Some of the features of StereoCore™ PhotoLog of which we are very proud are the features related to borehole path surveys. It’s pretty well accepted nowadays that boreholes tend to deviate from straight line paths for all sorts of reasons. The torque supplied by the drill rig, the pressure applied to the drill bit and the type of rock being drilled through are major contributing factors to boreholes commonly not being straight (depending on their final depth – if it’s just a short borehole then a straight line approximation would be fine).
As such, many mines employ companies specifically for the purpose of surveying their borehole paths, so that any logged features can hopefully be accurately placed in their actual in-situ context. A path survey instrument typically takes plunge and trend measurements at various stations as it descends the borehole, so generally a complete path survey is supplied as a series of (depth, plunge, trend) measurements and one then has to reconstruct the path in 3D space from that by interpolation. If one succeeds in so doing then for any depth one can say what the (x, y, z) co-ordinates of the borehole are at that depth and also what the direction of the borehole is at that depth. The direction of the borehole is essential in order to take a structure (alpha, beta) reading and convert it into a (dip, azimuth) reading.
Importing path surveys
To import a borehole path survey, go to the Borehole Path tab after opening a project.
Click on Import Survey and select an Excel file containing the path survey data.
Fill in the form with details of the survey you are importing and click on import.
For the plunge convention you have the options of Positive Up, Positive Down or Tilt. Some instruments supply a path survey where for example a plunge of 50 degrees means that the borehole is sloping downwards at 50 degrees – this is Positive Down, whereas others have the convention that a borehole sloping downwards at 50 degrees gets a plunge reading of -50 degrees – this is Positive Up. Lastly, some instruments supply a “Tilt” reading, where instead of the slope being measured from horizontal it is measured from the vertical. In this convention a tilt of 0 degrees corresponds to a plunge of 90 degrees (in the Positive Down convention).
Some instruments read Trend relative to Magnetic North. If you specify (via the checkbox) that the path survey is magnetic, then the project’s magnetic declination (which can be set via Project->Settings) will be applied to the path survey, meaning that before use in calculations the borehole path will be rotated to True North.
If the path survey imports correctly you will see a message as below.
Some instruments will insert an error code into the relevant path survey row when they fail to get a reading at that depth, and if that is the case then that row will fail to be imported by StereoCore™ PhotoLog, so failure to import a row is not necessarily a bad thing. The program will report which rows it failed to import though, so you can go back to the Excel file and check that no mistake has been made.
The Import Path Surveys form does not close once you have imported a survey, because sometimes people do multiple surveys as the drilling progresses. To import multiple surveys from the same Excel file, click in the box at the top left of the form to change the sheet you are importing from.
Imported path surveys will show up in the Borehole Path tab.
Creating and editing path surveys
Sometimes you might want to enter path survey data by hand – for example, if you don’t actually have a path survey available for the borehole you might want to use the direction at the collar as a rough approximation for the path survey, just to see how it affects the dip and azimuth of your structure readings (getting dip and azimuth readings for structures is described later in this article). In order to do this click on Create Survey and fill in the survey’s name, collar plunge and collar trend.
Next, select the survey in the List Box and click on the Edit button.
You can now add path survey elements to the selected path survey.
Note that when a path survey is imported or created, the survey plunge convention is normalized to Positive Up, i.e. even if the tool used to create the survey data supplied a Tilt or a plunge Positive Down, once the survey is imported the data will all be Positive Up.
As described previously, some survey instruments output trend measurements relative to Magnetic North, not True North or Mine Grid. Therefore before the path survey can be used (e.g. to convert a structure (alpha, beta) measurement to (dip, azimuth)), the survey must be corrected to True North/Mine Grid. For any one project, the Magnetic declination at that location will be a constant. The Magnetic declination is defined as the azimuth of Magnetic North from True North. A magnetic declination of 0 degrees implies that True North and Magnetic North coincide, while a magnetic declination of 5 degrees implies that Magnetic North has an azimuth of 5 degrees as measured from True North. Magnetic declination for the project can be set via Project->Settings.
Setting the active survey and exporting structure dip and azimuth readings
StereoCore™ PhotoLog allows you to import multiple path surveys to a single project. When exporting structure data, a single survey must be chosen as the active survey, which is then used to convert the structure (alpha, beta) measurements to (dip, azimuth) readings. In order to set the active survey go to Project->Settings, and select the active survey from the drop-down list.
