lex and yacc: Tools Worth Knowing
This article is about how Linux was used to program a Sun machine to manipulate well-log recordings to support finding oil and gas exploration in Western Canada. It will describe the problem, provide enough background to make the problem understandable, and then describe how the two fascinating UNIX utilities lex and yacc were used to let a user describe exactly what he wanted to satisfy his particular need.
In the fall of 1993, I had been recently downsized and was approached by a former colleague for assistance. He, and another former colleague, both of whom were still with my last employer, were what is known in the industry as well-log analysts.
To understand what a log analyst is requires a little knowledge of oil and gas exploration methods. Energy companies, as they like to be known, employ several different categories of professionals to assist them in finding salable quantities of hydrocarbons. Chief among these are the geologists and geophysicists (of which I am one) who study the recordings made in bore holes, or process and examine seismic recordings to identify what are popularly known as “plays” or “anomalies”.
Bore holes are simply the holes left when a drill rig moves off the drilling platform. Generally, these holes are logged by service companies who lower instruments called tools into the hole, and who then record on magnetic tape the readings made by those instruments.
There are many different types of tools, including sonic (which measures the time needed for a pulse of sound energy to travel through the rock wall from one end of the tool to the other), density (a continuous recording of the rock wall density), and gamma ray (a measure of gamma radiation intensity in the rock). These are just a few of the types of measurements that are made, recorded and called logs.
The various logs are examined by geologists to gain an understanding of what was happening when the rocks forming the bore hole were laid down, and what has happened to them subsequently as shallower rocks were created above them.
Geophysicists are more inclined to study seismic recordings which in essence are indirect measurements of the properties of the rocks forming the subsurface. Geophysics and Linux will not be discussed here, but you may find Sid Hellman's “Efficient, User Friendly Seismology”, Linux Journal, August 1995, Issue 16 of interest.
While seismic recordings provide much greater volumes of interpretable data over large areas, well logs are definitive measurements made at single locations, sometimes close together, and sometimes not. Geologists often correlate adjacent well logs to create cross sections of the subsurface, much like seismic recordings would provide. Detailed interpretation of individual logs, however, is often left to the log specialists.
My two acquaintances were log specialists who wanted an additional tool to assist them in the processing and interpretation of individual or combinations of logs. Specifically, they wanted to tell the computer to perform arithmetic operations on individual or some algebraic combination of logs.
For example, they might need to scale a specific log by an arbitrary amount, say 1.73. In another case, they might want to divide one log by another, and then add the result to a third, all before adding a constant and then raising the resulting values to some arbitrary power.
Keeping in mind that logs are composed of individual sample values taken as frequently as every few inches (or centimeters as they are here in Canada and many other places in the world), these example requests would mean a reasonable amount of computation, multiplying every sample of thousands of meters of recorded log values by 1.73, in the first example. The simple scaling problem isn't particularly difficult, but identifying the desired log could be.
The energy company for which my acquaintances worked was using a simple filing method for all the log curves (a curve corresponds to all the recorded samples for one tool in one bore hole) wherein each curve was identified by a name. To this was added some additional information on units and so on, plus all the samples for all the curves for the well. All the data were stored as ASCII. (The file format is known as Log ASCII Standard format, or LAS version 2.0, and although the names for curves were generally the same from well to well, that was not guaranteed.)
As a result, more complicated combinations of curves required a fairly sophisticated and robust mechanism for arbitrary name recognition, while the desired algebraic operation was being described. Given such an interesting challenge, I recognized an opportunity to put some relatively little-used tools to work: lex and yacc.