CA1271554A - Multipulse acoustic mapping system - Google Patents

Abstract

ABSTRACT
An oceanographic mapping system comprises a narrow beam acoustic transmitting transducer, a beam scanning system and controls for controlling the timing of the transmitted pulses and the movement of the bearing scanner. A receiver is also provided to receive the echoes generated by the acoustic pulses and a range calculated as provided for determining the delay between the transmitted pulses and their corresponding echoes. A mapping system is also provided which communicates with the range calculator and the beam scanner and is operable to process the range and bearing signals that generate the profile of the target area. The method of mapping includes the step of transmitting a series of acoustic signals before the first echo generated by a signal of the transmitted series is received by the receiver, thereby to increase the speed at which mapping can be accomplished.

Description

~27~rj54 This invention relates to improvements in oceanographic mapping systems.
In particular, this invention relates to an improved method of oceanographic mapping which greatly increases the speed with which mapping can be accomplished.
Prior Art Knowledge of the depth of a water body (channels, rivers, harbours, lakes, oceans, etc.) is necessa~y for many operations (navigation, dredging, etc.) Sometimes it is also desirable to know underwater shapes of natural (icebergs) or man-made objects.
Underwater mapping can be accomplished by transmission of an acoustic pulse towards a mapped surface (or object) and is based on observation of the reflected echo. The angular position of the reflecting surface can be determined from transmitter/receiver beams orientation and the range is obtained from the echo delay time.
Acoustic beams can be deflected by several physical or electronic means to scan the object. There are a variety of systems (sonars) performing such operations. The simplest is a depth sounder which measures the water depth directly beneath the sensor. Beam scanning and bottom mapping is accomplished by linear motion of the sensor and repeated soundings.
For a large area to be mapped, the major limitation of such a system arises from its inefficiency. For this reason multibeam systems have been developed which can perform a swath survey.
However, these systems require a dedicated vessel and are expensive to operate. A towed system which utilizes a phase information for a swath survey has recently been introduced. The principle of operation of this system, however, makes it ineffective for mapping bottoms with large slopes.
For complex profiles a system has been developed with a narrow conical beam which is mechanically scanned over the desired angular directions. For each direction a single acoustic pulse is transmitted and the echo received before the beam ls moved to a new position. The target range is related to the echo delay time. Due to the relatively low velocity of sound propagation in water and the number of measurements required, the scanning process can take a . ~, I '. . ., ~2'~
04666-2 GWH:jy very long time to be completed. Another practical limitation arises from the fact that the position of the scanning head must be kept stationary during the mapping process or proper reference points must be established.
Conventionally, mapping of a target area is accomplished by transmitting an acoust;c pulse toward a target area and receiving the reflected echo and determining the distance to the target area based on the bearing of the pulse and the pulse transmission time. The sequence of events is such that when an acoustic pulse is emitted, transmission of the next pulse is delayed until the echo of the transmitted pulse has been received or the delay time is sufficient to ensure that the range of the transmitted pulse is greater than the maximu~ range of the target area to be mapped. Due to the relatively low velocity of sound propagation in water and the number of measurements required, this type of scanning process is very time consuming.
In addition, with these known systems, it is difficult to maintain the scanning head in a stationary position during the mapping process or to establish proper reference points.
Summarv of Invention I have found-that it is possible to considerably reduce the mapping time by transmitting a series of acoustic signals before the echo genera-ted by a signal of this series is received by the receiver. That is to say rather than delay the transmission of successive signals until the preceding signal is received, I transmit a series of signals and then delay the transmission of the subsequent series until the signals of the previously transmitted series have been received.
It is also possible to reduce the time required to generate an oceanographic map by transmitting a series of acoustic signals toward the target area before the first echo generated by a signal of the series of signals is received by the receiver.
According to one aspect of the present invention, there is provided in a method of oceanographic mapping wherein the range and bearing of target area is determined by scanning the

