Standards for Hydrographic Surveys

4 Depth Measurement

4.1 Introduction

The navigation of commercial vessels requires increasingly accurate and reliable knowledge of the water depth in order to exploit maximum cargo capabilities safely. It is imperative that depth accuracy standards in critical areas, particularly in areas of marginal under-keel clearance and where the possibility of obstructions exists, are more stringent than those established in the past and that the issue of adequate bottom search is addressed.

4.2 Sensor Calibration

4.2.1 Multibeam Echo Sounding System (MBES)

Field procedures prior to any survey must be undertaken to determine any residual biases and the corrections that will be used to fine-tune the calibration of the MBES. These field procedures are commonly referred to as a “Patch Test” and involve logging data while the survey vessel is run over specific lines over different types of bathymetric relief at differing speeds, reciprocal directions, and offset to identifiable targets. The aim of the patch test is to determine any residual roll angle, pitch angle, azimuth angle, and time offset of the MBES with respect to the motion reference unit. The patch test is also conducted at the end of the survey to confirm that the system has not changed during the course of the survey. A patch test must also be conducted whenever there is a change of significant mechanical, hardware, or software components of the system.

More information on patch tests calibration can be found in “The Calibration of Shallow Water Multibeam Echo-Sounding Systems” by André Godin.

4.2.2 Multitransducer Vertical Sweep System (MTES)

At the beginning of the survey season with a multi-transducers system, it is essential to calibrate every components of the system. The Hydrographers will have to measure and position the components on the launch like: establish the spatial coordinates (xyz) of the GPS antennas, moving sensor, each transducers including the individual draught of those transducers. They will then have to calibrate components like the moving sensor, the gyro, etc. over a known area and analyze the results. It is recommended to establish a ground truth area to validate the system and compare the results with other vessels.

On a daily basis, it is necessary to calibrate the system with a “bar check” on at least one transducer to determine the sound speed in the surveyed area. It is also required to measure the draught variation of the vessel (vary with the fuel and water expense and after re-supplying).

4.2.3 Single beam Vertical System (SBES) and multitransducers (MTES)

The “Bar Check” is the field procedure for calibrating the MTES and SBES and involves a metal cone or plate device lowered to a maximum depth of 60 meters and recording the true depth versus the measured depth and compiling a depth correction table that will be used later to correct the measured depths. The bar check may be used to determine the correct draft entry in a MBES if the size and shape of the vessel permit. This methodology should be used at least once a day and possibly at the end of the day to ensure that no problems occurred during the day.

4.2.4 Sound Velocity Profile Sensors

These sensors are to be factory calibrated according to the manufactures schedule and specifications or sooner if the data has become suspect.

4.3 Sound Speed Measurement

4.3.1 Introduction

The speed of sound in the water column shall be measured either directly, using a sound speed sensor, or indirectly calculated from conductivity, temperature and pressure measurements. In planning the measurement of sound speed profiles, the type of acoustic survey instrumentation as well as other potential uses for the sound speed data must be considered.

4.3.2 Single Beam or Multitransducer System Survey

The measurement of sound speed profiles for the use of a single beam survey is desired to correct for the sound speed propagation differences caused by changes in sound speed through the water column. This results in a vertical correction only. Sound speed profiles are to be taken at an interval dictated by the variability of conditions in the survey area. Where possible, the entire sound speed profile shall be applied directly by the echo sounder. If only a single value is accepted by the echo sounder in use, a calculated harmonic sound speed shall be used. Bar checks shall be done at a frequency sufficient to validate the sound speed being used.

4.3.3 Multibeam Survey

The measurement of sound speed profiles for a multibeam survey is required to correct for the sound speed propagation and ray path variability through the water column. This results in a vertical and across-track correction. Sound speed profiles shall be measured at a sufficient frequency to ensure that the horizontal and depth accuracies for the order of the survey as defined in Table 1 are met. If a continuous profiling system is available, sound speed profiles shall be measured at the maximum rate that logistics and vessel traffic allows.

