TRANSDUCER DEVICE FOR ACOUSTIC LOG

Abstract: An acoustic log for vessels includes a transmitter which sends 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. The log further includes a transducer device (12) with five crystals (1-5) serving as transmitter and receiver elements for transmitting and receiving acoustic signals, and being arranged such that combinations in pairs of the crystals define twelve measuring directions regularly distributed around revolution.

Description: Transducer Device for Acoustic Log The present invention relates to a transducer device for acoustic log, including a transmitter sending a signal within a lobe region and at least two receivers, provided for receiving signals reflekted from a reference for determining the velocity component of a vessel relative to the reference in the measuring direction determined by the two receivers. Such devices in which the velocity determination takes place with the use of correlation technique are already known from US A, 3 991 398, US A, 4041 293 and US A, 4068207. 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 correlation technique, which is carried out by the transducer device in accor- dance 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, signals are assumed to be continuous. Two signals S^t) and S2_t) mutually time-shifted by the time t Q , see Fig. 1, and a sample system where the sample is taken with the interval t s are used. The following product is calculated for each new sample. E = {S 11- S 23 )(S 12- S 22> The devel opment of this product gi ves: E = ( S llSi2- s ll s 22- s 23 s 12 s 23 s 22 ) = = AKF ( t s )- F ( t s )-KKF ( t s )+AKF ( t s ) , i . e. E = 2AKF (t- )-2KKF (t,/2 ) 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 s = tø/2. Another interesting property is that for t s =t Q /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 s being controlled such that AKF = KKF, which results in the velocity v being obtained as v = l 0 /2t s where 1Q denotes the measuring base, i.e. the distance between the crystals used, as will be described in more detail below. In the evalution of the time delay t Q 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 0 /2. 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°, 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 90°, 60°, 150° and 60° 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 addi¬ tion 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. An acoustic log with an embodiment of the transducer device in accordance with the invention, choosen as an example, will now be described in more detail in connection with Fig. 1 7. On the drawings, Fig. 1 illustrates two time-dependant, shifted signals for illustrating the correlation calculation required for determining the velocity, Fig. 2 illustrates examples of the autocorrelation and cross correlation functions, Fig. 3 illustrates the difference between the autocorrelation and cross correlation functions, Fig. 4 is a block diagram of an acoustic log with the transducer device in accordance with the invention, Fig. 5 illustrates a preferred embodiment of the transducer device in accordance with the invention, Fig. 6 illustrates the velocity determination using the transducer device of Fig. 5, and Fig. 7 illustrates an example with transducer devices for two logs arranged in the same outer casing. In Fig. 4 there is illustrated a block diagram of an acoustic log with a transducer device in accordance with the invention. The system comprises four function blocks, namely a speed indicator 6, a depth indicator and administration unit 8, a I/O unit 10 for generating transmission pulses and receiving echo pulses, and the transducer device 12. This log operates according to the same principles as the log described in the above-mentioned US A, 4041 293 for determining two orthogonal velocity components. The velocity measuring function is based on the fact that the distance to the reference is known. This distance is therefore measured at the same time as the velocity is measured. Distance measuring functions as a locked regulating system. The transducer device 12 in accordance with the invention is illustrated in Fig. 5. The device includes five crystals 1-5 arranged in a special pattern so that measuring can be performed in all 30° directions around a revolution. In embodiment illustrated in Fig. 5 the crystal are numbered 1-5. In such an embodiment, all the crystals 1-5 can serve as receiver crystals for the velocity determination while only crystals 1-3 can serve as transmitter crystals. Transmission takes place via the crystals 1, 2 and 3 to obtain a given lobe width. In the illustrated example, the crystals 1 and 2 are assumed to lie in the longitudinal di ection of the vessel. The different directions are measured with the following crystal combinations: crystals directions As will be seen from Fig. 5, the measuring base, i.e. the distance between the two crystals used at each time, is constant and comperatively small, which increases measuring accurancy, as already mentioned. Since the velocity can be determined in twelve directions, a revoution can -be divided into twelve sectors, see fig. 6. The velocity in each sector is determined by determining the velocity vector in the two adjacent directions. The expression for the relationship between the velocity vectors in the longitudinal direction v x and transverse direc¬ tion v„ can be determined. In section 0, the relationship will be v x = o Other velocity vectors can also be used for increasing the accurancy. For depth measurement only the receiver crystals 4 and 5 are utilised. In the embodiment illustrated in Fig. 5, the crystals 1-5 do not take up much more than half the space within the outer casing 14 of the transducer device 12. Accordingly, if the illustrated and described log is a log for measuring velocities over shallows, there is root inside the casing 14 also for a transducer device 16 having, for example, two crystals 18, 20 for measuring the velocity relative to the water. This embodiment is illustrated in Fig. 7. The two logs then preferably operate at different frequencies. The embodiment illustrated in Fig. 7 of the second log, e.g. one intended for velocity measurement relative the water, thus comprises two crystals 18, 20 for measuring a velocity component. Of course, the transducer device of the second log can be constructed in a corresponding manner as the one described in • connection with Fig. 5. In this way velocity measurement in twelve directions is possible with the second log as well.

Claims: Claims 1. Transducer device for acoustic log, including a trans¬ mitter 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, c h a r a c t e r i z e d in that five crystals serving as transmitter and receiver elements for trans¬ mitting and receiving acoustic signals are arranged such that combinations of pairs of the crystals define twelve measuring directions, regularly distributed around a revolution. 2. Device as claimed in claim 1, c h a r a c t e r i z e d in that 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 said circular arc from the central crystal to each of the other crystals successively form the angles 90°, 60°, 150° and 60° to each other. 3. Device as claimed in claims 1 or 2, c h a r a c ¬ t e r i z e d in that three crystals are formed as both transmitter and receiver crystals, while two crystals are formed only as receiver crystals. 4. Device as claimed in claim 3, c h a r a c t e r i z e d in that said crystals formed only as receiver crystals are disposed also for depth measurement. 5. Device as claimed in any one of claims 1-4, c h a - r a c t e r i z e d in that it is arranged in an outer casing accomodating a transducer device for a second acoustic log.

Inventors: MASRELIEZ KARL-GUSTAV, MASRELIEZ, KARL-GUSTAV
Applicants: CONSILIUM MARINE AB
Application year: 1988
Main classification: G01S7
IPC codes: B63B49, G01S15, G10K11