Water Well Journal
Issue link: http://read.dmtmag.com/i/482464
lecting an azimuth reading with your compass to make sure you have ac- counted for the local magnetic declina- tion in your work area. Magnetic declination information is normally lo- cated at the bottom of the topographic map or it can also be found in published isogonic charts or online at NOAA's National Geophysical Data Center. Object Height One of the more important questions to answer when working on a geologic problem is vertical displacement of a given structure. To aid in this process, it is important to have a topographic map with you. As seen in the example (Figure 3), armed with a topographic map, basic mathematical equations, and combining the slope/angle function of the pocket transit, it is possible to define the height of geologic features with relative accu- racy. The basic equation for finding the height of an object is: Level Ground: Height of object = (Tan A + Tan B) × Distance Example: (Tan 5° + Tan 2°) × 1000 feet = (0.0874 + 0.0349) × 1000 feet = 122.3 feet Sloping Ground: Height of object = (Tan A – Tan B) × Distance Example: (Tan 5° – Tan 2°) × 1000 feet = (0.0874 – 0.0349) × 1000 feet = 52.5 feet (The Brunton Company 2000) Strike and Dip One of the methods for unraveling the geologic mystery is the determina- tion of strike and dip of key geologic structures. The strike line is a horizontal line perpendicular to the dip direction, or expressed by Robert Compton in his Manual of Field Geology: Strike is generally defined as the line of intersection between a horizontal plane and the planar surface being measured. . . . Dip is the slope of the surface at right angle to this line. (Compton 1962) Apparent dip is the angle of dip at less than right angle to the strike line. A simple method of determining the dip direction is simply pouring water onto the feature under investigation. The direction the water flows off the feature is the drip line. The drip line is the max- imum angle of dip of the feature and can be equated to the dip direction. Remem- ber the dip direction is a right angle from the strike. When writing the strike and dip (Fig- ure 4) information in your field book, it is important to use a logical approach so others can make a meaningful interpre- tation of the information. There are many variations in methods of recording this data. One of the more common methods of recording data from an azimuth com- pass is to use a three-digit number to represent the striking direction of a geo- logic feature and a two-digit number to represent the dip angle of the feature. For example, it can be expressed as 050/05. The "American Right Hand Rule" for measuring strike and dip is expressed by stating the dip is located to the right side of the strike line. Therefore, in the ex- ample, placing the dip at a right angle from the strike on the right side of a strike line trending from 050° to 230° on the compass, the strike would be recorded as 050° with the dip on the right side at a dip angle of 05°. To further clarify the method of recording data, the addition of the quad- rant in which the dip occurs can be placed at the end of the string. So for the example, the strike and dip would be FIELD NOTES from page 29 30 April 2015 WWJ waterwelljournal.com Figure 2. Magnetic declination shown on a USGS topographic map. Figure 3. Height measurement techniques.