Sonic Logging for Dummies

What a sonic log looks like...

Although our staffers had seen the site in passing a few times, they'd not paid it a great deal of attention. After all, someone who claims, "I'm a wise geek!" is right up our alley. That was before yesterday, when we noticed that one of our repeat DotDs had almost 200 bylines at the site; and we know from experience that she fakes her freelancing. That's why the team added WiseGeek.com to their scan list. Here's our first – but probably not our last – nominee from that site: Paul Reed, exposed by our staff geologist as he was trying to explain "What Is Sonic Logging?" (and failing miserably). The post, unedited, is now ar WiseGEEK niche site AboutMechanics.com, for some unknown reason.

We'll start this out by explaining what sonic logging isn't: sonic logging has nothing to do with seismic waves, nor does it have anything to do with two- or three-dimensional areas. Sonic logs are used by the petroleum industry to estimate the porosity and density of the rocks penetrated by a borehole (what laymen call a "well"). Reed got some of that right, but got more of it wrong. Here's what he says:

"The transmitter sends sound pulses in short bursts, which enter the rock surrounding the borehole; some of the sound reflects quickly back to the receiver, and some enters the surrounding rock and is diffracted, which means it changes direction from the outgoing sound. As the diffracted sound returns to the receiver, the time difference between the transmitted and returning sound is recorded."

That whole "diffraction" bit? it has nothing to do with sonic logs; Paul's conflating sonic logging with seismic exploration. Reed's next tangent?

"The second type of sound waves to return to the receiver is S-waves, or shear."

Here, Paul conflates sonic logging with seismology. Yes, there are shear-wave tools, but they are not sonic logs! Finally, Reed wanders off in the direction of, "Why use sonic logs in the first place?"

"Sonic logging is effective for determining the characteristics of a borehole because sound travels differently depending on the rock or soil surrounding the transmitter. The first sounds to return to the receiver are p-waves, or pressure waves, because they typically have the highest velocity, or speed. P-waves will travel faster in higher density rock, and slower in less dense sand or soil, which is called more porous." His bushwa about "less dense sand or soil" notwithstanding, this is the closest Reed gets to what sonic logging is. What he completely missed while performing a copy-reword-paste job of some other, far more authoritative, source (WiseGeek doesn't publish references) is a couple of things:

1. Sonic logging simply measures transit time (ΔT) over a known distance, from source to receiver
2. The more dense the rock in the interval, the faster that transit time
3. "Density" combines the bulk density of the rock with the density of the fluids in the pore space. If we know what fluids are filling the pores and the bulk density of the rock, it is mathematically simple to derive an estimate of porosity.

Reed hid these simple facts under a flood of bull because of his complete failure to understand whatever he read and reworded. He was assisted in his dumbassery by an "editor" (clearly not a fact-checker!) who also owns a journalism degree. For pretending to have researched and understood the concept of a sonic log – and doing a lousy job – Paul is our first ever WiseGeek Dumbass of the Day! Mazel tov!

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