this Workhorse was used o investigate the Chesapeake Bay plume as it advected down the coast of North Carolina.
11Coastal Dynamics Tidally generated eddy off Three Trees Point Photo and plot courtesy of Prof. Parker MacCready (University of Washington).Here is an example of an Ocean Surveyor on a pole-mounted installation on the University of Washington’s RV Barnes. Professor Parker MacReady at UW was interested in the dynamics of a tidally driven eddy that forms off Three trees Point in the Puget Sound. He developed a novel technique to explore this regular feature. He went out in the Barnes every day for seven days to repeat circular transect at a specific spot. Each green path above represents a days worth of going around that circle. Each day’s data then has its time renormalized to high tide, and then the data can be combined to investigate the formation and decay of the eddy. This plot shows the eddy at two hours after high tide.
12Vessel Mounted ADCPs JAMSTEC RV Kaiyo off of Mindanao, Philippines The RV Kaiyo is relatively new swath vessel in the JAMSTEC fleet. It is equipped with an Ocean Surveyor OS38, and I am going to show some data form its initial shake-down cruise. It is here operating off the coast of the Philippines, and I have put the track of interest onto the chart.ClickHere is what a lower frequency system would have seen along this track. The Mindanao Current is visible here in blue.However, here is what they were able to see at the full range of the OS38. There is a very large, subsurface eddy here, with currents approaching 80 cm/s and roughly 100km in diameter.
13Courtesy Charles Flagg, Brookhaven National Laboratory Volunteer Observing ShipsThere are a number of Volunteer Observing Ships now equipped with ADCPs making repeated transects in various parts of the world.ClickThe Oleander is a container ship making weekly runs from the US to Bermuda. It is operated by the Bermuda Container Line, and has been equipped with an ADCP since It was the first VOS with an ADCP, and its VOS activities are coordinated from the US.The Nuka Arctica is a container ship operating between Denmark and Greenland and operated by the Royal Arctic Lines. Its VOS activities are coordinated from Denmark.The First Jupiter is a coal/ore transport vessel operating between Japan and various points in the western Pacific. Its VOS activities are coordinated from Japan.The Explorer of the Seas is a passenger liner operating two cruise tracks from Miami Florida. Its VOS activites are coordinated from the US.The main point is that the ADCPs have been setup to gather data independently of the ship’s operations – making them low visibility and out of the way of the people manning the vessels.Courtesy Charles Flagg, Brookhaven National Laboratory
14Volunteer Observing Ships Explorer of the SeasThe Explorer of the Seas actually has two ADCPs on board, a 150 kHz and a 38 kHz. This is a single transect across the Gulf Stream
15Courtesy Charles Flagg, Brookhaven National Laboratory Volunteer Observing ShipsOleanderAn example of the type of data being gathered from VOSs. This shows how the position and magnitude of the Gulf Stream change with time.Courtesy Charles Flagg, Brookhaven National Laboratory
16Recovery of the Ehime Maru Vessel Mounted ADCPsRecovery of the Ehime MaruImage courtesy of the US NavyImages courtesy of Texas A&MVessel mounted ADCPs have also been used in salvage operations – most notably perhaps in the recovery of the Ehime Maru. This was the Japanese Fisheries training vessel that was sunk when a US submarine surface beneath her. She was sunk in water too deep for standard salvage operations, and needed to be raised to shallower water for the recovery effort. She was later laid to rest in deeper water. This chart shows where she was originally sunk, where she was moved to for the recovery effort, and where she was ultimately laid to rest.ClickHere is one of the first images of the vessel as she laid on the ocean bottom.ADCPs were used in the recovery effort in two ways:The state of Texas operates the Texas Automated Buoy System, or TABS, to provide oceanographic data for oil spill modeling. The state of Texas loaned two of these buoys equipped with ADCPs to deploy at the shallow water salvage site. The overarching goal was to have detailed knowledge of the current structure at the shallow site in order to protect sensitive marine areas should the vessel discharge any pollutants during the salvage effort. The second was an Ocean Surveyor that was mounted on a pole from the coast guard vessel Sumner in order to monitor the deep currents while the vessel was lifted and transported. The Ehime Maru was lifted by the Rockwater heavy lifting vessel to suspend roughly 100 m above the bottom during transport. The Sumner stood by to provide current information to the Rockwater during the whol lift and transport procedure.No lift and salvage operation of this magnitude had ever been undertaken to this date, and it was by all accounts a tremendous success. All but one of the missing students were recovered for proper burial, and because of the availability of the current measurements, no environmental impact was recorded in the operation.Image courtesy of the US Navy
17Ancillary Data Zooplankton Migration Here is zooplankton migration as before, but this time from a moving vessel, Again it is is mainly the RSSI profile that shows the behavior. The vertical velocities indicate it a bit as well as the zooplankton swim up and down, but there is also quite a bit of stripiness that is no doubt associated with the vessel motion.
