The Borehole Surveyor


Mankind's need to obtain more information from holes drilled into our planet has created an increasingly important job function - that of the "borehole surveyor". In fact, most holes are not considered to have been completed unless directional and other survey data are available. And for good reason- no hole will follow the path projected by the planner and there is more information obtainable from boreholes than has been routinely acquired. Yes, when it deviates from the projected path it can be brought back. But you need to know which direction the hole is off track and by how much; and someone has to decide if and when the deviation is too great for the purpose for which the hole is being drilled.
Who are these surveyors?
In all but the survey intensive projects on high operating cost rigs, the person is a part-time surveyor with other duties in the program. It could be the geologist, geotech, engineer, drill superintendent, or driller, for example. These surveyors obtain their knowledge from proprietary manufacturer's literature, on the site instruction by experienced members of the program staff or manufacturer's service staff, and hands-on experience. There are precious few academic programs that provide much information.
What kinds of borehole surveying programs are there?
Let's look at few examples to provide some depth to the subject: The costly projects, such as ocean platform oil well drilling, employ a professional surveyor or obtain the services of a surveying company. These surveyors obtain their training through "apprenticeship type" programs with either the exploration, production, drilling or survey service companies. Their responsibilities may include a number of in-hole procedures involving surveys and certainly the provision of survey plots. Programs involving large drill footages that need frequent or regular survey procedures usually require that the surveyor be a member of the drill-site team to achieve maximum cost effectiveness, and in multidrill projects, a number of survey instruments may be employed for the same reason. Efficiency in this context is a combination of factors like having an instrument and surveyor available when the specific drill is ready for a survey rather than having to wait for one or both to arrive, or having a surveyor and/or instrument sitting idle at the drill waiting for the drill to complete the hole to the survey depth while other drills are waiting for one or both to arrive. These factors have to be evaluated for the specific program, but the instrument type chosen should provide the data required efficiently, cost effectively and should be used with a clearly understood plan of procedures. More on planning later. The instrument chosen has to be a "work-horse" in this environment performing exactly what is needed to be done and not adding any unnecessary steps or demands. The knowledge required by the planner is fairly extensive and may be supplemented by consulting the manufacturers of surveying instruments. Some borehole surveying instrument manufacturers offer in-house, client's site, or drill site training; and all provide some form of instructional information. Just enquire. A small drill program for site engineering, exploration or scientific purposes may only require a few directional and/or tool facing surveys. Here, instrumentation that is easy to use and available at reasonable rental rates would be the prevailing consideration if no previous surveying experience is available amongst the project staff. Contract surveyors are another option and under certain circumstances may be the least expensive route. The contract survey can also be used to train your staff for the next job. Again shop around.
What kind of planning is involved in survey procedures?
Borehole surveying is a distinct operation and must be viewed and executed with the same professional dedication as that used in drilling the hole. If those planning steps are not taken, you can rest assured that the necessary mental and equipment preparation will not be there to get the directional, core orientation or other survey data accurately and cost efficiently. Consider which of the questions below apply to planning your survey project: What kind of accuracy do you need for the project? Here the instrument or the survey train (application hardware) may be the accuracy limiting item. If the borehole has a target, will the hole hit the target and who will decide when the hole deviates sufficiently so that corrective procedures or abandonment have to be invoked? Surveys at regular intervals as the hole progresses will allow the hole to be plotted and excessive deviation recognized at the earliest and least costly stage. Is the instrument and the survey train suitable for the application and drilling conditions? There is a wide variation in the ease of use or time required for a survey and this may depend on either or both the instrument and the survey train used for the specific data required. Consider how disruptive the surveys are to the drilling procedures. Compare data and supplementary advice from manufacturers or service providers for your specific requirements. Are there special conditions at or in the borehole that need to be addressed? These, for example would be temperatures above 85° F or below -20° F in the bore; water, steam, oil or gas discharges that could set up vibrations in the survey train preventing the instrument from stabilizing even at low velocities; and boreholes where the wall rock is not coherent. Are site procedures understood by the staff? This could include how the survey train is assembled and armed, how the lowering cable or rods are marked so that hard impacts with in hole-tools or the bottom of the hole are avoided, how the cable is marked so that the survey train is not wrapped around the drill rig when it exits the hole at high velocities, and how the readings are obtained and processed.