If you now go to Project->Export to Excel you will see that the Dip and Azimuth columns of the structure log are filled in with values for each structure.
Comparing path surveys
StereoCore™ PhotoLog has a number of tools which are designed to assist with comparison of borehole surveys. The problem is as follows. Firstly, we want to know that the survey is accurate, i.e. that the reported data values correspond to the actual path of the borehole. Secondly we wish to choose the best survey to use as the active survey for the borehole. In order for any of these tools to be useful, of course we require that the borehole has at least two surveys – otherwise we’re simply stuck with the one that we have (or in the worst case the collar inclination and azimuth). Fortunately most tools at least supply one each of an In-survey and an Out-survey – the In-survey taken as the tool is lowered into the borehole, and the Out-survey taken as it is withdrawn.
A crude technique for comparing just two borehole surveys which end at the same depth is to reconstruct both paths and measure the distance between the two surveys at their respective end-points. Say that the two surveys both go to a depth of 250m and the separation is 2.5m. Is this good or bad? Well, hard to say, but one limitation of this crude technique is immediately apparent and that is that the separation measured is dependent on how deep the surveys go. If they have a separation of 0.5m but only go to a depth of 25m this is clearly a worse deviation than 2.5m going to a depth of 250m. So what we do is normalise the separation to a depth of 1000m by dividing by the borehole depth and multiplying by 1000. So if the separation is 2.5m at 250m then normalised to 1000m the separation is 10m.
How do we calculate the 1000m separation for two path surveys of differing lengths? For this, in StereoCore™ PhotoLog, the standard is simply to calculate the 3D position of the borehole at the depth of the shorter of the two surveys for both path surveys, and calculate the separation as the distance between these two points, normalised to 1000m.
Next, in order to do this for multiple surveys (not just two) we calculate the 1000m separation for all pairs of surveys. This calculation is reported in the 1000m separation table of the Borehole path tab in StereoCore™ PhotoLog.
The table highlights the maximum 1000m separation for the project in yellow. Above, you can see that both BH001_Path_Survey and Test have a 1000m separation from Test 2 of 87.24m.
StereoCore™ PhotoLog has two visual aids to comparing borehole path surveys as well, both found under the Borehole path tab. The first is simply a tool which shows 2D cross sections (Eastings-Northings, Eastings-Depth and Northings-Depth) of the path surveys. You need to be a little bit careful with this tool because the scales are not equivalent on both axes.
It can give you at least some idea of the visual appearance of the path(s) in 3D space.
The other tool is a tadpole plot of the data, with plunge shown on the vertical axis, depth on the horizontal axis and trend shown as the direction of the tadpole tail. This allows quick visual comparison of the raw direction data for different surveys.
By selecting the borehole survey and choosing its colour one can show the different surveys more clearly.
Choosing the interpolation method
StereoCore™ PhotoLog offers two options for the algorithms you can use to reconstruct your borehole path. The first is the cubic spline. This is more or less the standard for interpolation in most mathematical applications because it’s got a lot of useful properties like being a mathematically smooth curve and “not too badly behaved”. It is the default option for path interpolation which is selected by the program. The second option is what I am calling a “Linear spline”, what that means is that the discontinuous data points used for interpolation are connected by a straight line. How do you choose which to use?
Basically, 99.999 percent of the time, you just forget about it and use a cubic spline. The only possible disadvantage of a cubic spline is that it can be vulnerable to outliers at the end-points of the survey, if you suspect this is the case, switch to a linear spline. A linear spline is “old reliable”. Sure, you sacrifice a little accuracy, but in exchange you get reliability, it will return a decent approximation to the borehole path without any fuss.
To set which path reconstruction option you wish to use for a particular survey, go to the borehole path tab and select the survey in the list box, then choose the interpolation method from the drop down menu.
StereoCore™ PhotoLog measures a structure’s (alpha, beta) angles and having the borehole survey data available then makes it easy to obtain the (dip, azimuth) for each structure. In addition, StereoCore™ PhotoLog has some built in functionality available to allow the user to assess the quality of the borehole survey data available, if more than one survey has been done for the hole. There is scope for improvement on the existing features, but we feel that the basic path survey features offered by StereoCore™ PhotoLog are useful as they stand, and hope that many users will benefit from them.