- 2 -1~'7~ 55a~
04666-2 GWH:jy target area, by means of a narrow beam acoustic transducer having beam width in the range of 1~ to 15, within a predetermined angular sector as said narrow beam is advanced in a predetermined direction and wherein a series of acoustic pulses are transmitted, one at each of a series of angularly spaced points within said angular sector and wherein after a delay which is indicative of the range of the target area a receiver receives a series of echos from the target area corresponding to the transmitted series, the improvement of a high speed method of mapping wherein, an assumption is made regarding the maximum bottom depth of the -mapping site and thereafter a first sequentially timed series of acoustic signals are transmitted in a predetermined order before the first echo of the sequential series of echos generated by said first sequential series of signals is received by the receiver and wherein a further sequential series of signals is not transmitted until after the last echo of said sequential signal is received a further sequential series of signals is transmitted.
According to a further aspect of the present invention, there is provided an oceanographic mapping system in the for determining the bearing and range of a target area, comprising:
a narrow beam acous-tic transmitting transducer having a beam width in the range of 1 to 15 and which is operable to emit a first sequential timed series of acoustic pulses, within a predetermined angular bearing sector; beam scanning means communicating with the transmitter and operable to change the bearing of the beam between successive pulses; receiver means operable to receive an echo generated by each acoustic pulse and to precondition the received echoes; control means communicating with the transmitter to cause it to emit a first series of sequentially timed pulses at predetermined timed intervals which terminate before the first echo of the sequential series of echos generated by the first sequential series of pulses is received by the receiver means and to prevent the transmission of a further sequential series of pulses until the last echo of said first series of pulses is received by the receiver means; range .; ~ - 3 -,~

04666-2 GWH:jy calculator means communicating with the receiver and control means and operable to determine the delay between the transmitted pulses and their corresponding echoes; mapping means communicating with the range calculator to receive a series of range signals and communicating with the beam scanning means to receive a corresponding series of bearing signals and being operable to process the range and bearing signals to generate a profile of the target area.
Brief Description of Drawinqs The invention will be more clearly understood after reference to the following detailed specification read in conjunction with the drawings wherein;
Figure 1 is a diagram illustrating a towed multipulse device constructed in accordance with an embodiment of the present invention.
Figure 2 is a block diagram of a multipulse system according to an embodiment of the present invention.
Figure 3 is a diagram illustrating the timing of a typical series of pulse transmissions and the receipt of their respective echo signals.
Figure 4 is a diagram illustrating the direction of the transmitted pulses.
Figure 5 is a block diagram similar to Figure 2 further illustrating the transmitted and received sequences.
Figure 6 is a diagram illustrating the limiting spatial distribution of reflectors.
Figure 7 is a further block diagram illustrating the system of the present invention.
With reference to Figure 1 of the drawings, the reference numeral 10 refers generally to a "fish" which is towed by a vessel 12 at a predetermined depth in a body of water 14 for the purposes of mapping the bottom 16 of the bed of the body of water. The fish 10 i5 of any conventional construction such as that commonly used for oceanographic mapping purposes. The fish 10 supports a narrow beam acoustic transmitting transducer 18 and a broad-beam receiver 20.

3a -55~
04666-2 GWH:jy The narrow-beam transmitter may be a conventional narrow beam transmitter of the type commonly used for oceanographic mapping which has a beam width in the range of 1 to 15 and similarly the receiver 20 may be a conventional broad receiver.
The transmitter 18 has a scanning sector extending through the angle Q . In use, the narrow beam 22 which is emitted by the transmitter 18 scans the target area 24 by sequentially emitting a series of pulses Pl, P2, P3--PX while scanning the full sector. The scanning can be achieved by mechanically redirecting the output of the transmitter or by an electronic scanning system or the like.
As previously indicated, it is common to map a target area by emitting a pulse from a transmitter and thereafter indexing the transmitter to a second position of a scan, receiving the emitted pulse and thereafter emitting a subsequent pulse and repeating this process for each of several bearing settings of the transmitter as it scans the target area.
The multipulse system of the present invention is illustrated diagrammatically in Figure 3 of the drawings wherein it will be seen that a series of pulses Pl, P2, P3 and P4 are emitted at time spaced intervals and the last pulse P4 of the series is transmitted ~efore the first echo El is received by the receiver and similarly the last echo E4 is received by the receiver before the next pulse Pl of the next series of pulses is initiatedO
I have fond that rather than following the procedure wherein the pulse Pl is emitted and the echo El is received before the pulse P2 is emitted, I can emit a series of pulses and provided the last pulse of the series is emitted before the first echo El of the echo series is received, it is possible to greatly increase the speed of the mapping process.
Because variations in the terrain of the target area will result in variations in the two-way transmission time between the ;., , ~' ''! ~r^ .

