Communications – electrical: acoustic wave systems and devices – Echo systems – Speed determination
Patent
1990-07-23
1992-02-18
Pihulic, Daniel T.
Communications, electrical: acoustic wave systems and devices
Echo systems
Speed determination
367 99, G01S 1500
Patent
active
050899961
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a transducer device for an acoustic log, including a transmitter sending a signal within a lobe region and at least two receivers, provided for receiving signals reflected from a reference for determining the velocity component of a vessel relative to the reference in the measuring direction determined by the two receivers.
2. Background of the Discussion
Such devices in which the velocity determination takes place with the use of correlation technique are already known from U.S. Pat. Nos. 3,991,398, 4,041,293 and 4,068,207. Two similar, but mutually time-shifted signals are then generated, the time-shift of which depends on the distance in the measuring direction between the two receivers and on the velocity of the vessel.
The principle for velocity measurement using a correlation technique, which is carried out by the transducer device in accordance with the invention, is described in the above-mentioned patents. A short description of the principle is given below, and for the sake of simplicity, the signals are assumed to be continuous.
Two signals S.sub.1 (t) and S.sub.2 (t) mutually time-shifted by time t.sub.0 (see FIG. 1), and a sample system where the sample is taken with the interval t.sub.s are used.
The following product is calculated for each new sample. ##EQU1##
The development of this product gives: ##EQU2## where AKF denotes the autocorrelation function, and KKF denotes the cross correlation function.
These functions are illustrated in FIG. 2. The functions AKF and KKF intersect at the point t.sub.s =t.sub.0 /2. Another interesting property is that S.sub.11 -S.sub.23 =0 for t.sub.s =t.sub.0 /2, which signifies that the estimate of E has little variance about the intersection point.
A velocity measuring system is provided by the sampling interval t.sub.s being controlled such that AKF=KKF, which results in the velocity v being obtained as crystals used, as will be described in greater detail below.
In the evaluation of the time delay t.sub.0 between the receiver signals the difference between the autocorrelation function and the cross correlation function, AKF-KKF, is thus used. This function is illustrated in FIG. 3 and its zero passage thus gives t.sub.0 /2.
SUMMARY OF THE INVENTION
The object of the present invention is to provide a new transducer device for an acoustic ship's log operating according to the methods described above, which transducer device is of compact configuration so that the total dimensions will be small, and gives better measuring accuracy while at the same time a minimum of transmitter and receiver crystals are required.
This object is achieved by a transducer device of the kind mentioned in the introduction with the characterizing features stated in claim 1.
With the device according to the invention it is thus possible with only five crystals to measure the velocity in twelve different directions regularly distributed around one revolution, i.e. the directions are successively displaced by 30.degree., which gives optimum measuring accuracy.
In accordance with an advantageous embodiment of the device in accordance with the invention, the crystal arrangement includes a central crystal, the remaining four crystals being arranged around a circular arc round the central crystal, so that the radii of the circular arc from the central crystal to each of the other crystals successively form the angles of 90.degree., 60.degree., 150.degree. and 60.degree. to each other.
A short measuring base is achieved with this realization, i.e. a small distance between two crystals, which increases the measuring accuracy, since the overlapping surface of the lobe regions of the two receiver crystals then will be large. In addition a constant measuring base is obtained for all twelve measuring directions. If a bottom measuring log is realized according to this embodiment, there is also space inside the outer casing of the transducer device for e.g. a relative measurement log.
BRIEF DESCRIPTION OF
REFERENCES:
patent: 3991398 (1976-11-01), Andermo et al.
patent: 4041293 (1977-08-01), Kihlberg
patent: 4068207 (1978-01-01), Andermo et al.
Consilium Marine AB
Pihulic Daniel T.
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