# Impedance/admittance (acoustic) models for outdoor ground and seafloor

Summary:
Can seafloor impedance get a complex value such as ground (porous materials)?
There are some mathematical models (eg Delany-Bazley) to calculate impedance of porous materials and these formulas give complex impedance values as a result. But I could not find any model to be used for seafloor (underwater ground modeling) or I don't know if existing models may be used. I usually got real values for impedances of seafloor.
Are there any physical explanation for seafloor impedance being a real value (not having an imaginary part) or can seafloor impedance also be modeled as a complex number?

anorlunda
Staff Emeritus
I assume you mean electrical impedance.

Yes of course. Line-to-ground capacitance is highly significant for overhead lines and is it much higher for underground or underwater cables.

For large scale power transmission, HVDC has big advantages over AC.

There was a lot of research on sea bottom impedance when trans-Atlantic single-wire telegraphs were first proposed in the 1800s. Would that be helpful to you?

bustun
Thanks for your explanation but I mean acoustic impedance.

I apologize for not making it clear.

berkeman
Mentor
Oops, I also thought you meant electrical transmission line impedance. What does imaginary acoustical impedance mean? Does it mean some transverse component to the normal longitudinal sound transmission?

No, as far as I know, it is explained as the phase between pressure and velocity at the ground (or the impedance plane).

berkeman, thank you very much for your help.

I have read the second reference already but the first and third ones are new for me.
These papers are dealing with the subject in detail but as I explained they are gettin real values for seafloor impedance and I am interested in to understand if impedance can be a complex value for seafloor.

berkeman
Mentor
I am interested in to understand if impedance can be a complex value for seafloor.
Yeah, I'm only seeing real numbers for the seafloor references like this one:

https://www.sciencedirect.com/topics/engineering/acoustic-impedance

Do you have an example of a reference for regular soil that shows complex values? It may take a non-isotropic medium to generate any phase shift to make the impedance complex...

Baluncore
Acoustic impedance can be complex if the surface layer is springy and has mass. The storage of energy in the surface as compression of a gas, and the momentum of mass, produce a complex impedance. The self resonant frequency of the layer is important. The complex impedance of the layer is highly frequency dependent near the resonant frequency.

bustun and berkeman
Thanks Baluncore for your neat explanation.

Do you have any comment on ground in contact with air having a complex impedance whereas seafloor has real impedance value? Two references are in the following.

https://asa.scitation.org/doi/10.1121/1.2338288
https://www.researchgate.net/public...ediments_off_the_coast_of_Southeastern_Hainan

Does this have relation with water being denser than air and canceling springy nature of surface layer? (This is just my simple interpretation, I would be pleased to hear yours.)

Baluncore
Does this have relation with water being denser than air and canceling springy nature of surface layer? (This is just my simple interpretation, I would be pleased to hear yours.)
Fundamentally, it comes down to liquid water being incompressible, while air is compressible.

A terrestrial soil has particles with mass, and unless it is saturated with water, usually contains compressible air between the soil particles.

Sediment on the sea floor is saturated with water, any gas is dissolved or rises to the surface, so there is no energy storage mechanism available to phase shift the acoustic signal energy.

bustun and berkeman
As I understand from your explanation, it is normal to expect a real impedance for seafloor layers while terrestrial soil can have a complex impedance.