18Courtesy of Gwyn Griffiths (SOC) Ancillary DataAvoidance BehaviorON the left we have our friend the RSSI profile, on the right is the vertical velocity at the same time. ON the bottom is the actual boat velocity. AS you can see, when the boat moves, there is a large, downward vertical velocity, and the region of high RSSI tends to descend. These are fish that panic and swim downward when the boat gets underway – no doubt scared by the noise of its screws. Gwyn Griffiths at SOC first showed this data.Courtesy of Gwyn Griffiths (SOC)
19ADCP vs Current MetersSo what is an ADCP? It is probably simplest to think of a string of current meters on a mooring line.ClickFrom real data, this is the type of current structure you can expect to resolve with three strategically placed current meters.From the same data set, here is what you would resolve with six strategically placed current meters.Ten meters – this is starting to get expensive….How about 75 current meters? This is an extreme example deliberately chosen because of the complicated current structure, but it is apparent that an ADCP is the only economical way to resolve the current structure to this level of detail.
20Moving Platforms Speed over ground Altitude The ADCP measures relative velocity. That is, the ADCP can not distinguish if it is sitting still in 3 m/s current or if it is on a platform moving 3 m/s through still water. To measure the actual water velocity from a moving platform, the platform motion itself must be somehow measured and removed.ClickIf the bottom is in range, and is stationary, we can use a feature called “bottom tracking”. I am showing the return signal strength intensity as measured along each of the four ADCP beams. This is basically a measure of the strength of the echo, and you can see that the bottom is echoing quite strongly in the data plot I am showing. We can measure the Doppler shift of the returned signal in this strongly reflecting area. We assume that the bottom is not really moving, and that in fact this shift is due to the vessel’s speed. We can then subtract the vessel speed from the measured velocity to get the true current velocity. As side note here, there are large number of ADCP owners out their who evaluate the echo intensity profiles to glean information about sediment concentration or estimate the biomass in the water column. We will go into those uses quite a bit more in the third course of this series.So, what happens when we can’t see the bottom? We need an alternative input from the vessel. Usually this is in the form of a GPS string from the ship’s navigation. We can difference the consecutive position fixes to obtain the vessel’s velocity.
21Vessel-Mounted ADCP Package ADCP TransducerADCP Transducer CableADCP Electronics ChassisADCP Data Acquisition SoftwareWhat does RDI provide with a vessel mounted ADCP?The transducer, which will be either a phased array or piston design. More on that later.A transducer cable. This is a significant cable, with 26 conductors carrying mostly analog signals in shielded, twisted pairs. The default length is 30 m, but we can make them as long as you like.Also included are the electronics chassis along with the software to acquire and play back data.
22Phased Array / Piston Transducers MarinerOcean SurveyorI have mentioned phased array and piston transducers several times, and am finally getting around to explaining the difference. The phased array transducers are shown on the top left. These are our Ocean Surveyor ADCPs, with the 38 kHz system in the center. It is about 1 m across. The phased array systems generate four beams simultaneously from this flat-faced transducer. The traditional, piston type ADCP is shown at bottom right. This is the Mariner ADCP. It has four distinct transducers, each of which generates one beam.