Continuous monitoring is required to determine if a change in the sound speed profile has occurred. Monitoring is done in two distinct means, the data and observable water conditions. Data monitoring consists of watching for refraction effects in the data. These will include mismatch in the overlap of survey lines and a trend towards an arcuate ping profile. Observable water conditions consist of effects that give an indication of a change in the sound speed profile. These include, but are not limited to, an observation of: a change in measured surface sound speed, an inflow of fresh water, or a sediment plume, wind/wave action causing surface mixing, significant rainfall, traversing of currents, surface water temperature change, etc. Any such indications shall result in a new sound speed profile being measured.

4.3.3.1 Surface sound speed

The surface speed of sound shall always be measure and applied in real time for a multibeam sounder whether it is an arcuate array (e.g. barrel array) or a flat, electronically steered array.

4.3.4 Oceanographic Purposes

Sound speed profile measurements shall be recorded with the sensor details, UTC time and geographic position of measurement. When sound speed is measured directly, it is desirable to measure temperature as well to enable the calculation of salinity for oceanographic purposes. When sound speed is calculated from conductivity, temperature and pressure, these values shall be retained along with the calculated sound speeds.

4.4 Sounding Density

4.4.1 Introduction

In planning the density of soundings, both the nature of the seabed in the area and the requirements of the users have to be taken into account to ensure adequate bottom search.

It should be noted that no method (not even 100% search, although desirable) guarantees the reliability of a survey by itself. Furthermore, it cannot disprove the existence of hazards to navigation with certainty; in particular, the existence or non-existence of isolated natural hazards or man-made objects such as wrecks between survey lines if conducting a SBES survey, or in the absence of redundant overlap in the case of MBES surveys.

If 100% seafloor search is required, it is recommended to use MBES/MTES or SBES combined with accurate sidescan sonar to achieve your results

4.4.2 Line Spacing

For SBES surveys, appropriate line spacing for the various orders of survey is proposed in Table 1. The results of a survey have to be assessed using procedures developed by the Project Manager responsible for the survey quality. Based on these procedures, it has to be decided whether the extent of bottom search is adequate and whether the line spacing shall be reduced or extended.

These procedures may include an appropriate statistical error analysis which shall take into consideration interpolation errors, as well as depth and positioning errors of the measured depths (see § 8.5 Error Sources and Budget).

For MBES, line spacing is replaced with percentage of coverage or density of soundings per square grid cell. To ensure adequate sounding density in shallow waters (<50m water depth) which are deemed critical to navigation, it is recommended that surveys use 200% coverage or 100% overlap. The sounding density should be at least 5 pings per cell to achieve the desired resolutions as described in table 1 of the CUBE Bathymetric data Processing and Analysis (CHS February 2012).

4.4.3 Shoal Examination

A shoal is a distinct rise of the seabed, which could be a hazard to navigation. Considering the draught of some modern ships, any isolated indication of shoaling of less than 50m may be of sufficient importance to warrant an examination for a possible shoal. A 10% rise in the seabed depending on the depth, the relative character and the navigation type (maximum draught, etc.) of the surrounding area may indicate the existence of a shoal or some other serious hazard to surface navigation and therefore be investigated.

One method of shoal examination, when operating a SBES, consists of running a detailed pattern of sounding lines over the shoal area. The line pattern and density is determined by the surrounding bathymetry, the system used and the navigational characteristics of the area. Another method is to sweep an area for 100% bottom coverage by either a mechanical or electronic sweep system.

Depending on the bottom characteristics, systems used and the client’s needs, the Project Manager will determine if the shallowest depth at each shoal examination shall be verified and bottom sample obtained (mechanical or inference method).

For MBES surveys, ensure that there has been sufficient redundant data over the peak of the shoal to ensure the least depth has been accurately determined.

4.4.4 Depth Measurement over hazards

Determination of the general seabed topography, tidal reduction, detection, classification and measurement of seabed hazards are fundamental hydrographic surveying tasks. Depths above hazards need to be determined with, at a minimum, the depth accuracy as specified for Order 1a in Table 1.

For wrecks and obstructions that may have less than 50 m clearance above them and may be dangerous to normal surface navigation, the least depth over them shall be determined either by high definition sonar examination or physical examination (diving). Mechanical sweeping may be used when guaranteeing a minimum safe clearance depth.

All anomalous features previously reported in the survey area and those detected during the survey shall be examined in greater detail and, if confirmed, their least depth is to be determined. The Project Manager responsible for survey quality may define a depth limit beyond which a detailed seafloor investigation, and thus an examination of anomalous features, is not required.

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