Floating sensor to detect very low frequency pressure signals

A floating sensor system is provided to detect very low frequency pressure ignals (down to 0.01 Hz). This system detects pressure fluctuations or pressure signals of interest in the ocean or other body of water in the presence of unwanted pressure signals generated by surface wave induced motion. A drifting sensor surface float follows the surface waves and in turn moves a suspended pressure sensor vertically, such that it detects the wave motion as a change in static pressure which then constitutes a noise source. A correlation circuit and a logic circuit discriminate between a first composite signal, comprised of the pressure signals of interest and wave motion noise, and a second reference signal, comprised only of the wave motion noise to provide an output signal transmitted to a monitoring station.

BACKGROUND OF THE INVENTION 
This invention relates to sonobuoy type, floating sensors for detecting 
very low frequency sounds and especially relates to oceanographic 
detectors for detecting pressure fluctuations caused by slow moving 
objects and the like in the presence of surface-wave motion. 
DESCRIPTION OF THE PRIOR ART 
Fay, U.S. Pat. No. 4,388,711, shows a method of canceling a flow noise 
received by a towed hydrophone module. Fay uses signals from a pair of 
hydrophones towed in tandem, inverts one of these and then adds the 
signals together, after appropriate phase shifting, if needed. The 
composite signal thereby created is thereafter added to the signal 
received by the towed hydrophone to cancel out the noise components 
attributed to water flow over the towed hydrophone and thereby yielding 
the signal of interest. 
Rennick, et al., U.S. Pat. No. 4,232,381, shows a noise-cancellation scheme 
for a system which receives acoustic information. This scheme cancels out 
unwanted engine noise received along with an acoustic signal. An engine 
rotation sensor generates pulses to create an independent engine noise 
frequency and harmonics signal. This separate engine noise signal is 
subtracted from a composite input signal having information and noise 
combined to generate a noise free acoustic signal of interest. 
Theodoulou, U.S. Pat. No. 4,222,266, shows an electronic filter for use in 
determining the position of a body traveling through water, (i.e. a towed 
hydrophone), relative to a datum, such as mean sea level. The filter is 
connected to an accelerometer and a pressure transducer that, 
respectively, produce an acceleration signal indicative of the traveling 
body's relative acceleration and a pressure signal indicative of the 
body's vertical displacement, i.e. depth in the water. The filter combines 
the acceleration and pressure signals to isolate a signal proportional to 
noise generated by the pressure transducer. A correction signal is added 
to the received signal. This correction signal takes into effect the 
changes in depth of the traveling body (towed hydrophone) and also changes 
in acceleration of the movement of the traveling body caused by the towing 
ship. 
Berni, U.S. Pat. No. 4,345,473, shows an accelerometer adaptable to marine 
survey equipment and used in combination with submerged hydrophone signal 
processing to cancel the effects of a surface reflected wave. The 
accelerometer measures the vertical component of acceleration independent 
of the devices orientation or pressure. This accelerometer is used in 
conjunction with signal processing circuitry in towed hydrophones to 
compensate for erroneous signal readings attributable to pressure 
variations produced by ocean surface to air reflected artificially 
generated seismic waves. 
Ballard, et al., U.S. Pat. No. 4,310,904, shows an ambient sea noise 
elimination system wherein the output of a directional hydrophone and an 
omnidirectional hydrophone are filtered and summed. The composite signal 
representing the effective receiving pattern below approximately 300 Hz is 
a vertically oriented dipole pattern having a null in the horizontal 
direction and the effective receiving pattern above 300 Hz is a cardiod 
having a null pointing vertically, either upwardly or downwardly. The 
composite signal is used for correcting unwanted ambient sea noise. It is 
applied in a summing circuit which provides compensated acoustic signals 
having ambient sea noise substantially removed therefrom. 
