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When well logs were first recorded digitally, each well logging contractor designed its own format (in the 1970s). These formats, like the seismic formats for digital data, reflected the technology of the times: they were built around 0.5" tape recorded at 800bpi. Today, of course, this is totally obsolete: but the formats endure.
The best known of these early digital formats is Schlumberger's LIS (Log Information Standard). I have a copy of their 1981 version of this format. It consists of a 42 page description of the format, written in impenetrable technical jargon, followed by 224 pages of appendices, mainly tabulating mnemnonics used in the standard.
LIS is a binary standard, like the SEG seismic data standards. Its details are closely dependent on the physical medium used for the recording, and it must be programmed at the lowest level ("bit twiddling") on any computer that uses the standard. But seismic data is fundamentally different from log data. Seismic data records very large numbers of channels of data representing measurements which only differ in their location and time of recording, while log data records a relatively small set of data channels simultaneously, but with each channel representing a different type of measurement. Log data formats must be able to handle many different kinds of measurement, but do not have the same data rate pressures which constrain seismic formats.
When log analysts started to use personal computers rather than just working with paper logs or on company mainframe computers, they did it by digitizing paper well logs. This seemed a waste of time, because the well logs were recorded digitally, but personal computers were not equipped with 0.5" tape drives, and even if they were, no-one had written a program for reading LIS, or for writing LIS on a PC.
A solution came from Canada. The Canadian Well Logging Society set up a Floppy Disk Committee in the late 1980s to define a format "to supply basic digital log data to users of personal computers in a format that is quick and easy to use".
The LAS format was released in 1989. It first came into widespread use as version 1.2 in about 1991. LAS is designed to be written as a file on an MS-DOS diskette, but because it is entirely in ASCII format, it can be handled by any computer software which uses ASCII coding for text. The latest version is 2.0, released in 1992. A complete package defining the format, together with three test files and two MS-DOS programs (REFORMAT, for converting back and forth between versions 1.2 and 2.0 and doing some simple processing of LAS files; and CERTIFY, which verifies that a file meets the LAS standard) is freely available for downloading from CWLS. The format is easy to understand, and flexible enough for almost any purpose involving well logs. ASCII files are generally regarded as being inefficient, but well logs usually are such small data sets that file size is not a problem. For example, an LAS file with nineteen curves over 3000 ft of well bore, with a sample interval of 0.5 ft, is only 0.8 MB in size, less than 80% of the capacity of a standard 3.5" diskette. In fact, the project involving this file had 141 wells, with all of the available logs in a single file for each well, and this was the largest file.
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