Sonar to detect holes on the floor

Hey everybody, I’m currently trying to implement a sonar system to be able to move my rover around and avoid obstacles, the first part went well, but I realize that I don’t know how to detect holes on the ground, I’m using HC-SR04 sensors that have an effectual angle of 30 degrees from the center. Someone has an idea of how to detect if there is a hole in any point let’s say 1m in front of the sensor.