Arduino / Raspberry Pi Sonar

Hey folks,

I'm working on building an underwater sonar / fish finder. My plan is to use an arduino (or straight avr chip) for signal generation and data capture, and then my pi for data processing and visualization.

One question I was wondering about was the speed at which I would need to capture my signal. The plan is to output a 200-300us 40khz pulse every 20 milliseconds (exact timing tbd). Based on my understanding I would only need to capture my data at 80khz if I was interested in the actual signal, however in this case I'm only interested in detecting the pulse. It seems that I should be able to capture at a much lower rate and still detect my signal?