~ ~7~55~

DS25-~666-2 emission of a pulse and the receipt of its echo, I space the acoustic pulses of each of the series of pulses from one another by a time Tl which is known to be sufficient to ensure that the sequence in which the echos El, E2, E3, etc., are received, is the same as the sequence in which the corresponding pulses are transmitted. This objective can be easily achieved in circumstances where the profile of the target area is approximately known or can be predicted. In circumstances where reasonably accurate prediction cannot be made, the timing can be adjusted in use to achieve this objective.
In order to determine the range of a target area, it is necessary to know the two-way travel time of each pulse and the bearing of each pulse. Clearly the travel time of the pulse which is emitted at the outer most end of the scan will be greater than the travel time of the pulse emitted vertically downwardly in circumstances where the target area is flat. If, however, the travel time and bearing of the pulse is known, it is a simple matter to calculate the actual depth. For a given received echo, the bearing of the corresponding transmitted pulse can be determined by assuming that the echoes will arrive in the same order as corresponding pulses were transmitted as described earlier. In addition, it is possible to distinguish successive acoustic pulses within a series of pulses by ensuring that the frequency of adjacent pulses is distinctly different from one another.
While the pulses within each series of pulses may each have a different frequency, it is possible to distinguish the pulses within any one series merely by employing pulses which have two different frequencies and thereby permitting nonadjacent pulses to have the same frequency. In these circumstances, the pulses which have the same frequency are sufficiently spaced from one another to ensure that their echos are received in the order in which they are transmitted.
With reference to Figure 2 of the drawings, it will be seen that an oceanographic mapping system according to an embodiment of 5~

the present invention, comprises the transmitter 18 and receiver 20 previously described. The transmitter 18 emits acoustic pulses, the timing of which is determined by a timing and frequency control device 30. A timing signal is transmitted from the timing and frequency control 30 through the line 31 to the transmitter 18. The timing and frequency control 30 serves to control the timing of successive pulses and the frequency of the pulse which is transmitted. A timing signal is also transmitted through the line 33 to a range calculator 32. A further timing signal is transmitted through the line 35 to the beam scanning system 34. A signal is transmitted from the beam scanning system through the line 37 to the transmitter 18 which serves to adjust the bearing of the transmitter between successive pulses. A corresponding bearing signal is transmitted through the line 39 to a mapping system 40. The mapping system 40 also receives a range signal from the range calculator through the line 41. The mapping system 40 may be adapted to provide any required mapping format.
It will be apparent that the mapping system of the present invention can be used for the purposes of mapping the profile of a surface which extends in a plane other than the plane of the bottom of the body of water. For example, the device of the present invention can be used for mapping the profile of an iceberg simply by reorienting the transmitter to transmit pulses in a direction toward the submerged iceberg.
It will also be apparent that in some circumstances, the two-way travel time of one or more of the pulses of a series of pulses may be greater than the delay time T2 between the initiatiny pulse of each series of pulses. Where, for example, the pulse P2 is directed into a very deep chasm, the echo E2 might not be received within the time period T2. In these circumstances, the receiver can be readily adapted to disregard the echo E2 when it is received.

~'7~5~

If in use it is found that the echo El is received before the ~inal pulse P4 of a series of pulses is transmitted, the operator can adjust the timer 30 to reduce the number of pulses in the series or to provide that the time delay Tl between successive pulses is reduced to restore the required timing to ensure that the final pulse P4 of the series is transmitted before the ~Eirst echo El is received.
It will be understood that the number of pulses in any one series may be varied depending upon the anticipated time delay between transmission of the first pulse and the receipt of the corresponding echo. It is anticipated that substantially more than 4 pulses will be transmitted in any one series and only 4 pulses have been illustrated in Figure 3 in order to simplify the description of the timing sequence and not for the purposes of limiting the scope of the invention.
The system consists of a pencil-like narrow-beam transmitter and a fan-like broad-beam receiver. The transmitter beam can be sequentially scanned over a certain angular sector which is within the receiver beam coverage as illustrated in Figure 1. There are N selected angular positions of the beam. At each selected position ~n,n = 1, ...,N a pulse is transmitted in the ~n direction as schematically shown in Figure 4. For simplicity, we will assume that the angular and time separation of these sequential beams is constant, that ls ~ n - ~n; n = 1,..., N (1) where ~ is the angular separation between two adjacent beams se~a~at~?d by a time T.
A pulse transmitted along the beam n is reflected by a target (scattering surface) situated at a radial distance rn (range) in ~n direction. To avoid the overlap between the transmitted and the received sequences as well as the ambiguity in range calculations, the following assumptions have to be satisfied:
a. The last pulse in the transmitter sequence is transmitted before the echo due to the first transmitted pulse is received.

