An orientation to the fascinating Multibeam systems for underwater mapping!

Other than communication, sound has brought upon far reaching consequences upon human endeavor for exploration. Multibeam has ever been a fascination among hydrographic surveyors and engineers. It was pioneered in 1970, but lately has evolved into mature commercially viable system.

Hereby is presented an orientation of incredible multibeam systems and the factors influencing its image compilation. Although the following discussion does not have the scope to provide a definitive response, it may shed light on some of the issues.

On the decibel scale, everything refers to power, which is amplitude squared. 0.0 dB corresponds to about the normal threshold of hearing and 130 dB to the point where sound becomes painful to humans.

Multibeam systems broadly can be categorised into two:

Interferometric (determine angle as a function of travel time) with swath width of 10-12 times of the depth are good for overall bathy and are less expensive (i.e. GeoAcoustics) while beam forming (measure travel time as a function of angle) use mill cross arrangement with swath width 3-4 times of depth is viable for detailed bathymetric and target identification (i.e. Reson, Simrad).

Bernard Mill, who first used this technique in 1954 referred as Mills Cross technique in a two-dimensional radio telescope.

Each arm of the cross was 1500 feet (450m) long, running N-S and E-W, and produced a fan beam in the sky. The voltages of the two arms were multiplied to form a pencil beam with high side lobes. The beam could be steered in the sky by adjusting the phasing of the elements in each arm.Received echo beam forming systems utilize two orthogonally mounted transducer array (projector and hydrophone). The transducer transmits a pulse that is flat (disk) shaped. The receiver receives a return signal that is also flat (cone) shaped and by placing the TX and RX arrays perpendicular to each other and using signal multiplexing a “beam” is formed.

 

In Interferometric system multiple horizontal acoustic elements are housed in a single unit for port and starboard to compare the phase of each incoming echo sample, thus allowing the angle to be calculated.

A limitation here is that at nadir the geometry of the returned echo makes it difficult to resolve phase shifts which may lead to ambiguous depth measurements… In practice however with the current state of art technology, this limitation stands more or less resolved.

Interferometry makes use of the principle of superposition to combine waves in a way that will cause the result of their combination to have some meaningful property that is diagnostic of the original state of the waves. When two waves with the same frequency combine, the resulting pattern is determined by the phase difference between the two waves.

 

Interferometric sonar , also referred to as Phase Differencing Bathymetric Sonar (PDBS) systems, use the measurement and comparision of phase at each of several horizontal line arrays (receive elements )to determine the angle from which the acoustic return originates (backscattered sound waves from the seabed. To conclude Multibeam systems don't actually use the interferometric method of superposition of signals.

 

In acoustics, beam steering is used to direct the audio from loudspeakers to a specific location in the listening area. This is done by changing the magnitude and phase of two or more loudspeakers installed in a column where the combined sound is added and cancelled at the required position. This type of loudspeaker arrangement is also known as a line array.

In beam steering echo sounders, the echo footprints get large as the angle of incidence increases; interferometric systems give high resolution images to the outer edges of the swath. However with advent of technology, Beam steering echo sounders come with the option of equiangular and equidistance transmissions so settlement upon a better system is disputed.

The data quality is determined by the reflected (actually refracted) received beam brightness (amplitude) and colinearity (alignment).

 CTD (Conductivity, Temperature and Depth) conclude the velocity of sound (speed of sound in water).

Temperature is the prominent factor affecting velocity. 165 meters depth change has the same effect as 1 degree centigrade temperature change. Thermoclines (sudden temperature changes) also occur near the surface.

Salinity increases little with depth.

Pressure increases rapidly but water being non compressible has little effect on velocity.

On Reson displays a sad face indicates local sound velocity less than reality and smiley face indicates local sound velocity more than reality. However such faces may also be due to indifferent spreading or absorption values in Reson Ocean Menu.

Absorption is due to the loss of acoustic energy to heat while propagation. Higher the freq. - higher the absorption.

