The Reflection Coefficient R between two velocity layers is expressed as:
R = (ρ2V2 - ρ1V1) / (ρ2V2 + ρ1V1)
Where ρ = density and V = velocity. The quantity ρV is the seismic impedance of the material. The Reflection Coefficient is therefore the difference in seismic impedance over the sum of seismic impedance of two materials. From the above equation, it is apparent that R will be a positive number when V2 > V1, and a negative number when V2 < V1. A positive R means that the polarity of the reflected wave will be the same as that of the incident wave. A negative R means that the polarity of the reflected wave will be the opposite of the incident wave.
It should also be apparent that the larger the contrast in seismic impedance, the larger the amount of incident energy that is reflected (and the smaller the amount that is transmitted).
The above assumes normal incidence. For incident angles other than 90o, the equation is more complex.