Large field parties note the change in atmospheric pressure on a barograph or a recording barometer which is left in the office, the changes as noted by it being used in figuring the errors due to such changes, or as a check on recordings as figured in the field.
The rest the surveying methods at our disposal are mainly the well-known surveying methods, which require a knowledge of surveying. The .following paragraphs will not take up these methods in detail, but are intended merely as a grouping and a short review of the various systems in use by the oil and gas geologist.
Hand Level. The hand level, also called the Locke Level, is shown in Fig. 21. The bubble of the small level tube C can be seen through the opening D, by means of a reflecting prism. A cross hair placed in the main tube AB serves to fix the object observed, and when this hair bisects the reflection of the bubble, the line of sight is horizontal.
The height of the observer's eye, when he is standing erect, must be known. Thus if the height of the eye above the ground is five feet, the object observed through the hand level when it is held horizontally, will be five feet higher than the point on which the observer is standing.
Clinometer. If a hand level is equipped with a movable spirit level and an attached vertical circle graduated to degrees, it is known as a clinometer. When the line of sight is given any inclination and the level turned to a horizontal position the angle of inclination is determined. A clinometer may also be used as a hand level if the bubble is set parallel (at zero degrees) to the main tube. (Fig. 22.) Hand levels and clinometers are useful instruments in follow ing outcrops from place to place in a country of low dips and few exposures. By standing at an outcrop on a hillside, the cor responding level on an adjoining hillside may be determined and another outcrop of the same formation looked for. Measuring vertical distances between strata may be done with a hand level rapidly. The angle of inclination or dip of a stratum may be found by the use of the clinometer.
Engineer's Spirit Level. Levels most commonly used in work consist of a line of sight, attached to a bubble vial on a vertical axis which may be firmly attached to a standard or tripod. By means of leveling screws the bubble is brought to the center of the vial, making the line of sight level. The instru ment is used to determine the difference in elevation between two or more points accurately. A rod, graduated to feet and tenths are used in conjunction with the level. The rod is held vertically on a point whose elevation is known (generally a B.M.) or is assumed, the level set up and the intersection of the line of sight on the rod is noted; this is known as the back-sight (B.S). Adding the back-sight to the elevation, the height of instrument (11.I.) is determined. The rod is then placed on a point whose elevation is desired and the intersection of the line of sight on the rod is again noted. This operation is the fore sight (F.S.). The elevation of this unknown point is calculated by subtracting the fore-sight from the height of- instrument. Thus we have for determining elevations the following formula;: If the distance between two points is too great either vertically or horizontally or both, to admit of this simple process, two or more settings of the level arc required so as to secure a connected series of rod readings, the algebraic sum of which gives the desired difference of elevation between the two points. (Fig. 23.)
Only one back-sight is taken from one set up of the instru ment, but any number of fore-sights may be made; care must be taken in using any of these fore-sights for a turning point that the proper one be so recorded and the rest considered as extra or internal sights (I.S.).
Adjustment of Wye Level. Line of collimation adjustment is as follows: By means of tangent screw and leveling screw bring the intersection of the cross hairs upon a well defined and distant point. Revolve the telescope in the wyes one-half a revolution. If the intersection of the cross hair is still on the point sighted at, the line of sight coincides with the axis of collimation; if not, the adjustment is for one-half the apparent error which is done by moving the capstan-headed adjusting screws, being careful to move them in the opposite direction to which it would appear they should be moved. The apparent error shown by reversing the telescope is double the real error.
The second adjustment is to make the axis of the level tube bubble parallel to the axis of the collars, and, consequently, parallel to the line of collimation. • Level up the instrument over a set of leveling screws, open up the wye collars and revolve the telescope, first in one direction then in the other. If the instrument is in adjustment the bubble should stay in the center of the vial; if moving first towards one end and then to the other, the error is to be corrected by bringing the bubble nearly to the center by means of the capstan-headed adjusting screws at the end of the level tube, which regulate its lateral movement, and repeat the operation until the bubble will remain centered during the partial revolution of the telescope.
This will take care of the lateral adjustment; for the vertical adjustment proceed as follows: level up the instrument, and take the- telescope out of the wyes and turn it end for end, if bubble remains in center, adjustment is correct, if not, bring bubble half way back by the adjusting nuts at one end of the tube and the remaining half by leveling screws.
The third adjustment, or wye adjustment, consists in leveling up the instrument, removing the telescope and turning it end for end in the wyes; if bubble is out, one-half the error is to be corrected for-by- the adjusting nuts on one end of the wyes, the other half by leveling screws. Repeat the operation over the other set of leveling screws.
Sometimes an adjustment is made to make the cross hairs horizontal. It is horizontal if both ends as well as the center of the horizontal hair will cut any object sighted at, when the telescope is revolved on the vertical axis; if not, the reticule adjusting screws are to be loosened and the reticule to be rotated until the hair is horizontal.