NOAA Protocols for Fisheries Acoustics Surveys and Related ...

More documents

Recommendations

Info

degradation are external to the echo sounder, where as system performance accounts for the echo sounder electronics. Additionally, performance degradation is often not observed until the resulting effects are greater than the TS threshold. Routine monitoring of data by scientific personnel during data collection is necessary to ensure a high standard of data quality. Techniques Noise Acoustical A common type of acoustic noise is a discreet spike caused by another echo sounder or sonar operating within the frequency bandwidth or a harmonic of the scientific echo sounder. The solution is to identify the source of the interference and shut it down. A list of all acoustic systems with associated operating frequencies can aid in identifying the interfering system. Interference can be eliminated if acoustical instrumentation essential for safe ship operation is synchronized with the survey echo sounder. Removal of acoustic noise during post-processing is sometimes possible, but difficult, so eliminating it during the survey is always preferable. Electrical Electrical noise can be of many types. Electrical interference caused by improper grounding or other electrical systems can cause low-level voltage interference, spikes, or cyclical interference. A low level voltage introduced to the echo sounder can be amplified with range by the TVG function, and may pose a problem only in the deeper parts of the survey area. Problems can be reduced or eliminated by ensuring proper grounding of the scientific echo sounder, by using an uninterruptible power supply (UPS) for the scientific echo sounder, and by eliminating electrical interference during data collection. Electrical interference not eliminated during data collection should be removed during post-processing, either manually or with signal processing techniques. If signal processing techniques are used, care should be taken to ensure that target strength distributions are not modified, or correction factors may be required. Bubble Attenuation Bubbles can have a strong effect on propagation and transmission of sound. Due to the high acoustic impedance between air and water, bubbles are efficient scatterers of sound. Bubbles can increase attenuation (loss of signal strength) and potentially increase the probability of misclassification of gas-bearing organisms. Bubbles near the sea surface are generally associated with increased sea state and/or the position of the transducer relative to the vessel’s hull. The transducer location on the hull must be chosen to minimize potential problems caused by bubbles. To prevent degradation of survey data, it is necessary to slow vessel speed or suspend acoustic survey operations when sea state causes unacceptable bubble attenuation. Currently, this decision is based on the judgment of the scientific field party chief, but explicit criteria need to be developed. In some cases, bubble backscattering can be removed from target strength data during post-processing, but this will not correct signal loss from targets of interest. Transducer Motion Excessive transducer motion is associated with increased sea state. Transducer motion affects bottom tracking, target strength and volume backscattering measurements. ‘Dropouts’ (i.e., reduction or elimination of TS values over one or more pings) observed on the echogram 38
are a clear indication of excessive transducer motion. If the vessel is outfitted with a motion sensor, these data should be recorded. Motion sensor data may be used for objective decisions on acoustic data quality or for making corrections to the acoustic data. When sea state or vessel motion is excessive, as judged by the field party chief, survey speed must be slowed or operations must be suspended. Bio-fouling Bio-fouling can occur on hull-mounted transducers or protective coverings that stay in the water for long periods of time. Accumulation of material on the transducer will reduce the transmitted and received sensitivity, and this reduction may not be recognized by system performance procedures, although it should be detected by calibration. Hull-mounted transducers and protective coverings should be checked and cleaned regularly, and at a minimum before each field season. Error Bio-fouling will cause a systematic degradation in echo sounder performance as the biofouling increases. Transducer motion effects increase with increasing sea states, but the overall effect on abundance estimates requires investigation. Near-surface bubbles can be removed from the data during post-processing, however the resulting effects of bubble attenuation