Overview: Formation Evaluation (May 2007)
Authors: Kennedy Martin 
Published: February 2015

Realistically, only a limited number of measurements can be made in a borehole. Most logs function by stimulating the formation with some form of energy and recording how it responds. Some of the best analytical techniques use beams of charged particles, but putting a particle accelerator downhole is impractical, and we must make do with electrically neutral forms like electromagnetic radiation, neutrons, and sound waves. Neutrinos are electrically neutral, but because they need a swimming pool full of ozone-destroying chemicals as a detector, prospects for their use are not good.
Of the sensible sources of energy that have been exploited, electromagnetic radiation appears in the greatest range of devices and frequencies (e.g., resistivity, nuclear magnetic resonance, the density log, and some casing-inspection devices). Neutron-based methods can provide a lot of data about rocks and fluids, although somehow they never seem to get the acclaim they deserve.
That leaves sonic logs. At its simplest, this tool was the easiest to understand: create a sonic pulse at one end of the tool and measure the time it takes to travel a known distance along the borehole. At the other extreme, a full-wave field explanation of how the tool works is best left to the specialist. The first tools appeared 50 years ago and used a single monopole transmitter and two receivers to measure formation velocity. Normally, this arrangement provides a reliable measurement of compressional velocity, and it is still used throughout the former Soviet Union.
The business end of these early tools was approximately 5 ft long; the latest models are significantly longer but are superficially similar. Like automobiles, however, the big changes are under the bonnet ("hood" if you prefer). The latest models use an array of approximately 10 receiver stations. Each station consists of 8 individual receivers arranged azimuthally around the tool axis—an increase from 2 to approximately 100 receivers in 50 years. The number of transmitters also has increased, and, more importantly, the simple monopole source has been joined by dipole, quadrapole, and even a miniature vibroseis source. This technology comes at a high price in both operating charges and the volume of data generated (of the order of gigabytes for 1000 m of logged interval).
The incentive for this exponential increase in complexity is that of all the log types, the sonic has proved most versatile. The sonic log started its life as the link from well to seismic section and/or a porosity tool. Now, it is increasingly used as a way to characterize the stress field, quantify anisotropy, and, hence, to help design development wells that are less prone to failure. An excellent example of old technology evolving to answer modern problems.
Formation Evaluation additional reading available at the SPE eLibrary: www.spe.org
SPE 100353 "Multiscale Pore Structure Characterization by Combining Image Analysis and Mercury Porosimetry" by H. Han, University of Waterloo, et al.
SPE 102435 "NMR Petrophysics in Thin Sand-Shale Laminations" by C.C. Minh, Schlumberger, et al.
SPE 102175 "A New Method for Improving LWD Logging Depth" by C.R. Chia, SPE, Schlumberger, et al.
Available at the OTC Library: www.otcnet.org
OTC 18199 "Controlled Source Electromagnetic Imaging in Areas of Complex Geology" by L.M. MacGregor, OHM Ltd., et al.