Depth Conversion, Time & Velocity

Depth Conversion involves the seismic reflection method of measuring an acoustic sound wave and the time it takes to it to travel from one source to another to measure the actual depth whilst also determining the surface’s velocity.

How is depth determined?

Depth needs to be determined when looking for oil and gas reserves in the earth’s surface. So why don’t we just drill as soon as we know its there? Simple. Not only can it be impossible to do some in some cases, but it can be extremely time-consuming, costly and even dangerous.

Depth is determined by Geoscientists placing several receivers across the earth’s surface in the area where they want to measure depth. This is done scientifically at even intervals, north, south, east and west.  They will place a source at one end and a reflector at the other and measure the time it takes from when the sound wave is produced, goes through this net-like field of receivers, is reflected back through the receivers and is back at the source. Much like an echo is to our ears in a cavern.

However, depth is not only determined by measuring time but also by a rock’s velocity.

What is Velocity?

The earth is made up of several layers of rock and unlike a cross-section diagram that you may have seen of the earth in a textbook, the earth’s layers vary and are made up of several types of different rocks.

You may have even noticed whilst walking along the beach that some rocks are harder and others softer and chalky to touch. Those softer rocks are more porous than harder ones and when thrown can easily break. This is because they are two different types of rock and vary in rock velocity.

The speed of each rock can be determined by using a geological hammer and hitting each rock. Fast rocks go “ding” whilst slower rockers “thud” or “squelch”. The slower the rock the more porous it is and therefore easier to drill into.

It is important that depth conversion is done before any drilling is carried out. Like a lot of things, there is not just one single methodology when it comes to determining depth conversion. When using depth conversion, you may want to consider the area’s history as well as other poorly positioned wells too.