23TransducersSo we talked a little bit about the different types of transducers we make last week, but we are going to go into quite a bit more detail here. On the left is one of our old piston type transducers. On the right is the phased array we make today. Both of these instruments are 75 kHz, so the size reduction allowed by the phased array is obvious.The cartoons underneath show why the size of the phase array is so much smaller. A piston transducer creates one beam, so you need four of them to establish the traditional ADCP beam geometry. The phased array actually generates all four beams from the one transducer. You can see in the photo that the phased array 75 is about the same size as one transducer in the piston 75.From last time we know that lower frequency sound translates to increased range. Since the space afforded to mount the ADCP can be quite limited, the phased array generally gives more bang for the buck. In fact, we designed the 75 kHz phased array so that it would be relatively easy to fit into the mounting structures created for 150 kHz piston ADCPs. That change alone nearly doubles the range without requiring any substantial modification of the vessel.There are several advantages to the phased array. The most obvious one is the size savings that come with it. Because of the reduction in size that the phased array allows, the 75 kHz Ocean Surveyor can fit into wells previously designed for 150 kHz piston transducers.
24Cabling Options Wet end: Right Angled or Straight-in Connector Dry end: Loose or AttachedSo, you’ve picked your ADCP based on frequency, size, available funds, etc. What decisions are left to get the system onto the vessel?One of the first questions we are going to ask you is about the cabling. You have the option of a right angle or straight-in connector. The choice is undoubtedly based on how you intend to mount it. This end of the cable is molded, so making the right choice is important.You can also choose whether or not you want us to send you the dry end already attached to the cable or leave it loose. Often on the larger ships the cable will need to snaked through various parts of the ship to get from the transducer to the data acquisition computer. This is easier to do if the connector is not yet installed.
25Vessel-Mounted ADCP Package ADCP Electronics ChassisSerial Comms. InterfaceAttitude Interface (Synchro/Stepper)ADCP Data Acquisition SoftwareADCP Setup & ControlGPS NMEA InterfaceAttitude NMEA InterfaceThe chassis does the actual Doppler processing. It also provides the Attitude interface for analog gyros.The software runs on your dedicated computer, and brings in the GPS string from the ship’s navigation system. If you have a serial gyro, it will be processed by the software rather than in the chassis.
26Vessel-Mounted ADCP Package Supplied by the user:Vessel Heading InterfaceVessel GPS InterfaceDedicated ComputerSea Chest (or well) to install the ADCPThese are thing that we do not provide. We assume that your vessel will have GPS and gyros for heading/attitude. The GPS interface is necessary if you will be operating in water depths to deep for bottom tracking to work. The gyro interface will be needed in nearly all installations, only the higher frequency Mariner systems have heading information, and this is from their magnetic compass. On most large vessel installations, the magnetic compass gets hopelessly lost.You will need to dedicate a computer to the ADCP for data acquisition. This computer can also integrate the GPS and gyro information if the gyro has an NMEA output.You also need some sort of structure to mount the ADCP. Most commonly on vessels of size, the ADCP is mounted in a sea chest or well. There are some vessels that use a pole mounted system – particularly for the higher frequency systems.
27Block Diagram of Vessel Mounted Setup Vessel GPSSerial (NMEA)OR Synchro/ StepperVessel ComputerSerial (NMEA) OR SynchroVessel Pitch/RollSpeed Log DataRaw DataVessel HeadingADCP ChassisHere are the basic building blocks of a vessel mounted ADCP. The transducer, the chassis, the computer, Vessel GPS and Heading information. The GPS NMEA output should be sent directly to the computer.ClickIf the gyro is analog, then it should be connected to the chassis. If it is serial, then it should be connected to the computer.From these inputs the software on the computer can generate a speed log output.Many researchers choose not to correct for pitch and roll on the assumption that these motions average out over a typical current measurement of interest. If you want to correct for pitch and roll, then you can supply this information as wellSimilarly to the gyro, the actual connection depends on of your attitude information is serial or analog. With analog outp[uts going to the chassis while serial NMEA outputs go to the computer.And lastly, from all this information the computer assembles and output the raw data.ADCP Transducer
28Choosing the Frequency Depth Below Keel (meters)Range (meters)RDI’s ADCP Answer21-20WH Mariner 1200kHz42-40WH Mariner 600kHz2512-150WH Mariner 300kHz5025-400OS 150kHz10050-700OS 75kHz200OS 38kHzOK, so now we are ready to start talking about some of the choices involved in selecting the most appropriate ADCP. The first one is the range you want to measure. Higher frequency sound attenuates more rapidly than lower frequency sound. Also, higher frequencies allow smaller bin sizes because of their smaller wavelengths. It is frequently the case that a tradeoff must be considered between maximum range and maximum resolution.Because of the larger bin sizes, and for other reasons, there is some distance from the head of the instrument in which no measurements can be made. For example, a 38 kHz Ocean Surveyor is pretty much useless if your vessel is never going into water deeper than 200 m or so. Several of the larger vessels have opted to install two systems, one for use in shallow water and one for deeper water. A dual system solution has the added benefit of getting high resolution upper water column measurements in addition to the lower resolution measurements of the low frequency system.It is actually unusual for anyone to order a Mariner version of the 600 kHz or 1200 kHz systems, as these are usually going on much smaller boats. Those researchers will instead typically order a standard Workhorse ADCP with bottom tracking, and will use our Winriver software for data acquisition. This software is explored at great length in another course, so I just mention it here for completeness.
