The echo amplitude tends to be gentle, corresponding to the range between the peaks in Fig. 10(a). That is to say, the range between the two peaks in corona bond represents the nugget. This article also extracts different frequencies (16–19 MHz) to test the specimen and the small wavelet packet analysis results (Fig. 10(b)) show that the repeatability is good. According to this characteristic, the nugget and corona bond can be easily distinguished