Water Well Journal

April 2015

Water Well Journal

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I f you read the first articles from the Field Notes series, you'll recall see- ing some of the fundamentals of geology. If you've forgotten, don't worry; there's not a test at the end. As a refresher, the following is an excerpt from the first installment of Field Notes from Water Well Journal, May 2013: Geology, as with most fields of study, is guided by key principles and theo- ries. . . . Several of these basic laws are the Law of Superposition, the Principle of Original Horizontality, and the Principle of Uniformitarianism. The Law of Superposition simply states: In an undeformed sequence of sedimentary rock, each bed is older than the one above and younger than the one below. (Tarbuck and Lutgens 2005) The second and equally important Principle of Original Horizontality says: It means that layers of sediment are generally deposited in a horizontal position. Thus, if we observe rock layers that are flat, it means they have not been disturbed and still have their original horizontality. (Tarbuck and Lutgens 2005) The Principle of Uniformitarianism states: The biological, chemical, and physical laws that shape our world today also operated the same in the geological past. (Tarbuck and Lutgens 2005) These principles help create the basis for a good portion of the fundamentals of geology. Of course as with all laws, there are the exceptions, but we will not digress into them here. For now, we'll stick to the main principles. If a formation is deposited in an orig- inally horizontal position and it remains unaltered, it should still be in a horizon- tal position. But if a formation is no longer in a horizontal position and is altered, how can you tell and what is the significance? Magnetic Compass– Clinometer One of the more common tools for measuring a formation's position in out- crop is the pocket transit (Figure 1). It is a combination magnetic compass, clinometer, and hand level (Compton 1962). The magnetic compass is used prima- rily to measure the orientation of geolog- ical surfaces and features in reference to north. The compass can be obtained in a quadrant or azimuth format. The quadrant format has the compass broken into four 90° quadrants, with north being 0° in the northern hemi- sphere and south being 0° in the south- ern hemisphere of the compass. The northern hemisphere is broken into two quadrants, the NW and NE quadrants. The southern hemisphere is broken into two quadrants as well, the SW and SE quadrants. The azimuth com- pass divides the radial of the compass into 360°, with north referenced as 0°. The azimuth compass is the instrument we will be focusing on in this article. The clinometer portion of the transit is coupled with a liquid level that can be referenced to the horizon from 0° to ±90° and is used to measure the dip angle of geological structures with respect to the horizontal plane (Coe 2010). These combined features become a powerful tool for unraveling the geo- logic puzzle when combined with an accurate topographic map. Magnetic Declination The magnetic declination is the degree of variation between magnetic north, grid north, and true north. Magnetic north is the uncorrected direction a magnetic compass naturally points. (Bates and Jackson 1984) Grid north is the direction of grid lines on a topographic map. True north is consid- ered the earth's rotational pole. Mag- netic north may vary considerably left or right of true north, based on local magnetic variations. (Coe 2010) Magnetic variations are broken into two segments, west and east. When con- verting true north to magnetic north, re- member the mnemonic device "East is least and west is best." This means when converting true to magnetic, the eastern variation is subtracted and the western variation is added to the heading. In the magnetic declination example (Figure 2), magnetic north is 6° 55′ east of true north. So remember when col- A Brunton Pocket Transit in the field in West Texas. FIELD NOTES continues on page 30 WWJ April 2015 29 Twitter @WaterWellJournl Figure 1. Examples of a pocket transit.

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