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Only two holes are required to determine strike and dip of planar structure observed in unoriented cores; strike and dip can be determined by using graphs. Data required for determination of strike and dip are the location, bearing, inclination, and depth (elevation) of holes at the point of penetration of correlative features and the acute angle of intersection of the planar structure with the plane normal to hole axis. Graphs necessary for determination of strike and dip are represented by the formula  
cos i = cos h sin a h sin d ± sin h cos d ,
where a h is azimuth of structure relative to bearing of hole, d is dip of structure, h is hole inclination, and i is intersection (acute angle) of structure with the plane normal to hole axis.

Two graphs, representing basic data from two core holes, are utilized to determine alternative combinations of strike and dip; a graphic solution or inspection involving the spatial relationship of the two points penetrated is required to determine the unique combination of strike and dip. Graphs may also be utilized to determine all possible combinations of strike and dip of a structural feature recorded in an individual hole.

A simple calculation may be made to determine the thickness of the structural feature penetrated or the spacing between parallel features. The formula for this calculation is  
T = L cos i ,
where T is thickness or spacing, L is length in core hole through or between structural features, and i is defined as above.

Caution is necessary in making any structural interpretation from the diminutive area prospected by a drill hole. However, if maintained in proper perspective, graphs and formulas can be of value for interpretation of geologic structure.

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