S~

b. The reflectors are spatially distributed such that an echo due to an earlier transmission always arrives before the echo due to consecutive transmission in the transmitting sequence.
Time representation of the transmitted and the received sequences is shown in Figure 5. For simplicity it was assumed that echoes have the same duration as the transmitted pulses. The end of the transmitted sequence of duration to ha~s been chosen as a convenient time origin.
In order to satisfy the assumption (a) the radial distance to the first reflector rl must satisfy rl~ toC = rO
2 (2) and ~t - to - Nt o N - 1 , In the above equations ~t is time spacing between transmitter pulses, t is pulse duration, tO,rO are constants, and c is sound velocity.
The radial range of each reflector can be calculated based on echo arrival times tn using the following relationship.

tn = 2rn + (n _ l)T _ to; n _ 1 N (4) c where T = ~t + t (5) The angular position of the reflector i9 given by equation ~1). The average angular velocity of the scanning transmitter bearns is given by ~= Q = ~c ~6) rO 2h~
where , ' Q = ~N - 1)~ (7) In order to satisfy the assumption (b) the Eollowing inequali~y must apply.

tn~l ~ tn~t (8) or in spatial domain rn+l ~ rn - ~ r (9) where ~r = c ~t = 2r~ - Ntc 2 2(N - 1) (10) Inequality (9) defines a spatial distribution of the reflectors which will satisfy the assumption (b). In the limiting case for which the received echoes start to overlap and rn+l~, rn we have rn+l = rn - ~ r (11) or equivalently in normalized form - (n-l); n = l,..... ,N (12) r ~r lS~i~

A contour of the limiting reflector radial distribution for the assumed values of parameters is shown in Figure 6. The first beam in Figure 6 was chosen in vertical to bottom direction and reference distance rO was assumed to be the water depth. For the assumed parameters ~ r = 9.2m.
FREQUENCY DIVERSITY
The restriction on reflector rad~al distribution imposed by inequality (9) can be eased if different frequences are assigned to adjacent beams and the received signal is processed by appropriate analog or digital filters (using, for example, Fast Fourier transform algorithm). Typically, the number of available different frequences M will be smaller than the number of beams N and some nonadjacent beams will utilize the same frequency. The effect of applying the frequency diversity on the ~r parameter, given by equation (10), is essentially equivalent to the reduction of the number of beams without actually changing it. In the limiting case when N = M the restriction of the reflector spatial distribution imposed by the assumption (b) is completely removed. The number of available different frequencies is determined by the bandwidth of the projector which is related to its central frequency ~O and quality factor Q. Assuming that the bandwidth occupied by a single pulse B = l/t, we obtain M = ~fo Q (13) For narrowband systems with Q 10 and equation (13) becomes M t~o (14) SYSTEM CONFIGURATIONS
There are many possible configurations for the system. The particular configurations suggested here are intended as a high-frequency, short range and low-cost acoustic mapping systems which can be deployed using a towed body or which can be suspended from the ship's or boat's side. The circular acoustic project (P) is housed in an oil filled drum driven by an electric motor (MR) equipped with shaft position encoder (ER) as shown in Figure 7. The ' ' ` ~ '`' .

.

