Instrumentation amplifier: resistor to ground for input bias current?
R8 and R18 serve an important function. They guarantee a zero volt input when the input coax isn't connected to anything. Without them, the inputs would float to unspecified (and probably unpleasant) levels. They do not try to reference the input measurably above ground. The INA111 has input bias currents of about 20 pA, so there is really no IR drop there.

R18 is not zero, just in case the cable is connected to a system with a ground slightly different than the INA111. Without it, such a connection could conceivably produce nasty ground loop currents.