Rules of Thumb for Geological Field Mapping Success

By: Erik Ronald, PG

Mining Geology HQ

20 March 2016

Following my February article on Rules of Thumb for Mineral Exploration Success, I’d like to share some fundamental aspects to ensure success when conducting geological mapping. These “Rules of Thumb” can be applied to either true greenfields mapping, near-mine brownfields work, or open pit bench mapping.  I’m not going to touch on underground mapping as that is a different animal entirely. This list is a compilation of notes from various publications, most notably Robert Compton’s classic text book “Geology in the Field“, inspirational mentoring from folks like Arthur G. Sylvester and Phil Gans, and my personal experiences of mapping in various terrains and pits across multiple countries and two continents.

Planning and logistics are a major effort and shouldn’t be taken lightly. One should treat a mapping campaign like a Project Manager and not like a weekend camper. There should be a clear understanding and agreement with management on the scope, resources, alternatives, objectives and timeframes to ensure success. Access rights, permissions, local guides, ability to collect samples, transportation, and other aspects must all be clearly organized prior to any campaign. There is a big difference in logistics required for mapping via a four-wheel drive in Montana versus horseback in Mongolia or a chopper in PNG.

1) Safety – I’d like to start with the safety aspect. Though some may see safety as non-critical to the technical mapping work, I strongly disagree. Your final product won’t be very good if you break a leg or end up as a pile of sun-bleached bones in the desert. Establish an emergency plan outlining locations and times. Always map with a partner, as this is good for both safety and to have someone to bounce ideas off or to argue about structural or timing relationships. Carry a satellite phone because wherever you are in the world, mobile coverage is garbage. Establish check-in times so others will know within a few hours if something bad has happened. Carry an appropriate field first aid kit and know how to use it. Finally, take enough water or a filter/tabs to last 24 hours at a minimum.

2) Scope – It’s critical to have a clear understanding of your goals. Is your plan to complete a generalized geologic map showing all units with major structures or are you after a specific attribute? Maps for mineral alteration, engineering geology, hazards, or oil & gas exploration can be quite different. This focus of scope can be a blessing and a curse so tread carefully. Focusing on one aspect typically means you can overlook another. There are many cases in both mapping and logging where the geologist failed to recognize the obvious. A good example is the case of a geologist logging core for an industrial minerals deposit and missing the visible gold, a true story!

3) Pre-field work – The work you do before you set foot in the field can be the difference between success and failure of your field mapping campaign. I used to go by the adage of three weeks of dedicated office work for each week in the field, others have suggested three months! Your field book should be packed with information from stratigraphic columns, to cross-sections, to unit descriptions before you leave your office. This time is important to familiarize the geologist with previous work in the area, review any internal or public reports or maps, contact the local Geological Survey, create preliminary maps via satellite imagery/aerial photos, and talk to anyone experienced in your area of interest.

Remote sensing work (hyperspectral, satellite or aerial photo images), if possible, should always be done well in advance of fieldwork. For those who can’t afford satellite imagery, an amazing amount of review can be done in Google Earth with some vertical exaggeration of topography and the Elevation Profile tool along with free information online. Any published geophysical data will be very supportive in your preliminary mapping. Remember that most places on Earth have at least a rudimentary geologic map so you’re not 100% shocked when your boots hit the ground.

Be sure to learn as much from others before you go into your field area. This can range from university professors, to water well drillers, to farmers or landowners who know their own property. It’s amazing what non-professionals observe and you can typically learn a great deal, if you listen. If there’s a sand pit, quarry or good road-cut nearby, be sure to add it to your list during a reconnaissance trip or early on when mapping.

4) Field observations – Be detailed, confident, and consistent in field observations, interpretations, contacts and any measurements collected. Ink your map in the field or at worst, do it that night in your hotel room/camp/cave before going to sleep. Realize that once you’re sitting back in an office a week or year later, you won’t suddenly have an epiphany and work out all your problems, so just do it then and there. The more detail in your observations in regards to landforms, erosion patterns, weathering habits, minerals, stratigraphic features, textures and anything else will go a long way when you see similarities the next day or need to recall subtle details to work out the greater geological puzzle. When making geological observations, don’t forget to use all five senses: does that shale have an oily feel? Can you smell sulfides in that diorite once it’s broken? What sound does the serpentinite make when you hit it with your hammer? Does that kaolinite-rich shale stick to your tongue? Lastly, don’t forget to take ridiculous amounts of photos, label them and describe each in excruciating detail – except for the photos back at camp on your last night there, the HSE manager doesn’t need to know about that night. No one needs to know about that night!

