Update to term: Pinger sub-bottom profilers

[louatbodc]

To enable distinction with request #17. We would like to propose the following updates to wording for term 394 - Pinger sub-bottom profilers.

Marine seismic sources that send sound pulses into the sub-sea floor sediments. The sound pulses bounce off the sea floor and subsequent buried sediment layers according to differences in their acoustic impedance (hardness) and in turn, their density. Pingers are linear, single channel, high frequency systems that typically operate at frequencies greater than 2 kHz and penetrate to several tens of metres.

This request duplicates a discussion in the L22 repository. Please see nvs-vocabs/L22#155

[louatbodc]

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