Hutchins, U.S. Pat. No. 4,091,356, shows a heave compensation system for an 
underwater towed seismic device, which towed seismic device contains an 
acoustic signal source and a hydrophone receiver. An accelerometer in the 
towed device produces a signal indicative of the vertical acceleration of 
the device. This acceleration signal is double integrated to produce a 
position signal which is used to adjust the signal provided by the 
acoustic signal source. A pressure transducer in the device produces a 
pressure signal indicative of changes in reflected seismic signals caused 
by the vertical movement of the seismic sensing device and not by changes 
in the sensed depth of the ocean bottom. 
The devices discussed above concern themselves, principally, with 
compensation for noise in towed systems or for noise filtering used in 
towed hydrophonic devices. The towed hydrophonic devices usually have 
active acoustic signal sources. These prior art devices focus on one of 
two problems. The first problem is the adding out or canceling of known 
repetitive noise to yield a signal of interest. The second problem is the 
canceling of the error of reflected seismic signals caused by a change in 
vertical position of a towed hydrophonic sensor. As a result, the signals 
which these systems are limited in dealing with are of relatively high 
frequency. 
Non-moving acoustic sensors are capable of detecting very low frequency 
pressure signals and very low amplitude signals. These non-moving acoustic 
sensors have taken one of two forms, these being either a floating sensor 
or a moored, submerged sensor. However, floating sensors have to contend 
with relatively low frequency noise attributed by wave motion as wave 
motions is sinusoidal with long periods. 
Previous non-moving pressure protecting systems have avoided the pressure 
signal errors caused by surface wave motion by mooring a sensor on the sea 
bottom. However, this design approach has depth restrictions and precludes 
open ocean sea bottom applications for battery powered radio transmitter 
devices. 
It would be desirable to have a surface floating acoustic pressure sensor. 
However, with this type of device surface wave motion becomes a 
significant factor. The use of vertical motion isolation systems, such as 
those described above in connection with towed acoustic (hydrophone) 
devices, is impractical for noise compensation of wave movement noise 
because of the very large magnitude of wave motion induced pressure 
signals. 
As an example, the cyclic, vertical, physical motion imparted to a floating 
acoustic sensor of one inch peak-to-peak is equivalent to a 168 dB 
relative to 1 micro pascal pressure signal. Typical, state-of-the-art 
vertical motion isolation systems can provide only approximately 26 dB of 
motion attenuation at 0.1 Hz signal levels. A sea state "5" condition is 
one where there are 12 foot waves. This wave motion would generate a noise 
signal of approximately seven inches of water or about 184 dB relative to 
one micro pascal pressure at the sensor. Thus, signals man-made, low 
frequency pressure would easily be masked by wave generated motion (noise) 
in open ocean floating sensor type devices. 
It is desirable to provide a floating sensor which detects low frequency 
pressure signals in the open ocean and which compensates for "noise" 
generated wave motion. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an open ocean type 
drifting buoy for detecting very low frequency pressure signals in the 
presence of unwanted pressure signals generated by surface wave induced 
motion. 
A second object of the present invention is to provide the device with a 
means for sensing surface wave motion and for generating a signal 
proportional to the amount of surface wave motion in amplitude and 
sinusoidal period. 
A further object of the present invention is to provide such a device with 
a logic circuit which discriminates between man-made signals and the 
previously received statistical information on wave motion generated 
signals of amplitude and sinusoidal period. 
The objects of the present invention are realized in a surface floating, 
drifting-type, buoy device. A surface float portion of the device includes 
a battery and radio transmitter. Connected to the surface float portion is 
a submerged weighted lower unit portion containing a pressure sensing 
hydrophone and accelerometer sensitive to vertical movement, as well as 
signal processing electronics. The floating portion of the device and the 
submerged portion of the device are connected by a length of suspension 
cable and signal transmission lines. 
The accelerometer position in the lower unit measures the change in 
position of that lower unit which equates to the change in position of the 
surface float. This device is connected to electrical processing circuitry 
which will provide a sinusoidal signal of amplitude and period 
representative of current wave motion, a reference signal. 
