An underwater micriphone is used to record sounds emitted by porpoises. The minimum intensity level the instru?

Firstly a couple of points about the way this question is worded. Note that J/s is an unconventional way of expressing power in units of Watts (W). Secondly, by describing the source of sound as a point source, your teacher is saying something about its shape or geometric form, but a more important point is whether the source emits sound equally in all directions i.e. whether it is isotropic. Your teacher probably wants to say that the source is isotropic, but has not thought clearly about what he or she is trying to communicate, and has consequently said the wrong thing; a point source is not necessarily also an isotropic one.

assuming that the source is a point isotropic source we can answer the question as follows:

The reference intensity level for sound is 10^-12W/m^2. 40.2 dB corresponds to an intensity ratio of 10^4.02, so the minimum detectable intensity is 10^(4.02 - 12) = 10^-7.98 W/m^2

Assuming that there are no reflections from nearby objects or from the surface or bottom of the water, the power (P) from the source will be equally distributed over the surface of a sphere of radius r where r is the distance from the source to the mic . The intensity at radius r will be P/(4*pi*r^2)

we can write 10^-7.98 = 0.0793/(4*pi*r^2), from which r can be determined. I leave the arithmetic to you.

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The asker has some problems in understanding to measure the noise of porpoises by an underwater microphone, because there are some differences to the measuring of sounds in air.
Great help in understanding: "Conversion of decibels (dB) between air to water":
http://www.pmel.noaa.gov/vents/acoustics/tutorial/8-conversion.html

In underwater acoustics, sound is usually expressed as a Sound Pressure Level SPL.
Sound Pressure Level SPL = 20 × log (p/po).
Reference sound pressure under water is po = 1 µPa = 0.000001 Pa.
Reference sound pressure level SPL under water is Lpo = 0 dB re 1 µPa = 1µN/m².
In 1970, the pressure reference level of 0 dB ≡ 1 µPa was chosen by the US Navy for their underwater work for sound in water.
Reference sound pressure in air is po = 20 µPa = 2×10^−5 Pa (Threshold of hearing)
Reference sound pressure level SPL in air is Lpo = 0 dB re 20 µPa = 20 µN/m².

On this scales, a doubling of the sound pressure means an increase of 6 dB. In order to interpret quoted sound pressure levels one must also have some indication of where the measurement applies. SPLs are usually expressed either as a received sound level at the receiver location or the sound level "at the source."
Comparing sound levels in air against sound levels in water must be done very carefully. First, by accepted convention, the reference pressure values are different by 26 dB. Second, due to the difference in impedance (the stiffness or density of the medium) between air and water, roughly a power level of 36 dB is required in air to produce an equivalent pressure level in water. Combining these two values, a 62 dB difference, or correction factor, between the two scales is required. Therefore, 62 dB must be subtracted from a sound level in water to produce an equivalent acoustic intensity in air.
It is misleading to compare the underwater sound with sounds heard in the air, like they do it here the wrong way: "Blue Whale - The Loudest Animal on Earth": http://www.enchantedlearning.com/subjects/whales/species/bluewhale/Loudest.shtml

Cheers ebs

PS: A guess - 160 dB sound level under water may be something like 98 dB re 20 µPa in air.
130 dB in air as the threshold of pain may be something like 192 dB re 1 µPa under water.
Help:
Specific acoustic impedance in air is Zo = 413 N∙s/m³.
Specific acoustic impedance in water is Zo = 1.5×10^6 N∙s/m³.
The exact value depends on temperature and pressure.
Zo = ρ × c = p / v