The catch is that because low-frequency waves can travel great distances, when they do bounce off cavities or objects underground, they often end up far from the detection equipment at the surface.

• 捕捉过程是这样的，因为低频波可以传输到更远的距离，当它们遇到地下的空穴或其他物体反射回来的时候，已经距地面的接收装置很远了。