The pressure hydrophone senses the man-made signals as well as wave noise. 
These sensed signals are processed in a correlation circuit device with 
the signals representing wave motion. The reference wave noise signal and 
the composite hydrophone sensed signals are compared and any lack of 
statistical correlation between the two provides a change in output from 
the correlation circuit. A logic circuit receives the output from the 
correlation circuit and provides an output representing wave noise or a 
signal reflecting the detected signal of interest. This output is 
transferred to the surface float portion of the device and transmitted to 
a monitoring station.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
A floating sensor to detect very low frequency pressure signals in the 
presence of open ocean wave noise includes a floating portion 11, FIG. 1, 
and a submerged portion 13 held at a desired depth by a length of 
suspension cable 15. Contained within the floating portion 11 is a battery 
and a radio transmitter which will be discussed further below. Connected 
to the radio transmitter and projecting above the floating portion 11 is a 
radio antenna 17, FIG. 1. An electrical signal and power transmission 
cable 19 connects the electrical components in the floating portion 11 and 
the submerged portion 13. 
The floating portion or buoy 11 is carried by the wave action of the ocean 
surface 21. These ocean waves 21 have a sinusoidal form which causes the 
buoy 11 to rise and fall with the wave motion. This wave 21 motion causes 
pressure differences, by changing pressure head, at the submerged portion 
13 hydrophone. This changing pressure at a submerged portion 13 
hydrophone. This changing pressure at a submerged point caused by surface 
waves constitutes unwanted noise of sufficient magnitude to mask a 
man-made pressure signal 23 which is received by a hydrophone located in 
the submerged portion 13. 
The components housed in the floating portion 11 are shown in FIG. 2. The 
antenna 17 is connected directly to a radio transmitter circuit 25. This 
ratio transmitter 25 may be selected to transmit in the AM or FM bands and 
may carry pulse modulated or frequency modulated information. The radio 
transmitter 25 as well as the electrical components in the submerged 
portion 13 are powered from a battery 27 housed in the floating buoy 11. 
Again, the battery 27 can be any of those available in the marketplace, 
including the longer life lithium batteries or cadmium batteries. As an 
alternative, a solar charger may be included with the battery 27. 
As recited above, an electrical signal and power transmission cable 19 
electrically connects the floating buoy 11 with the suspended, submerged 
portion 13. This submerged portion 13, of course, is weighted, to 
compensate for any buoyancy it may have, in order that it may be submerged 
to the full length of the suspension cable 15. 
Positioned within the submerged housing 13 is an accelerometer 29. This 
accelerometer can assume the design considerations of some of the prior 
art devices described herein above. Accelerometer 29 provides an 
electrical signal output as a function of vertical acceleration, both with 
upward and downward movement. 
The output from the accelerometer 29 is amplified through a preamplifier 
circuit 31 and then connected into a double integration circuit 33. The 
electrical output from the double integration circuit 33 is an electrical 
signal which is equivalent to the instantaneous position of the submerged 
housing portion 13, as well as that of the floating buoy 11, as these two 
portions 11, 13 are connected by the suspension cable 15. 
The signal 35 represents the wave motion noise created by changes in 
pressure at the location of the submerged housing portion 13 and the 
hydrophone device housed therein. A hydrophone, or acoustic pressure 
sensing device 37 senses the wave 21 motion noise as well as the presence 
of a pressure signal of interest, signal 23 of FIG. 1. An electrical 
signal output from hydrophone 37 is sent through a preamplifier circuit 
39. The output from the preamplifier circuit 39 is an amplified signal 41 
representing the composite of the desired pressure signal 23 and the wave 
21 motion noise. This composite signal 41 as well as the wave motion noise 
signal 35 provided out of the double integration circuit 33 are entered 
into a correlation circuit 43. 
