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THE BAROMETRIC METHOD OF GEOLOGIC
SURVEYING FOR PETROLEUM MAPPING.

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The following paper outlines the barometric method1 as it has been applied by the writer in geologic mapping for petroleum. The method is used to greatest advantage in regions where stratified rocks have low dips and where the structure is best represented by contours, or isobaths. Where beds are more steeply inclined, strikes and dips are commonly obtained by compass and

1 The barometric method, as originally devised, was first described by M. R. Campbell, who applied it in investigations for coal. See Trans. Am. Inst. Mg. Engrs., Vol. 26, pp. 298–315 (1896).

clinometer and are plotted, together with the distribution of the formations, to make the ordinary geologic map.

In all geologic mapping, as is well known, rock formations or rock structures are plotted in reference to selected points, or “stations," which must be located in their proper relations to one another as regards direction, horizontal distance, and vertical distance, or difference of elevation. In the term, "barometric method," stress is laid on the fact that relative elevations are determined by means of an aneroid barometer instead of by vertical angles or levelling. The other two quantities of direction and horizontal distance, which together may conveniently be called the horizontal control, may be obtained by pacing, odometer when an automobile is employed, or any good method that is sufficiently accurate for the particular work in hand. On the assumption that the reader understands these methods of preparing the horizontal control, we shall not attempt to explain them in this paper.

THE ANEROID BAROMETER.

There is no need here to describe the mechanism of the aneroid barometer. Suffice it to say that the instrument is delicate and that it should always be handled with care. It should be carried where it is not severely jolted. It should be read while it is resting horizontally on a support or in the palm of the hand, but it should not be grasped. It should be set so that "31 inches" on one scale is directly opposite "o feet" on the other scale, and should not be turned from this position for different localities.

Aneroids are made with various ranges of altitude. Those registering to 5,000 or 6,000 feet are most serviceable in regions where elevations above sea level are not over 4,000 or 4,500 feet. For greater altitudes an instrument made with proportionally higher registry should be employed. Under no circumstances should a barometer be used near the limits of its range.

For satisfactory results the barometer must be reliable. Few aneroids are good enough for geologic surveying. Consequently, before an instrument is selected to keep, it should be carefully tested. This may be done in several ways, as follows:

1. Gently tap the glass face with the fingers. The needle should jar slightly, not too freely, and should return to the same point at which it stood before being disturbed. If it stops now at one place and now at another, after successive tappings, while it is held always in the same position, it is unreliable.

2. Read the instrument, carry it to an upper floor in a building, or thirty or forty feet up a hill; read it; return to the starting point and read again. Two or three minutes after arriving here the needle should record the same elevation as at first, provided the ascent and descent were made within 5 or 6 minutes. Go up to the upper station once more, wait a minute or two, and read. Any considerable differences in the elevations recorded at the same station, especially if these differences are unrelated, usually indicate a poor barometer. The pause after reaching the station is to allow for a possible lag.

3. Take out the new barometer on a day's traverse and compare its readings with those of another barometer known to be reliable. In doing this observe that both barometers may not show the same elevations, estimated from zero, nor equal amounts of rise and fall, but both should rise together and fall together, and after the readings have been corrected (see pp. 155 to 166), the elevations obtained by both instruments should be approximately the same for the same station.

ADDITIONAL EQUIPMENT.

In addition to the barometer, the geologist will need a compass, a watch, a coördinate-ruled notebook, and a base map upon which to mark the positions and elevations of stations along his traverse. The compass should be corrected for the local declination. When two men work together in the same area, their watches should be set together. The notebook is most serviceable when ruled to fifths or tenths of inches. Any fairly accurate county, state, or property plat will generally serve for a base map. There is no need to have one that shows topographic features, although it is well to have streams indicated. If the geologist can not procure a satisfactory map of some sort, he should make or have made a

simple outline map showing a few prominent landmarks in their correct positions, so that he can "tie" his traverses on to these. Where the land is sectionized he can often sketch his own base map along with his geological work.

SYSTEM OF RECORDING STATIONS.

The writer has found it most convenient, for note-taking, to designate days by letters and stations by numbers. Thus, in a given area, the first day will be "A," the second "B," and so on until the investigation is finished. The next area will be commenced "A" again. If more than 26 days are spent in one district, the 27th day will be "AA" or "2A," the 28th "BB" or "2B," and so on. The stations on the first day will be "A1," "A2," A3," etc.; on the second day, "BI," "B2," etc. These symbols are plotted on the base map and are recorded in the notebook, as will be explained presently.

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GRAPHIC METHOD OF NOTE-TAKING.

According to the method devised by Campbell, the notes may be recorded in the form of a profile section. Fig. 20 illustrates To explain this, let us assume that the geologist is at Station AI. He has a sufficiently clear idea of the area to know that Sta. AI, his starting point, is rather low and that most of the stations he will visit will be above the level of Sta. AI. The barometric elevation at this station he finds is 2.460 ft. Consequently, holding the notebook with its length right and left, he chooses a horizontal line rather low on the page, in order to leave room above for higher points, and he labels its left end "2,400." Letting I inch equal 100 feet, the other horizontal lines are labelled accordingly, upward and downward from the first line. (See Fig. 20.)

Suppose that he arrives at Sta. Ar at 8 A.M. On the intersection of the 2,460 line and a vertical line at the left of the page, he marks a small cross or a dot to represent AI. Let the direction from AI to the next station be due north. Along this same vertical line he writes, “A1, 8.00 A.M., No.," and any other nec

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essary information. If, as we may suppose, AI is on the top of a bed of limestone, this is plotted by the usual symbol. (See Fig. 20.)

He now proceeds 110 double paces to Sta. A2, a point here assumed to be on a ten-foot bed of sandstone, at an elevation which he reads as 2,500 ft. He arrives at A2 at 8.10 A.M. Letting the horizontal scale of the section be 1 inch equals 100 double paces (equals approximately 10 mile), he plots A2 according to the adopted scales. As before, he writes on the vertical line through

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FIG. 20. Geologic profile section of a portion of a traverse. The diagram rep

resents a page in the notebook.

the point for A2, “A2, 8.10 A.M., N10E.," the direction being from A2 to A3. To save time that might be lost later in measuring distances in the notebook, he records the number of paces from A1 to A2. He represents the sandstone bed by the convention for that rock.

In this manner he continues from station to station, recording whatever information seems to be of importance and sketching to topographic profile along his traverse. (See Fig. 20.)

Every effort should be made to settle the correlation of the

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