Final answer:
The statement is true: when a sonar system is imaging at shallow depths, the pulse interval is short to ensure timely echo detection. This fast pulse rate facilitates higher resolution in detecting slight differences in depth, and the ultrasound frequency must be high enough to distinguish these differences, while still being within the normal range for diagnostic use.
Step-by-step explanation:
The statement that when a sonar system is imaging at shallow depths, the time from one pulse to the next is short is indeed true. This is because at shallow depths, the echo from the pulse will return to the sonar system more quickly than it would from a greater depth. The system must emit pulses frequently to get timely feedback and accurately image the environment. This, in turn, imposes limits on the time resolution of the system. The time resolution must be fine enough to distinguish between echoes coming from targets at slightly different distances. For instance, to detect a difference between tissues that are 3.50 and 3.60 cm beneath the surface, the minimum time difference the system should resolve corresponds to the difference in times it takes the sound to travel to and from each depth.
For an effective time resolution, the period (T) of the ultrasound should be shorter than this minimum resolving time. If the minimum resolving time is the difference in echo times for the two depths, then the frequency (f), which is the inverse of the period (1/T), must be high enough to differentiate this time difference. Diagnostic ultrasounds have a certain range of frequencies that are considered normal, and the required frequency for resolving 0.100 cm details should fall within this range.