Correlation circuit 43 is of a design previously found in information 
processing circuits for determining the statistical correlation between 
any two electrical signals. 
The output of the correlation circuit 43 is connected into a logic circuit 
45. Logic circuit 45 discriminates upon the correlation information 
received from the correlation circuit 43. The signal provided by the 
correlation circuit 43 to the logic circuit 45 indicates the correlation 
"value" of the composite signal 41 to the wave noise signal 35, i.e. 
whether the composite signal 41 deviates from the wave noise signal 35 and 
by what value. 
Logic circuit 45 provides an output signal 47 to the radio transmitter 25 
via the electrical cable 19 for transmission to a monitoring station. The 
output signal 47 from logic circuit 45, therefore, either represents a 
sine modulated tone indicative of only wave 21 motion noise or a signal on 
the modulated tone which represents that portion of the composite signal 
41 which does not statistically correlate, within the design statistical 
values, to the wave noise signal 35, i.e. the signal 23. 
The goal of the system is to detect man-made, low frequency pressure 
signals and to discriminate wave motion noise received by the hydrophone 
37 housed within the submerged housing portion 13 at the end of the 
suspension cable 15. The suspension cable can be of any length, and as an 
example, a length of 100 feet is of particular interest. As the waves 21 
move the surface float 11 vertically, the lower unit 13 is driven in the 
same vertical motion by the surface float via the suspension cable 15. The 
resulting vertical accelerations are sensed by the accelerometer 29. At 
the same time, the pressure hydrophone 37 senses the change in static 
pressure due to the vertical displacement of the device and in the absence 
of any other pressure signals, processes an output analogous to the 
vertical displacement. Thus, the output of the double integrated 
accelerometer leg of the circuitry, i.e. signal 35, and the output of the 
hydrophone leg of the circuitry, i.e. signal 41, are the same if the only 
input to the hydrophone 29 is surface wave motion "noise". When these two 
signals 35 and 41 are input into the correlation circuit 43, the output 
will equal 1, or perfect correlation. The logic circuit 45 would then send 
an appropriate signal to be transmitted by the radio transmitter 25 
indicating the absence of any pressure signals 23, and a signal level due 
to surface waves 21. 
If a pressure signal 23, unrelated to the wave 21 induced vertical motion, 
is present, it will be detected only by the pressure hydrophone 37. The 
signal 35 from the accelerometer leg and the signal 41 from the hydrophone 
leg will then be uncorrelated and the logic circuit 45 will provide a 
signal 47 indicating a correlation less than 1, i.e. a non-surface wave 
related pressure signal present. Output signal 47 will have fluctuations 
which will increase as the correlation value between signals 35 and 41 
decreases. The logic circuit output signal 47 can appear as a modulation 
representing the pressure signal 23 riding on a carrier frequency, with 
the carrier frequency signal representing the wave 21 motion noise. 
The advantage of this type of buoy system over earlier designs is that it 
eliminates the need for expensive and complicated vertical isolation 
suspension systems. This allows the present invention to operate in 
frequency ranges where vertical isolation is not feasible. In addition, 
the present invention transmits a signal indicating the presence or 
absence of the pressure signal 23, as a result of a simple, single 
correlation process carried out by the electronics. The signal transmitted 
by the antenna 17 is either pure carrier wave signal (the wave 21 noise), 
or a modulated carrier wave signal with the modulation representing the 
signal 23. The present invention, therefore, is implemented in simple and 
inexpensive circuitry. 
Changes can be made in the above-described invention without departing from 
the intent and scope thereof. As an example, the location of the 
accelerometer and the correlation circuit can be changed to the surface 
float unit in lieu of the submerged lower unit. While the lower location 
is preferred, since a more accurate measurement of hydrophone motion is 
determined at that location, reallocation of electronics is still within 
the scope of the invention. Likewise, other types of modifications can be 
made. It is intended, therefore, that the above-description be read as 
illustrative of the invention and not be interpreted in the limiting 
sense.