You should include a common mode choke at your input, preferably at a physical position that prevents RF coupling onto your board. The obvious location on the schematic would be to the left of R11 and R12. Your sensor wire, at 20m long, will be a good antenna for frequencies ranging from 1MHz to 500MHz and beyond, so you have to provide suppression that is effective over this broad frequency range. C7 and C10 are both critical for this. I think that C6 is also useful to extend the bandwidth of bypassing beyond what C7 can provide. C8 seems not that useful unless it is intended to be placed right beside TVS1 and act as its bypass, but with proper placement C7 and C6 should do that job. C9 and C10 are a bit redundant and assuming the component placement is very close, you could change C9 to 100 pF. I might also suggest that a 100 pF cap across D1 is a good idea to prevent rectification of any RF that gets that far. Is there really any point in including D1, by the way?
I wonder if the resistance of R4 is a bit high to be practical. If this is an outdoor application, you should expect some leakage between your sensor terminals due to humidity or other moisture and with such leakage, you will get a false alarm.