Short Answer: No, this is NOT advised.
Long Answer: Doing surveys in the rain is both a safety hazard for persons operating the equipment and most often leads to survey problems:
1. The risk of shock from wet equipment: Possible shock hazards from batteries (12V), generator (120/220V 50/60Hz 10A) or power cables, resistivity cables (400V/2A), the instrument or anything near these items are more likely in wet weather.
2. Increased risk of lightning strike: The cables attract lighting and channel the strike back to the instrument. (Which is often where operators will be standing!)
3. Collecting noisy data and/or damaging the equipment. Rain percolating through the near surface may cause enough charge imbalance in the ground to create significant SP noise compared to non-rain days. Though most of AGI's hardware is weather-resistant when the connectors are closed, it is not weather-proof.
4. Connectors in Rain. It is nearly impossible to conduct field surveys without opening and closing connectors with wet hands or exposing them to splashes. This allows water into closely spaced conductors that need to carry up to 400V while transmitting near pins that need to receive mV level signals. This can lead to cross talk noise and system stress or damage.