Difficulties in the indirect experimental inference of the low‐frequency intrinsic acoustic attenuation in marine sediments.

Abstract

Intrinsic attenuation is ideally measured for plane compressional waves in an unbounded homogeneous medium. For typical marine sediments, at frequencies below 5000 Hz, sufficient realization of this idealization cannot be achieved. Reported indirect measurements typically have source and receiver in the water column and require hypothesized theories and previously measured parameters. An extended theory‐based discussion is given of various experiments with substantially different designs that have led to widely different results. Relevant comparisons are the attenuation at 1 kHz and the general dependence on frequency over the range 100–1000 Hz. Data from a single experiment are typically amenable via approximate graphing as being proportional to frequency raised to some power n, usually between 1 and 2. Some theoretical notions say that n should be exactly 2, others that it should be exactly 1 or perhaps only slightly greater than 1. It is pointed out that most experimental designs involve questionable assumptions. If the true exponent is 2, then the attenuation at very low frequencies is going to be especially difficult to infer. Other physical mechanisms can enter into an indirect inference of attenuation, and mechanisms that would lead to a perceived linear frequency dependence seem to be ubiquitous.