Copolymer films such as poly(vinylidene fluoride-trifluoroethylene) P(VDF-TrFE) have lower acoustic impedance compared to their ceramic counterparts, allowing for a better acoustic match to tissues in the human body. Because of this, copolymer ultrasonic transducers are capable of yielding the desirable characteristics of broad bandwidth and short pulse duration that allow better image resolution to be achieved. In the past, such transducers in the frequency range from 40 to 80 MHz have frequently been fabricated by spin coating the copolymer film onto a flat substrate and then applying the film to a curved backing using an adhesive layer. The adhesive layer may cause spurious signals at these frequencies, in addition to the film damage that may occur as a result of such processing. In order to avoid these problems, a copolymer film can be directly spin coated onto a curved substrate. The resulting devices had an operating frequency of over 40 MHz and approximately a 75% bandwidth. The potential of several approaches that could be further explored to increase the level of performance of such devices is also discussed.
The operation of the Large Aperture Seismic Array (LASA) has provided vast amounts of seismic data. In the study described here arrival time data was first used to obtain values of station residuals to Jeffreys-Bullen P-wave arrival times. Having found that the apparently unrelated residuals and their scatter were large, the residuals were recomputed relative to the P-wave moveouts from site A0. It was found that the residuals are a strong function of azimuth and distance to the epicenter. In an attempt to see if a plane wave approximation would reduce the residuals to a simple speed and azimuth correction, the data was replotted relative to the plane wave moveouts. In this representation the data still exhibited strong azimuth and distance dependence indicating that the residuals were not small enough to allow only a speed and azimuth correction. When the average values of the station corrections were plotted, they indicated that the anomaly causing the residuals may be in the shape of a syncline with its axis trending northeasterly and passing nearly through the center of LASA.The aperture of LASA provides the ability of measuring the actual values of dT/dA, the slope of the travel time curves. The results of many such measurements were plotted and revealed a distinct separation of the data into two groups of separate velocity curves. Data from the northwest gave one curve while data from the southeast defined another curve. Further investigation revealed that as the LASA aperture was reduced the separation became more severe. The corresponding errors made in determining the azimuth were large and increased as the LASA aperture was reduced. The data on velocity measurements and azimuth errors seem to conform to the picture of the anomaly under LASA that was obtained from individual station correction data.To examine the anomaly in further detail, a collection of stations that lie entirely on each limb of the anomaly were used. The group of stations on the southeastern limb produced speed and azimuth errors that were completely consistent with the general picture of the anomaly beneath these stations. The group of stations on the northwestern limb produced azimuth and speed errors similar to the other collection and in general agreement with the picture of the anomaly beneath these stations.
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