A bubble leak test for a bottom tank plate involves introducing air or gas into the tank and observing the surface of the plate for the presence of bubbles. The process can be performed using the following steps:

1. Clean the bottom tank plate surface thoroughly and ensure that is dry.
2. Seal all openings in the tank except the one being tested.
3. Introduce air or gas into the tank through the opening.
4. Observe the surface of the bottom tank plate for the presence of bubbles.
5. If bubbles are observed, mark the location of the leak on the plate and confirm the location by other method.
6. Repair the leak by welding, sealing or other appropriate method.
7. Repeat the test until no leaks are detected.

This test commonly used in the oil and gas industry to ensure the integrity of tanks and prevent environmental contamination.