1. In rotary-type borehole drilling wherein a bit is attached to the lower end of a drill string that is rotated while the downward force on said bit is controlled, the method of developing a surface drilling log which comprises the steps of measuring the revolutions of said bit, measuring the depth of said bit in the borehole, measuring the weight on said bit, determining the tooth dullness of said bit, and determining a characteristic of the formation being drilled by combining said measurements and said tooth-dullness determination.

9. In rotary-type borehole drilling, wherein a bit is attached to the lower end of a drill string that is rotated while the downward force on said bit is controlled, a method of developing a surface drilling log which comprises the steps of measuring the revolutions of said bit by increments of a revolution, measuring the depth of said bit in the borehole by cumulating incremental displacement in a downward direction only and calibrating said cumulative measurement to correct the total periodically, measuring the weight on said bit and converting said weight to pulses, the frequency of which is proportional to said weight, determining the tooth dullness of said bit by taking the ratio of the number of revolutions of said bit to a predetermined number, and determining a characteristic of the formation being drilled by correlating said depth measurement with a count of said weight pulses during said revolution increments over a predetermined interval of said depth and with said revolutions as modified by said dullness determination.

10. In rotary-type borehole drilling, wherein a bit is attached to the lower end of a drill string that is rotated while the downward force on said bit is controlled, the method of developing a surface drilling log to indicate at least one parameter of the formation being drilled, comprising the steps of measuring the revolutions of said bit by increments of a revolution, measuring the depth of said bit in the borehole by cumulating incremental displacement of said bit in downward direction only and calibrating said cumulative measurements by algebraically adding one of said increments after a predetermined number thereof, measuring the weight on said bit during said increments by converting said weight to pulses, the frequency of which is proportional to said weight, determining said parameter by correlating said depth measurement with a count of said weight pulses during revolution increments over a predetermined interval of said depth, and recording said parameter on a record medium while translating said medium in accordance with said depth of the bit while drilling.

11. A surface drilling log system for use with rotary-type well-drilling equipment having a bit attached at the lower end of a drill string and including means for rotating said string and bit while controlling the downward force on said bit, comprising in combination means for measuring the depth of said bit in the well, means for measuring the revolutions of said bit in drilling the well, means for measuring the weight on said bit as the well is drilled, means for determining the dullness of said bit as the well is drilled, and means for correlating said measurements and determination to provide a log of characteristics of the formation being drilled.

16. A surface drilling log system for use with rotary-type well-drilling equipment having a bit attached at the lower end of a drill string and including means for rotating said string and bit while controlling the downward force on said bit, comprising in combination means for measuring the depth of said bit in the well, means for measuring the revolutions of said bit comprising means for generating gate signals at predetermined increments of said bit revolutions, means for measuring the weight on said bit during said increments of the revolutions as the well is drilled comprising means for generating variable-rate pulses at a rate dependent upon said weight, means for correlating said measurements to represent a parameter of the formations being drilled comprising means for counting said variable-rate pulses during said increments to produce a product of weightimmes-revolutions, means for recording said correlation of said measurements on a record medium as it is advanced, and means for advancing said record medium in accordance with the depth of said bit.