~.~

electrical connections to the transducer are provided through a slip ring (SR). Motor, transducer, and slip ring are housed in a single unit which also contains a roll sensor (RS). The unit can be mechanically stabilized against the pitch and possibly yaw of the platform. The controller controls the revolutions of the electric motor and provides suitable timing information to the transmitter (TR) based on the readings from the roll s,ensor. The echo signal is received by a receiving hydrophone array H, and is conditioned at the receiver RR.
A standar~ gray-scale recorder (GS) can be used to access the quality of the received data and to display the mapped contours. The received echo sequences will appear on the record as the position-varying-traces each corresponding to the individual echo. Such a display will provide immediate information on the profiled surface and may be properly interpreted even if the assumption (b) is violated.
The received signal can be further processed at the other processing stages to produce a corrected contour depth map. The controller can be programmed to facilitate diverse beams and probing sequencies. Typically, the first beam will be directed vertically towards the bottom as depicted in Figure 4 to perform starboard (or port) scan only. Two rotating projectors (and two receiving arrays) might be required to perform simultaneous starboard and port scans.
In suitable orientation the system can be used to survey underwater objects such as icebergs.
Another interesting application of the system is high density depth sounding performed from a fast moving platform. In this application, only one beam is projected in the vertical direction and multiple pulses are used to produce a dense depth pro~ile. By using two or more fre~uencies, a continuous high density survey can be conducted.
If full frequency diversity is applied (N = M) the system can be used as a fast obstacle avoidance sonar with a possible 360 coverage.
The concept of a multipulse acoustic mapping system i ~.~7~5~
.~

described above can be used in several appllcations. The system can operate as a portable, low cost mapping device with relatively simple display and small required bandwidth.
Various modifications of this invention will be apparent to those skilled in the art without departing from the scope of the invention.

Claims (5)

04666-2 GWH:jy The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows.

1. In a method of oceanographic mapping wherein the range and bearing of target area is determined by scanning the target area, by means of a narrow beam acoustic transducer having beam width in the range of 1° to 15°, within a predetermined angular sector as said narrow beam is advanced in a predetermined direction and wherein a series of acoustic pulses are transmitted, one at each of a series of angularly spaced points within said angular sector and wherein after a delay which is indicative of the range of the target area a receiver receives a series of echos from the target area corresponding to the transmitted series, the improvement of a high speed method of mapping wherein, an assumption is made regarding the maximum bottom depth of the mapping site and thereafter a first sequentially timed series of acoustic signals are transmitted in a predetermined order before the first echo of the sequential series of echos generated by said first sequential series of signals is received by the receiver and wherein a further sequential series of signals is not transmitted until after the last echo of said sequential signal is received a further sequential series of signals is transmitted.

2. A method as claimed in claim 1, wherein the transducer is intermittently activated such that the acoustic pulses of each of said sequential series of pulses are spaced from one another to ensure that the order in which the echos are 04666-2 GWH:jy received corresponds to the order in which the pulses were transmitted.

3. A method as claimed in claim 1, wherein the frequency of successive acoustic pulses within each sequential series of pulses are distinctly different from one another and wherein the frequency of each echo which is received is measured and the echoes and pulses of the same frequency are matched with one another in order to determine the range and bearing of the target area.

4. A method as claimed in claim 1, wherein, within each sequential series of transmitted pulses, nonadjacent pulses have the same frequency and said pulses of the same frequency are sufficiently spaced from one another to ensure that their corresponding echos are received in the order in which the pulses are transmitted.

5. An oceanographic mapping system for determining the bearing and range of a target area, comprising:
a) a narrow beam acoustic transmitting transducer having a beam width in the range of 1° to 15° and which is operable to emit a first sequential timed series of acoustic pulses, within a predetermined angular bearing sector, b) beam scanning means communicating with the transmitter and operable to change the bearing of the beam between successive pulses, c) receiver means operable to receive an echo generated by each acoustic pulse and to precondition the received echoes, 04666-2 GWH:jy d) control means communicating with the transmitter to cause it to emit a first series of sequentially timed pulses at predetermined timed intervals which terminate before the first echo of the sequential series of echos generated by the first sequential series of pulses is received by the receiver means and to prevent the transmission of a further sequential series of pulses until the last echo of said first series of pulses is received by the receiver means, e) range calculator means communicating with the receiver and control means and operable to determine the delay between the transmitted pulses and their corresponding echoes, f) mapping means communicating with the range calculator to receive a series of range signals and communicating with the beam scanning means to receive a corresponding series of bearing signals and being operable to process the range and bearing signals to generate a profile of the target area.