How to do dip and strike on a brunton compass?

Question

A few years ago i learned how to record dip and strike of rocks for paleomagnetism. I just bought a brunton compass and need a little refresher. What i do remember, the strike is how the rock is oriented from North. And the dip is the degree which the rock bed is tilted. However, on the compass there is the other set of degrees, like when i am on 90 it reads 0. and when i am on 0 it reads 270, ect. I have forgotten what this second set of degrees is. This isnt the stike is it and i just got it confused did I? I remember they are perpendicular to each other but am a little disoriented myself and need a refresher. Any good sites to recommend or any advice or neat techniques or tricks for using this.?

Answer 1

Okay, here's a picture I'll refer to:

http://www.moontrail.com/details/brunton/geotransit/brunton-geotransit-face300-.jpg



The strike is the orientation of the intersection of the rock plane and a horizontal plane. So let's say you have a bed that dips 45 degrees to the west. It has a north-south strike. You will measure the strike with the part of the compass labeled "C".



The numbers are written "backward" so that the needle will point to the correct number. In other words, if you are facing north and turn toward the east, your needle will move counterclockwise relative to the compass, which is why "90" is written to the left rather than the right. (I hope this makes sense- I'm not sure, but I think that's what you're getting at with the number confusion)



When you lay the compass flat against the strike of the bed (using the bubble level "A"), the needle will point to the azimuth of the strike. So in the example from the first paragraph, your needle would point to 0 and 180. You can report either number, because they mean the same thing, unless your professor/boss has a convention for you to follow.



To measure the dip, lay the side of the compass along the dip and move the clinometer ("H") until the bubble ("D") is level. Read the dip from the clinometer. (There should be a lever on the underside of the Brunton that controls the clinometer).



When you report dip, you have to say what direction it's dipping. For example, a bed with a north-south strike can dip either west or east, so you have to either specify which or use a consistent convention (like the right-hand rule).



Before you do this in the field, make sure it is set to the proper declination for your area:

http://www.ngdc.noaa.gov/geomagmodels/Declination.jsp



If none of this makes sense, just go here:

http://www2.gsu.edu/~geohab/pages/geol4013/bruntonCompass.htm



Good Luck

Answer 2

jobinoj has it right. But, when measuring dip be sure to place a clipboard or something similar on the bed, and place your brunton on the clipboard. That will eliminate errors due to irregularities in the bedding.

Answer 3

Certainly. A Craftsmen 10in. digital level with laserTrac® is on sale at Sears for $38.24 while supplies last, and it makes for a fine inclinometer. .