CTD Oceanographic Tags

Transcription

1 CTD Oceanographic Tags The first telemetry tag that links a marine mammal s behavior with its physical environment. Features: Oceanographic quality temperature & salinity profiles Detailed individual dive and haulout data. Near real-time data relay Up to 50,000 full length Argos data transmissions Option to forward CTD readings direct to Argos GTS Introduction Developed originally for the southern elephant seal, the CTD Oceanographic tag provides an essential tool to allow researches to better understand the behavioral characteristics of sea mammals in relation to the environment in which they live. This is realised through the combined monitoring of the mammal s detailed dive and haulout information with the key oceanographic parameters, temperature and salinity. Intrinsic to the tag is a miniaturised CTD sensor capable of measurements that compare favorably with standard oceanographic tools. Coupled with effective data sampling and compression techniques, detailed and accurate temperature and salinity profiles are generated and relayed back in near real-time for analysis.

2 Physical Characteristics Housing: Standard Sensors: Telemetry: Longevity: Shelf Life: Dimensions (max): Mass: Solid epoxy body, rated to 2000m Pressure: o Range: dBar. o Accuracy: 2dBar, ± ( %*reading)/ºK o Resolution: 0.05dBar Temperature: o Range: -5 º to 35 ºC o Accuracy: ± ºC o Resolution: ºC Conductivity: o Range: 0 to 80mS/cm o Accuracy: ± 0.01mS/cm o Resolution: 0.002mS/cm ARGOS Satellite System Up to 1 year with 50,000 ARGOS transmissions more than 5 years (in standby) 120mm x 85mm x 60mm - (excluding antenna) ~ 600g Conductivity (salinity) sensor Pressure sensor (not visible) Argos antenna Battery Temperature sensor

3 Tag Operation Our focus is on answering scientific questions and we are always prepared to consider software adjustments to meet a particular need. Most of the key parameters that control the tag s behavior may be readily changed. For illustration we show [in square brackets] the default values. Location Location is obtained as a by-product of the transmission of data through the ARGOS satellite system. CTD tags typically generate about locations per day at mid to high latitudes. Data Sampling Samples of depth are recorded every [4] seconds. Each tag contains its own specific calibration information, which allows it to immediately convert its sensor readings into real-world units (e.g. metres, ms/cm). Depth is automatically reset to zero whenever the wet-dry sensor detects the surface. Behavioral States The tag continually monitors the sensor data it is collecting and maintains a three-state model of the animal s activity, determined from surface sensor/depth sensor/ time interactions. For example: As the tag monitors transitions from one state to another, it constructs data records of several complementary types and prepares them for transmission.

4 Behavioral Data Types Dive Record Dive records are constructed at the end of each dive. When a dive begins, all the samples are accumulated at until the end of the dive is detected, creating a full-resolution dive profile. The tag then calculates the [4] internal points in the profile that give the best fit to the entire profile. Dives are transmitted in groups of consecutive dives Haulout Record Haulout records contain the start- and end-times of unbroken periods spent in the Hauled Out state. These records require only a few bits to transmit but can account for long periods of the total data record. Summary Record One summary record is constructed for every [6] hour period. It contains summary statistics such as the number of dives; the proportion of time spent in each of the three states; mean, maximum and standard deviation of maximum dive depth; mean, maximum and standard deviation of dive duration. The length of the period may be set to 3, 4, 6, 12 or 24 hours, depending upon the species and particular research interest. Summary records are relatively compact to transmit, and it is normally possible to achieve an almost unbroken coverage. This provides a good feel for the overall behavior of the animal, against which more detailed data about individual dives can be set.

