Most water bores that are drilled by the rotary mud method have a length of slotted PVC or stainless steel screen at the bottom to allow the water in to the borehole.
Much as we want plenty of water to flow in we want to keep fine sands and silt out as much as possible. This is the job of the gravel packing envelope.
Around the screen, the drilled bore hole is a wider diameter than the bore casing itself. This area is called the annulus. The annulus is packed with carefully graded driller’s gravel to act as a primary filter.
A lot of care is taken to make sure this drillers gravel is perfectly suited for the task. Most drillers gravel used in Perth bores comes from near Carnarvon.