29Narrowband/Broadband The Ocean Surveyor also has the capability to alternate its mode of operation between broadband and narrowband processing. Very simply, broadband processing uses coded pulses to increase the precision of each measurement, at the tradeoff of decreased range. Narrowband processing uses uncoded pulses for maximum range, at the tradeoff of decreased precision. Ocean Surveyor allows you to alternate between the two measurement techniques so you get the best of both worlds.An Ocean Surveyor 75 kHz was installed on a the Research Vessel Endurance, and for comparisoin purposes the researchers left the old pitson transducer 150 kHz system in place.Shown here is a comparisoin between the two systems. The data presented was collected in the open ocean at 8-10 knots where the depth was greater than 2000 meters. As you can see the features from both systems are identical and there is less than 5cm/sec difference between the 2 instruments. The profiling range of the OS ADCP is approximately 725 meters and the NB ADCP is approximately 375 meters. - click -
30General Installation Concerns Engine noiseFlow disturbanceOn or off the centerlineAttitudeLarge or varying magnetic fieldsFixture ringingEngine/Prop noise. A cavitating prop can generate a lot of noise. The level of noise is a function of velocity (18dB/octave) and it will decrease profiling range and can cause large errors. The mounting location should be reasonably far from the prop and the ADCP should be oriented so that none of the beams are pointed toward the prop.Flow disturbance. A boat moving through the water disturbs the water column. If this disturbance is sufficiently large, the accuracy of the current measurement will suffer. Obviously, mounting the ADCP away from the boat minimizes this effect.Center line installation. Mounting the ADCP on the boat’s centerline is not required, but it has the benefit of reducing the effect of roll. When mounted on the side of the boat, roll changes the immersion depth of the ADCP and it introduces vertical velocities into the ADCP. If high enough, these can result in the need for higher ambiguity settings.ADCP attitude. RDI prefers that the ADCP be installed so that it is reasonably level. That is, Pitch & Roll are minimized. The most significant benefit of this is that the maximum beam radial velocity is minimized. It is minimized further if the ADCP is mounted such that Beams 1&3 are forward at 45 degrees to the boat’s heading. The result of this is lower ambiguity settings. The 45 degree rotation, however, should not, result in a beam pointing toward the prop.Compass. The location of the ADCP should be far away from any material that can affect its ability to measure heading. Magnetic materials should definitely not be used in the fixturing used to mount the ADCP.Fixturing. How you attach fixtures to the ADCP can affect a phenomenon called “ringing”. Ringing can get significantly worse if the ADCP is connected directly to metal fixtures.
31Installation Types Pole Gate valve Sonde Well Moon Pool Up-looking Here are some representative installations.The first is through a gate valve in the bottom of the vessel. A stem is typically attached to the ADCP to make it easier to raise and lower through the gate valve. It is important in this type of installation that the orientation of the ADCP is the same for each deployment.ClickNext up is a moon pool. Here is an ADCP mounted into a cage designed for lowering into the moon pool. Moon pools can be a bit large, so you need to be careful about the possibility of ringing. Especially if other activity is gong on in the pool.Here is a pole mount over the side of the Sumner. It is actually a bit unusual to use a pole mount for an OS38, but this was temporary installation to aid in the recovery of the Ehime Maru. Pole mounts are very common with the higher frequency instruments.Here is an acoustic well purpose built to house the ADCP. We can see it from above and below. This is the sailing vessel the Sea Education Association uses in their undergraduate training program.Acoustic wells and pole mounts are far and away the most common mountings used for ADCPs. But there are a handful of exotic installations as well.Here is sonde that the French built to house all the acoustical instrumentation in a single, streamlined gondola under the ship. The gondola has side scan sonar, echo sounders, hydrophones and two ADCPs of different frequencies.How about an up-looking ADCP on your vessel?Click.This was installed as part of the SCICEX experiments under the Arctic.