on target strength need to be studied. Most types of electrical and acoustical noise can be eliminated during data collection or post-processing. Noise that cannot be removed will bias target strength measurements and lead to errors in fish biomass estimates. Considerations Remediation If noise issues are found to be a problem, then analyses are required to determine the effects on target strength measurements. In most cases, noise can be removed from the data set by eliminating the problematic pings, but if the noise persists for long periods, removal is more difficult. If motion sensor data are available, corrections can be made to the acoustic measurements (Dunford 2002). Improvements A better understanding of the effects of transducer motion and bubble attenuation will improve our ability to make adjustments to the target strength data or make objective decisions on when conditions preclude collecting useful data. The timing of pulses emitted by the echo sounder can be adjusted (adaptive pulse timing) so that the necessary correction is minimized. (Dunford 2002). Systematic analyses are recommended to determine the effects of performance degradation on data quality. Results of these analyses will improve (or at least make possible) objective decisions during data collection. Data Management Target strength data, post-processed data, biological, and associated meta-data should be routinely archived during the survey. These data are downloaded to shore-based computers and 39
Page 1: NOAA Protocols for Fisheries Acoust
Page 4 and 5: Echo Sounder Parameters ...........
Page 6 and 7: Trawls ............................
Page 9: Introduction In response to Vice Ad
Page 12 and 13: Advanced Survey Technologies Progra
Page 14 and 15: Techniques Before conducting a cali
Page 16 and 17: Considerations Remediation Echo sou
Page 18 and 19: transmission loss. To correct for s
Page 20 and 21: unavoidably discarded, just as some
Page 22 and 23: factors such as orientation are not
Page 24 and 25: A schedule for calibration and main
Page 26 and 27: Techniques Noise Acoustical A commo
Page 28 and 29: ⎛ I ⎞ r TS ≡ 10 log10 ⎜ ⎟
Page 30 and 31: Considerations Remediation If a TS-
Page 32 and 33: Techniques Beam Pattern Transducers
Page 34 and 35: during vertical migration, will cau
Page 36 and 37: A schedule for calibration and main
Page 40 and 41: permanently archived for each surve
Page 42 and 43: Numerical Density to Biomass Densit
Page 44 and 45: An electronic ‘event log’ can b
Page 46 and 47: References Demer, D.A., M.A. Soule,
Page 48 and 49: Regional Protocols Because of the d
Page 51 and 52: Table of Contents Introduction.....
Page 53: Electrical ........................
Page 56 and 57: Center Background NEFSC The Northea
Page 58 and 59: S v Calibrations The NEFSC acoustic
Page 60 and 61: Operation Menu/Transmit Power Norma
Page 62 and 63: Software The echo sounder firmware
Page 64 and 65: ii. Minimum Sv for good pick: -50.0
Page 66 and 67: Net mensuration sensors are attache
Page 68 and 69: We remove backscattering by surface
Page 70 and 71: GPS The primary Global Positioning
Page 72 and 73: Electrical Electrical noise protoco
Page 74 and 75: Scientific Computer System (SCS) Fi
Page 77: Appendix 2 March 11, 2005 NOAA Prot
Page 80 and 81: Techniques ........................
Page 82 and 83: Center Background AFSC The Alaska F
Page 84 and 85: On-axis sensitivity and S v calibra
Page 86 and 87: Volume Backscattering Measurements
Page 88 and 89:
Mid-water juvenile pollock (max sa=
Page 90 and 91:
Biological Sampling Mid-water and n
Page 92 and 93:
Bubble Attenuation Vessel speed is
Page 94 and 95:
Data Collection Since 1990 to the p
Page 96 and 97:
sampling resolution of scattering l
Page 98 and 99:
A decision must be made as to wheth
Page 100 and 101:
design. If fish abundance in this a
Page 102 and 103:
Simrad. 2001. Simrad EK60 scientifi
Page 104 and 105:
104
Page 106 and 107:
Acoustic Data......................
Page 108 and 109:
ackscattering cross-section (σ i )
Page 110 and 111:
o For the 38 kHz transducer operati
Page 112 and 113:
• Two-way integrated beam pattern
Page 114 and 115:
Classification Techniques Single Fr
Page 116 and 117:
Performance Degradation Definition
Page 118 and 119:
Biological Data Data from catch pro
Page 120 and 121:
elation also assumes that backscatt
Page 122 and 123:
finer than the 13.5-18.9 nmi (25-35
Page 124 and 125:
Improvements - The annual hake migr
Page 126 and 127:
Techniques The primary source of th
Page 128 and 129:
Hamano, A., Sasakura, T., Kieser, R
show all

NOAA Protocols for Fisheries Acoustics Surveys and Related ...

Create successful ePaper yourself

Delete template?

Save as template?