5 Temperature and Conductivity Cast Records The tag uses an adaptive technique to choose [2] of the deepest dives to represent each [4] hour period. Temperature and conductivity samples are collected at [1] second intervals during the ascent phase of the chosen dives. This corresponds to approximately one sample per metre depth for most animals. Filtering and compensation for sensor time-constants is performed on board to provide the highest quality calculation of salinity. Temperature and Salinity profiles are compressed for transmission by choosing a combination of standard depths and best-fit points selected by the broken-stick method. The result is a set of [20] temperature-depth-salinity triples. Power Budgeting The power required for basic sampling and calculations is very small compared to transmissions and CTD measurements. The lifetime of the tag is therefore controlled by restricting the number of transmissions and CTD measurements attempted. Transmission targets may be set at several dates, specifying the maximum number of transmissions that should be made by that time. The tag continuously reviews the number of transmissions to be made each day to keep itself on track to meet the next limit. There is a complex trade-off between the number of CTD casts collected and the energy remaining to transmit them. We use a probabilistic model to choose a collection rate that maximizes the expected number of profiles received over the deployment.

6 The Instrumentation Group The Instrumentation Group within the Sea Mammal Research Unit is made up of biologists, software and hardware engineers who work together at all levels - from the biological objectives of the study through to formulating hardware and software responses to scientific and management questions. We have produced a number of important innovations in this field. Because we see our main role as furthering advances in knowledge, we try to make these innovations available to a wider community as quickly as possible. As a non-profit organization, we consider those who use our equipment as "collaborators" rather than "customers" and work with them to help tune the technological approach to their particular questions and to use our experience to help interpret the data. We design and build equipment. Working with the user, we create data collection software, we decode and archive the data and make it available for distribution. We also provide a basic data visualization tool (MamVisAD) to help visualize the data. When investigators think it is helpful, we can play a role in helping with analysis. We place an extremely high value on feedback on how devices are deployed, how effective they are thought to be and on suggestions for changes. This feedback is crucial in fostering new developments and approaches.

7 Example Data SEaOS Programme The southern ocean is a remote and dynamic system that strongly affects global ocean circulation and climate. Oceanographers consider it under-sampled but critically important. Data collected by the seals are being incorporated into global ocean databases and have significantly augmented that collected by more conventional means. The figures below show historical and contemporary conductivity-temperature-depth (CTD) profiles collected throughout the Southern Ocean. Data lodged in the World Ocean Database 2001 (WOD01) have been dramatically augmented by Argo float data collected since 2001, especially in the region north of 60 S. South of this latitude, ship-board sampling is hampered by ice and weather conditions, especially in the Austral winter, and Argo floats do not operate effectively within the packice. In 2003/04 and 2004/05, the SEaOS partners tracked Southern elephant seals from Macquarie, Kerguelen, South Georgia and the Antarctic Peninsula throughout their winter migrations between moulting and breeding. ( During these long foraging trips the seals ranged widely across the entire Southern Ocean, collectively circling the globe. Each animal relayed salinity & temperature profiles each day for on average 160 days, sampling along and across the main circum-antarctic fronts and water masses. Their choice of foraging areas also led to long temporal series from the marginal ice zone and even deep within it. Oceanographic temperature and salinity data collected by the seals has contributed enormous amounts of data from places and seasons which have previously been extremely data sparse, while providing new insights into how this species utilizes their deep ocean environment.

8 This figure shows tracks of eight southern elephant seals during their outward migration from Kerguelen towards the Antarctic. The directional tracks across the major frontal systems within the Antarctic Circumpolar Circulation system provide simultaneous records of salinity and temperature across the Indian Ocean sector of the Southern Ocean. These data would previously only have been available using the same number of research vessels, at extremely high cost. The following figure, representing the outward migration of Aspasia, a seal from South Georgia, shows how information gathered from the CTD tag can be used to explain how the physical environment affects the movements of southern elephant seals. The water temperature (shown as a coloured curtain) suggests she made a sharp left-hand turn as she intercepted the Polar Front, as indicated by the disappearance of the cold remnants of winter water around 200 meters depth. She then proceeded to follow this front towards the Antarctic Peninsula.