32Conceptual Well Design Here is a generic well. The transducer is mounted inside the well, which is typically sealed with an acoustic window and filled with water – perhaps with antifreeze. Some mechanism must be in place to fill and drain the well – especially if the well will contain enough water that the window can’t hold it when in dry dock. The transducer needs to be close to the window, and it can be a bit tricky to fill the well and make certain that no bubbles are caught under the transducer. Bubbles are murder on high frequency sound – if there is a layer of bubbles trapped by the transducer then the ADCP is not going to work very well, if at all.Click.I am going to put some beams into the water. The well needs to be large enough that the beams have an obstacle-free path to the water. To make certain that this the caseClickWe recommend that a fifteen degree exclusion zone be used when designing the well. That is, for an ADCP with a thirty degree beam angle, we recommend that the area in a forty five degree cone around the transducer be free of obstructions.
33Acoustic Windows 0.75 in 1.5 in 0.5 in 1.0 in We recommend an acoustic window primarily to reduce the effects of flow noise. Large vessels generate quite a bit of high frequency noise as the pass through the water, and the window will filter most of that out. Even though the window itself will attenuate the signal somewhat, this effect is dwarfed if the transducer is directly exposed to the vessel’s noise. Windows also help reduce fouling of the transducer, which is a good thing – but the primary reason we recommend them is to reduce the flow noise.So, what type of window do we recommend? We can supply a sheet of polycarbonate of the appropriate thickness. You will need to machine the window to the shape you want, and have the shipyard install it.We base our thickness recommendations on the attenuation of sound, which depends quite a bit on the frequency. I am showing a couple of plots here. The red line shows the operating envelope of frequencies generated by a 38 kHz transducer. The blue line is our modeled attenuation loss due to the presence of the material. As you can probably guess our main goal is to avoid getting the notches into our operating envelope. The weighted insertion loss is the attenuation in decibels for transiting one-way through the material – so we double it. One decibel loss is one nominal range cell. For the 38 that is 32 m lost per db.I am showing the results for three quarter inch and one and a half inches. As you can see both are pretty good.Click,But you need to be careful. Here is what happens if the sheet is one half inch thick or one inch thick. We have hit the notch, and the attenuation in both cases is huge. Interestingly, the 0.5 inch thick window is the worst of all.
34A “Typical” WinRiver Display And here is the kind of display you might see. I am showing a contour plot of velocity, the vessel track, a profile and some diagnostic data. The nice thing about WinRiver is that you have the ultimate flexibility in how you want to set this up. I often run it with a bunch of contour plots and nothing else.
35Communications TabLet’s run through the exercise of setting it up like we did for WinRiver. Here is how you establish communications with the relevant instruments. Everything is done from this screen.
36ADCP SETUP TabHere is how you can change the setup of the ADCP itself.
37AVERAGING TabIn addition to the raw data files generated by VMDAS, you can set it up to collect short term and long term average files.
38A “Typical” VMDAS Display And here is a typical display. Here I have chosen to show the velocity and signal strength profiles, along with the vessel track as determined by two different methods.VMDAS does not have the ability ot show contour plots, but it can automatically call another program that will do this for you.
39Data QA/QC Return Signal Strength Intensity (RSSI) Correlation Percent GoodError VelocityInter-beam Comparisons (e.g. fish screening)We have several quality control parameters that I list here, and we will go into quite a bit of detail on each of these.
40Data QA/QC Return Signal Strength Intensity (RSSI) Generally expected to decay with rangeIncrease with range indicates a solid object – e.g. the bottomLow scattering concentration can result in data drop outThe Return Signal Strength Intensity, or RSSI, is a measure of how much of our signal actually reflects back into the transducer. We measure this at each bin, so we get a profile of the RSSI to match up with the velocity profiles. Since we generally expect sound to decay with distance, we expect the RSSI to decay with range. It will flatten out at the farthest ranges, where there is basically no signal left. If there is a bump in the RSSI, then it is likely that there is a solid object or boundary at that range form the ADCP. Also, if there are insufficient scatterers, then the RSSI can actually drop below levels we consider reliable for making velocity measurements.The plot show velocity drop outs that are due to very low concentrations of scatterers. I will talk a bit more about this plot later in this course.
