What is one of the main advantages of gas-drive sampling devices?

One of the main advantages of gas-drive sampling devices is that they can be installed permanently without casing. This characteristic allows for a more flexible sampling approach in various geological settings. By eliminating the need for permanent casing, gas-drive systems can be more efficiently utilized in situations where traditional methods might be cumbersome or not feasible. This capability also enables easier access to groundwater or soil gases, facilitating environmental monitoring and data collection over an extended period. The absence of casing means that these devices can be set up in a way that minimizes disturbance to the surrounding environment, leading to more accurate sampling and data integrity.

The other options do not align with the primary advantages of gas-drive sampling devices. For example, stating that they can only sample very shallow wells does not reflect their versatility in sampling different depths based on the specific design and application. Saying they require a primary power source overlooks that many gas-drive systems can function independently of electrical power, which is a major benefit. Lastly, the notion that they are only suitable for small volumes is misleading, as these systems are designed to be scalable and can be adapted for varying volume requirements based on the purpose of the sampling.