if master and slave image have higher difference in back scattering than coherence will be low. because Coherence refers to the amplitude of the complex correlation coefficient between two SAR images and images have high difference than correlation will be less.

There is no direct relation between difference in backscatter of two SAR scenes that are used for SAR Interferometry (InSAR), and the interferometric coherence.

In fact back-scatter is governed by the physical, geometrical and dielectrical properties of the target whereas interferometric coherence solely depends upon the random dislolation of scatterers between the two SAR passes.

I will like to explain with few simple examples.

SAR backscatter is very high from human settlements due to its geometry that produces corner reflector effect. At the same time the interferometric coherence from human settlement is also very high (close to 1) because scatterers of human settlements are stationary and don't move between the two SAR passes. It has resulted in almost no random dislocation of the scatterers between 2 SAR passes.

Now consider the case of dry, bare and smooth agricultural field. Due to low dielectric constant of bare dry soil alongwith specular reflection due to smooth surface, this type of target gives very low backscatter. But again due to low or almost no random dislocation of the scatterers between 2 SAR passes, bare, dry and smooth agricultural soil will produce very high coherence.

Now consider a forest, it gives moderate (more than crop) backscatter in both the passes but due to random dislocation of leaves and branches between the two passes, produces low coherence.

Relatively calm waterbody gives low backscatter in both the SAR images, but due to continuous movement of water surface produces low interferometric coherence.

above 4 examples clearly indicat that

Interferometric coherence can be high for high SAR backscatter (e.g. human settlement) as well as for low SAR backscatter (bare, dry smooth agricultural fields).

similarly interferometric coherence can be low for low backscatter (calm waterbody) as well as moderate to high backscatter (forest).

Dear Hari Shanker Srivastava, thanks for the detailed answer.