Surface exploration only defines two dimensions of a mineral prospect or discovery.  Drilling is the stage of the exploration process where the third dimension (depth) is defined.  This is a very important process, because ultimately the information acquired from drilling will prove if an ore deposit will be mined.  There are essentially two types of drilling:  diamond core and rotary. 

Diamond core drilling uses a diamond bit to carve a solid column of rock.  The core is studied in detail, and then all or part of the rock column is sent to a geochemical lab for analysis.  If the rock is competent (solid) and the drilling goes well, the rock core may be present in its entirety.  Usually the rock is fractured and some is missing, so the recovery percentage must be calculated. 

Rotary drilling methods shatter the rock into bits and then pump the bits to the surface where they are collected for sampling.  The drill uses compressed air to force the fragments to the surface.  Recovery can be estimated by weighing the samples and comparing the weights to the weights for the ideal solid column of rock.  This can be calculated knowing the density of the rock and the dimensions of the cylinder of the rock.

Information gathered from both types of drilling is treated similarly.  The best way to present drilling data is by graphical methods so the information can be visualized more clearly and interpreted easier.  This includes both maps and drill-sections.  The depth dimension is shown in the vertical plane called the drill section, which is essentially a special type of cross-section.  The information plotted can be geochemical values, or geological  information, or both on the same section. 

A drill section shows the surface topography by creating a topographic profile, just as in regular cross-sections. The profile will show the “collar” locations, which are the tops of the drill holes.  The profile will then show various “intercepts” (depth ranges) for each significant geochemical zone or lithological unit that is encountered, as noted in the drill log.  By knowing the locations of the intercepts the geology of the rocks can be interpreted.  The closer the spacing of drill holes the more accurate the profile is likely to be.

Drill Data Collection

The geologist must carefully record the information from the rock core or rock chips obtained from every drill hole.  This is done by completing a “drill log”, which is a form that is filled out during the actual drilling process.  The drill log has basic information at the top, including hole number, inclination, bearing and plunge angle, date, and loggers name.  Then the log will show the depth scale on one side of the form.  The scale is usually in feet since the equipment used by most drilling companies, particularly the drill stems, are in standard lengths of feet.

The drill log typically has a series of columns to the side of the footage scale, each for a different criteria to be observed or logged.  The choices for the column titles are quite variable due to the many different characteristics which can be recorded.  Drill logs are generally tailored to each individual drilling project.  Some of the characteristics which are commonly recorded include:  1) lithology, 2) alteration, 3) mineralization, 4) structural features, and 5) geochemical values.  The angle of a structure is a measure of the acute angle between the structure and the core axis.  Any of these characteristics may be plotted on the drill section. 

Plotting of geochemical values on a drill section is commonly the first step to understanding the geometry of an underground ore body.  The geometry must be understood in as much detail as possible before mining can begin.  The volume of ore bodies which are of a tabular nature, such as veins or dikes, can be estimated by knowing the dimensions, including the true thickness of a mineralized zone, the dip length of the zone, and the strike length of the zone. 

Graphic Representation of Drill Holes

The drill holes are shown on the drill section as lines extending below points representing the collar locations.  The length of the lines are drawn to the scale of the drill section.  A vertical drill hole will plot as a vertical line in the drill section. The length of the line will correspond to the total depth of the drill hole, as stated on the drill log.  Some drill holes are “inclined” at a specified angle below the horizontal, and must be illustrated accordingly.  For the simplest case of plotting an inclined hole which happens to plunge directly down the plane of the drill section, the angle of the hole will be shown as the same.  If the hole inclines or angles away from the drill section plane, then the “apparent” dip angle must be used to illustrate the drill hole.

Apparent Dip:  Some angled drill holes plunge away from the vertical plane of the drill section.  If the drill hole plunges or dips away from the vertical plane, then the dip angle which will be observed in the drill section is called the “apparent” dip.  The apparent dip angle is always smaller than the true dip angle.  The apparent dip angle can be determined by looking up the true dip angle in the “Dwerryhouse” table (Table T4).  This table shows what the apparent dip angle will be as a function of the angle between the section line and the strike direction of the feature.  This requires measuring the angle between the strike and the drill section line in the map view. 

Projecting Drill Holes to the Drill Section Plane:  
It is often necessary or desirable to “project” drill holes into the plane of the drill section (Figure F14).  This is the process of illustrating a feature in a drill section or cross section, even though the feature does not actually occur in the drill section plane.  Drill holes which are not exactly on the section line will be “projected” onto the plane of the drill section.  Drill holes can be projected any distance, but the longer the distance, the less the reliability in the interpretation.  The drill hole is positioned on the drill section at the closest point to the original.  In most grid-based drilling campaigns these distances are usually kept under 200 feet.  

After the drill holes are plotted on the drill section, the geochemistry and/or geological data is plotted alongside the lines representing the drill holes.  It is then the goal of the geologist to interpret the geochemistry and geology details into the ground between drill holes.  This is why a good imagination is important.  The interpretation process may be simple and straightforward, or it may be difficult, depending on the complexity of the local geology.  Typically a color coding system is used to color code the geochemical values.  The geology in between the drill holes is usually illustrated with patterns or simple unit designators.  Faults are shown as special line patterns, as they are on geologic maps. 

Figure F14:   Projection of drill holes to closest point on the drill section line A-B.

Apparent Thickness

If an angled drill hole intersects an inclined, tabular-shaped layer or deposit, then the thickness of the zone as seen in the drill core or noted on the drill log will represent the “apparent thickness”.  The “true thickness” of the tabular body is the thickness measured perpendicular to the contacts of the body.  The reason it is important to determine the true thickness is because the true thickness must be known in order to determine volume (Volume = length x  width x thickness).  The size of a deposit has a direct impact on the commercial value of the deposit.