Of the available range of 0 to 120 dB/km, in case the exact value is not known, a value of 60 -110 dB/km for salt water and 70 dB/km for fresh water is recommended for a high freq 455 KHz high resolution Reson multibeam. While for Simrad 80 dB/km is recommended. However operator prudence must be employed. Operator prudence is much however much required.

Spreading loss is because the total amount of energy remains the same in the wave spreads out of the source. As the wave spreads with time and distance, energy per unit length of the wave is diminished. Intensity falls off for square of the distance, 6dB for each doubling of distance.

In Reson system, out of the range of 0-60 it’s recommendable to begin with 30.

Bottom absorption varies with freq., bottom type and grazing angle. Losses increase with freq. and grazing angle.

For 24 KHz with 15 degree grazing angle the approx. absorption is:

Rock 20dB/m2
Sand 30dB/m2
Mud 40Db/m2

Beam width depends upon the length of array and freq. Elements of the array is spaced half wave length apart. More elements in array, narrower the beam along with higher the freq., narrower the beam.

Beam steered in a direction other than perpendicular to the direction of array by applying a variable delay to the signal from each hydrophone.

The data transmission is primarily through User Datagram Protocol. It employs minimum protocol mechanism with no handshaking.

UDP provides two services not provided by the IP layer. It provides port numbers to help distinguish different user requests and, optionally, a checksum capability to verify that the data arrived intact.

A port is a software structure that is identified by the port number, allowing for port numbers between 0 and 65535.

Port 0 is reserved, but is a permissible source port value if the sending process does not expect messages in response.

The Internet Assigned Numbers Authority has divided port numbers into three ranges. Port numbers 0 through 1023 are used for common, well-known services.

Port numbers 1024 through 49151 are the registered ports used for IANA-registered services.

Ports 49152 through 65535 are dynamic ports that are not officially designated for any specific service, and can be used for any purpose.

Source port number field identifies the sender's port when meaningful and should be assumed to be the port to reply to if needed. If not used, then it should be zero.

 

Destination port number field identifies the receiver's port and is required.

In Reson 8160, 8101 UDP Ports are 1028 (Multibeam data,) 1029 (Latency) and 1034 (snippets data). Reson 7101 use UDP Port 8000 and 7125 UDP Port 7000.

Simrad EM3002 SIS/HWS (Seafloor Information System/ Hydrographic Work Station) has two network cards to connect to PU (Processing Unit) and the online acquisition computer.

PU UDP communication in Simrad system Host UDP 1 to 3 (Port 16100 to 16102) is for communication between Processing Unit (PU) and SIS computer. For communication with the online acquisition computer Data distribution module is employed. Host UDP 4 (Source Port 16103, 16104) and (destination port 2012, 2040) on QINSy is used to transmit raw and Motion applied Multibeam data from SIS computer.

However other UDP Ports numbers also can be assigned.

Multibeam requires MRU string with high update rate (25-100 Hz), Gyro HDT string (1-10Hz), Navigation ($GGA, $GLL and $VTG) or RTK input, online SVP, over the side SVP profiler profile and Time synchronization (PPS).

Time associated with event goes serially to acquisition software, usually a $ZDA string. Systems with high depth i.e. Reson 8160 don’t need a PPS BNC input but only a $ZDA string.

For PPS sync, position to Simrad Processing unit (COM1) should contain ZDA string along with GGA. (GGA, ZDA, HDT, VTG etc are transmission protocols of NMEA (National Maritime electronic Association)

Datum, position and heading have no effect in raw Multibeam data. So datum selection is not necessary.

In case an INS system is not being used (i.e. POS MV or F180) aid motion sensor with GPS $VTG (speed), $GGA, $GLL and gyro $HDT strings, this helps a lot with the 'race tracking' effect (i.e. helps it settle quicker after turns).

For high data rate applications it is necessary use Ethernet.

If you connect MRU to Simrad Processing unit (PU) Pitch, Heave and Roll will be applied in SimradMBE.fbf file so use SimradMBERaw.fbf for post processing. If you are using SimradMBE.fbf file, disable motion correction during post processing.

There is much more to Multibeam encompassing its installation, calibration, maintenance and troubleshooting which call for more future discussions. Cheers!