41Data QA/QC Correlation Measure of how much the scatterer distribution has changed between measurementsIn the absence of a boundary – usually sets the rangeWe also return a profile of Correlations to match with the RSSI and velocity profiles. To make our velocity measurements we are essentially comparing how much the distribution of particles reflecting our sound back has changed in a given time. Most of that motion is presumably going to be in one direction – with the flow. But there will be some relative motion between the particles as well. In addition, the actual distribution itself will change during the measurement because some scatterers will leave our measurement volume and different ones will enter it. The correlation is a measure of how much the distribution of particles reflecting the sound has changed during the measurement interval. The less change within the distribution, the higher the correlation.In deep water, where the bottom boundary is out of range, it is usually the correlation drop off that wills et the limits of our measurable range – not the RSSI. That is, while we expect the signal strength to decay with distance, we also expect more change within the measured distribution at the far ranges.
42Data QA/QC Percent Good Percent Good 1: Percentage of solutions that used three beamsPercent Good 2: Percentage rejected for high error velocityPercent Good 3: Percentage rejected for having fewer than three good beamsPercent Good 4: Percentage of solutions that used all four beamsPercent Good is really a measure of how much data was actually usable for making the measurement. The definition can be a bit confusing, but the main thing to consider here is Percent Good 1 and 4. The sum of these actually tells you how many measurement cycles were used n one war or another to create a given velocity profile. Like RSSI and Correlation, Percent good is measured at every range, giving you a profile of the parameter for comparison.
43Data QA/QC Error Velocity Fundamental Assumption: All beams are measuringthe same flow.Error velocity is a measure of flow homogeneity. RDI generally uses an extra transducer to make any measurements because the redundancy allows us to measure the flow inhomogeneity. The basic idea is that the four beam geometry will give us two orthogonal measurements to define the horizontal velocities, but also two independent measurements of the vertical velocity which can be compared. If these two measurements of vertical velocity disagree, then the most likely explanation is that at least one of the beams is measuring in a different flow field than the others – or that the flow is inhomogeneous.I am showing a model of a mid-depth eddy propagating through a 10 cm/s background flow. Since this is a model, we have complete control over it, and we specified that there would be no vertical velocity in this model field. We then played around with different ways an ADCP might measure this eddy, for example with one beam in the eddy and the other three out, two in two out, and so on. We then resampled at every point to generate the other three plots. The second plot shows the measured horizontal velocity as an ADCP steps through the eddy. Notice that it is not quite the same as the actual velocity. The third plot shows that we are measuring a vertical velocity around the edges of the eddy. Remember that there really is no vertical velocity. This measurement is strictly an artifact of the fact that our beams are in inhomogeneous flow. The bottom plot is the error velocity, and as you can see it pretty clearly marks the boundaries of the eddy.
44Fish ScreeningI mentioned earlier that solid object can cause a “bump” in the RSSI profile.Click.Here is a solid object, that is quite likely NOT passively following the flow. The fact that the ADCP has four beams can be used to get a good velocity measurement even when one beam is blocked like this. We compare the RSSI at a given range between beams, and if there is a significant difference we flag it as “bad”.
45VMDAS Data: Typical Display And so here is how the whole thing combines into a typical VMDAS display. ON the top I am showing a tabular output of the navigation information form the ship. Middle left is the velocity magnitude and direction. I chose this plot specifically, as it was taken in a deep eddy off the coast of the Philippines. Middle right are our QA profiles: RSSI, Correlation and Percent Good. This plot shows the mean RSSI of all four beams, the mean correlation of all four beams, and the sum of Percent Good one and two. The bottom plot show the vessel track, with some stick plots showing the mean currents during the track.
46Operational Limitations Weak Scattering LayersStrong Scattering LayersAcoustic InterferenceHeading Alignment ErrorsLarge Pitch and RollNow we are going to turn to some of the real world conditions that limit the ADCPs operations. These are the types of things you might encounter, and they typically rasie a lot of questions about whether or not the ADCP is working. I am going to step through each one and show how something in the environment that is affecting your measurements might show up in those measurements.