5) Samples – Human beings and especially ones training in Geology are excellent at noticing deviations from the norm, oddities and outliers. Your eye will always focus on what’s different in an area. These oddities can be good at times if you’re hunting mineralized veins or alteration features, but for general geology maps this can be troublesome. Ensure you collect “typical representative samples” for each geologic unit in addition to the more “interesting” or potentially high-grade samples you grab. The common, boring, and consistent sample of a formation or unit can be extremely helpful to drag back to your camp to gain an eye for what is “typical”, thus assisting with identifying the atypical.

6) Take time to sit and contemplate. Some of the world’s best mineral deposits were discovered simply because of where the field geologist happened to take their lunch break. The time to sit and observe an outcrop to the point of boredom can mean the difference between seeing that minor bit of alteration or just walking over it assuming it’s the same rock unit you’ve seen all morning.

7) Know your mental limits – This goes directly to the quality of one’s work at various times of the day and your physical & mental state. Personally, I’m useless before coffee in the morning and then again between 1pm and 3pm, after which I tend to get a second wind of energy. Any mapping, field notes, observations, measurements, or interpretations made in those low-energy hours should be reviewed or simply thrown out. Instead, I accept my hopelessness and schedule that time for a longer lunch, a long walking period, mindless work like coloring, or even a nap if conditions permit. I once had a field partner who couldn’t be consulted before 9am. Whether it was the lack of good coffee in the Mojave or simply being hung over from the previous night, the important thing to recognize is that not all field observations are of equal quality. If you’re not focused, don’t press the issue. The same goes for too many days in the field, heat, cold, flies, mosquitoes, the grumpy camp cook, or general burnout.

8) Break rocks. Everyone should already know this point but you can only determine rock types and mineralogy with freshly broken surfaces. Don’t simply walk over an area without leaving a trail of cracked rocks in your wake. Sometimes weathering patterns of different rocks appear similar or too much desert varnish will make everything look the same. Don’t be lazy, just check.

9) Draw pictures. A field book should be filled with sketches, drawings of intersecting veins, cross-cutting relationships, reconstructions of faults, interpretations of topography…whatever you need to bring that geologic puzzle together. I know many people are not great artists and I’m not saying you need to be, but simple line diagrams can do a lot toward understanding structure, veins, unit timings and relationships. Fossils observed in the field can be great markers for time constraints on units. If you’re like me and you can’t tell a Sarcopterygii from a Paramblypterus, then draw it accurately and ask someone later.

10) The right gear. This sounds straightforward enough but if you’re trying to carry too many gadgets, various rock hammers, map boards, iPads, laptops, food, water, GPS, cameras, gold pan, reference books, color charts, and rock samples around for multiple hours per day it’s likely you’re going to be physically and mentally exhausted resulting in reduced quality of field observations. Focus on the essentials: your brain, maps, hammer, hand lens, compass/transect, camera, GPS, and magnet. Be sure to grab the correct hammer dependent upon the rocks you’ll be working with. A chisel and crack hammer is required for continental shields, a chisel edge hammer when in sedimentary or softer units, and the classic geology hammer or geopick in most everything else. A few other handy items are good such as a small scratch kit and acid bottle. Depending on your specific goals of the mapping program or the terrain, you may swap that Brunton for a Mag Sus meter, a scintillometer if known radioactive units are in the area, or lug along your PIMA or portable XRF but in most cases these items become dead weight.

I’ll also include good boots, appropriate clothes for the climate and fauna (black flies and mosquitoes!), sun protection, a big enough lunch(s), and should the occasion call for it, the appropriate firearm when you encounter those pesky Grizzlies or radical fundamentalists.

Unofficial #11 – End the day with a beer and enjoy that your role as a field geologist has just allowed you to “work” in the greatest office in the world – outside.

Once you’ve returned from your field campaign, ensure ample time is provided to organize, catalogue, and report all observations immediately. The longer you wait to write up reports or findings, the worse they will be. Ensure all work is correctly archived, digitized or stored for maximum benefit.

I’ll leave you with a quote from S.W. Muller in the Journal of Geological Education (1983, v. 31) – “Field Geology is learned in the field; therefore one must go there as soon and as frequently as possible”.  I hope you’ve enjoyed this article or perhaps it simply brought back some good memories of past experiences.  As always, I encourage your comments, feedback, or suggestions. Best of luck to everyone on their next field mapping expedition!