Design Parameters of Differential Probe for Eddy Current

i want to design and manufacture differential probe for eddy current testing, to find cracks in pipes.

if any body can suggest any site how to calculate the dia, spacing, length etc. of the coil (differential), to make differential probe for different pipe sizes.

i know that the this coil is the fourth arm of a bridge and the eddy current due to some crack make changes in this primary coils impedence, and this change in Z can be analysed.

i have data acquisition system to analyse the data.