The vertical scale which has been found most useful in making graphic logs is 100 ft. to 1 inch. The width of each drawing should for convenience be between 1; to 3% inches.
Graphic logs are usually drawn on two kinds of material de pending upon their purpose, i.e., heavy cardboard or tracing cloth. The advantage of cardboard is that it can be more readily handled, and colored crayons can be quickly used to show various features. The advantage of using tracing cloth is that any num ber of duplicate copies can be quickly made, and furthermore several tracings can be quickly grouped in any desired position to make blue prints of cross sections. The advantages possessed by cardboard can be obtained by making blue prints or blue line prints on heavy paper.
More than ten thousand well logs have been drawn by the California State Mining Bureau on tracing cloth in accordance with the specimen shown in Fig. 19.
At the top of the drawing is shown the well name, its number and location.
The left hand side of the drawing notes all mechanical features, and particularly the amount of casing in the well and its condi tion.
The right hand side of the drawing names or describes the geological formations which a.re symbolized in the log, and notes special features such as oil, gas, tar, water and fossils. The sym bols shown here would, of course, need revision to be applicable to some other localities.
At the bottom of the drawing are brief notes on the production of the well at the time of its completion, and also at other par ticular periods.
The time required to draw such a graphic log was found to average about two hours. As a guide and assistance to the draftsman, a drawing, showing the scale and having vertical lines, was pasted on the drafting board. The drafting board was 5 in. wide and from 30 to 40 in. long. A strip of wood nailed to the left hand side of the board projected far enough above the board to serve as a guide against which the triangle rested. The symbols were designed to follow the conventional system as nearly as possible, and were all made with one drafting pen, which saved time.
Graphic logs are filed most conveniently in speCial drawers, slightly deeper than the width of the drawing, and wide enough to hold logs of average length without folding. Cardboard folders should be provided of proper size to contain a dozen or more drawings, thereby providing for their separation and classi fication according to location of wells or other convenient grouping.
Cross Sections.—A graphic log does not serve its full purpose unless it can be compared with similar logs of neighboring wells. A comparison can be had by merely placing the two drawings side by side with their upper ends even. However, such a comparison is apt to be misleading because it takes no account of distance between wells, differences of elevation and dip of strata.
The best method of comparing and correlating graphic logs is to place them side by side and at such distances apart that their relative positions will be the same as exists at the wells. Such an arrangement gives a cross sectional view of the underground conditions.
The horizontal and vertical scales should, in most cases, be the same, so that the picture presented to the eye is not distorted. The distances between wells can be scaled directly from a properly drawn map. The elevations of the top of wells furnish data for placing drawings in proper vertical position.
Many persons who realize the usefulness of cross sections have not given the subject sufficient study to enable them to arrange graphic logs properly. A frequent error consists in placing graphic logs in a single cross section, although the wells them selves are not located along a straight line on the ground. This error will frequently lead to such distortions that the resulting cross section is worse than useless. Elementary engineering training involves the study of geometry in three dimensions with the aim of solving exactly this sort of problem, so that further discussion of the subject seems superfluous.
Time and expense are saved in making cross sectional drawings by the use of graphic logs on tracing cloth. Such logs can be temporarily fastened in their proper positions upon a large piece of tracing cloth or even directly upon the glass of a blue-print frame. After sufficient blue-prints of a cross section are made the logs may be quickly rearranged for making prints of some other cross section. Such procedure saves making several dupli cate drawings of individual logs, which might not be fully cor rected or posted when changes occur at the well.