47Operational Limitations Weak Scattering LayersThese can cause velocity dropouts ata given depthThis is the plot I showed earlier when I was talking about RSSI. This is actually from a mooring, so it is a little bit of a cheat, but you can see it while underway. So, what I am showing here is the vertical velocity on top, the RSSI in the middle and the correlation on the bottom. Usually the first thing to be noticed is that the velocity has holes in it. Whne that happens, it is a good idea to look at RSSI and Correlation. You can see that the RSSI shows a pretty regular pattern of low and high values, that is mirrored pretty well in the correlation. You can also see that the vertical velocity is kind of stripy, alternating large downward velocities with large upward velocities. Notice that the downward velocities generally are at the beginning of the periods of low RSSI and Correlation in the upper layer, and the upward velocities are generally near the end. This is zooplankton migration. The bugs swim down when the sun comes up, presumably to avoid predation, and then swim back up at night to feed. In this particular instance, there was such a huge migration that it actually depletes the upper water of scattering points. This situation is actually pretty rare, but it can happen.
48Operational Limitations Strong Scattering LayersThese can cause high velocities in a layerthat seem to depend on the vessel’s speedThis one is pretty subtle. Basically, what happens is that you will see that you are getting strong velocities at mid-depth that depend on the speed of the boat. When we look at the RSSI, we see that there is a strong scattering layer at the depth where we see this. Aha! They must be related.They are, of course, but the explanation is a bit tricky. It is convenient to think of ADCP beams as flashlights, but there is actually energy that is propagating off the center axis in what are called sidelobes. Generally, the sidelobes can be ignored, except in the presence of a boundary or in the occasional very strong boundary layer like we see here. The problem is that this sidelobe energy can reflect directly back to the transducer along a shorter path than the sound in the main lobe. Because the sidelobe here is nearly vertical, it is measuring a smaller velocity than the velocity measured by the main lobe. So, our measured velocity becomes biased low.Now, I can hear you saying “but we see a velocity maximum”. Yes we do. That’s because we are looking at data from a ship that is underway. We are measuring the boat’s velocity somehow – either with GPS or bottom tracking, and it is generally a pretty big number. Recall that the ADCP is measuring relative velocity – that is, the combination of the vessel speed and the water speed. Since the total measured velocity in the area affected by sidelobes is biased low, removing the large vessel velocity results in a velocity maximum.Fortunately, these really strong scattering layers seem to be fairly rare.
49Operational Limitations Acoustic InterferenceThese can cause velocity drop outs indiagonal bandsWe talked about this briefly last time. What can happen is that your velocity will show dropouts in a regular, diagonally striped band as is shown in the top panel. Again, the RSSI profile has the explanation. This is a classic acoustic interference pattern. There is an instrument of similar frequency operating nearby and simultaneously with the ADCP.
50Operational Limitations Current MagnitudeCurrent DirectionWe also talked about this one briefly last week. This is data taken by a ship doing reciprocal transects. As you can see, the measured velocity depends on which way the ship is going. This is a classic indication that there is a heading alignment error. I bring this up again because heading alignment is not necessarily a one-time consideration. Especially if your ADCP is on any kind of a temporary mount (e.g. through a gate valve or on a ole of some sort).
51Operational Limitations Severe pitch/rollThis can limit the measurement rangeB1B2B3B4Sever pitch and roll can affect the range of your measurements. I am showing a cartoon of the four beams illuminating some scattering points. AS I rock the ADCP back and forth (click left, right, left, right, etc). You can see the beams can actually leave the measurement area. Ultimately, this means a complete change in the scattering points actually measured during the interval, which results in a loss of correlation.
52Operational Limitations Severe pitch/rollThis can limit the measurement rangePercent good is used here to indicate the loss of range when the ship is oriented into the seas (causing significant pitch and roll) on day 52.1.
53Vessel Mounted ADCPs JAMSTEC RV Kaiyo off of Mindanao, Philippines We have spent quite a bit of time discussing data qa/qc and highlighting some of the operational limitations on using an ADCP from a moving vessel. After spending so much time on the negatives, I thought it might be worthwhile to show you this slide from the last course again - just to show the good things that the instruments can do when everything comes together. Again, this eddy was measured through an acoustic window at 1,000m deep from a vessel traveling at 14 kts.