Editor’s note: The E-Geo News has been walking readers through the various steps required to turn physical data such as core samples into a functional geological map. This is the fifth installment in this series; core collection was covered in September, core describing in November, lab analysis in December, and outcrop scouting in February.
If geological mapping were a treasure quest, Robin Rupp would be a guide and translator.
A veteran IGWS staffer, she knows what data exist from the past several decades of projects and where it’s buried. A master’s-level geologist, she also knows how to decipher that data and can judge whether it contains reliable information.
While the Jasper bedrock mapping team, led by Don Tripp, has been in the field and core library for months gathering new data from rock cores and outcrops, Robin has been stationed at the IGWS office digging up older information stored in multiple databases and filing cabinets. Both are working to create digital maps of bedrock, but from different angles. Don’s team is looking for data on how the different rock types within the bedrock crop out along hillsides, road cuts, slopes, and valleys, and how those project downward, while Robin is searching for data on where the buried bedrock surface starts under soil, clay, and other “unconsolidated” (non-rock) sediment. Put together, their data will give as complete a picture as possible of what the bedrock in that area looks like.
This work is an example of how the IGWS’s long history of geological information-gathering serves the state well. Over its 186-year history, the Survey has amassed literal tons of physical data like rock and sediment cores, and paper or digital data like maps and databases. It may be older information, but that doesn’t mean it’s no longer useful.
Robin’s task is to mine decades’ worth of paper and digital sources for data points that could shed more light on what’s going on underground in a project area. The vast majority of the data that goes into a bedrock map is from historical sources like these, she said.
A major source is the IGWS’s Petroleum Database Management System (PDMS). It contains records relating to about 85,000 oil, gas, gas storage, service, dry, or stratigraphic test wells across the state, dating back to at least the 1950s, and it’s still growing. However, the completeness of those records varies. “Some exceptional petroleum records list all the lithologies the well was drilled through, and the top of the bedrock surface, while most of the records list only deeper units where oil and gas may occur," Robin explained. “Finding information on the depth to the top of the bedrock surface can be a little like looking for a needle in the haystack.” She also knows that what looks like bedrock to one person may not look like bedrock to another. “There can be these very soft shales or hard clays, and everybody gets fooled” into recording the wrong lithology in the well.
The most prolific data source is the Water Well Record Database maintained by the Indiana Department of Natural Resources’ Division of Water. (The IGWS used to be the Division of Geology under the DNR.) The database includes driller’s logs for 400,000 wells showing which type of soil or rock was hit at what depth in that customer’s quest to find water.
Road records from the Indiana Department of Transportation – some from filing cabinets at the IGWS office and others only at the INDOT office in Indianapolis – can also be searched. Bridge data is especially useful, as engineers drilled deeper for those than standard road borings.
The National Coal Resources Data System (NCRDS) contains lithologic records of coal exploration borings, which may contain depth-to-bedrock information.
Another source is SSURGO, a database of Natural Resources Conservation Service (NRCS) soil data. It can tell researchers what types of soil exist at the surface and usually provides information on the parent materials of the soil, giving clues to whether bedrock is close to the surface or not.
Decades’ worth of IGWS geophysical logs – which look like heart rate print-outs – can tell a trained geologist or geophysicist what types of rock or sediment exist at certain depths.
The IGWS also has two sets of seismic data (like earthquakes, but manmade). One is a legacy dataset dating back to the 1950s when IGWS geologists “were looking for depth to bedrock and using long lines with geophones and dynamite that they would set up on the road, and they would get reflections coming back from the bedrock,” which would tell them how deep the bedrock was, Robin explained. That’s what geologists call “active” seismic data. Geologists still do similar work, but with a small, "passive” device that does not involve dynamite; it instead measures frequencies that are created naturally. The mobile seismometer dataset dates to 2014 and is growing.
Do all these data sources sometimes contradict each other? “Of course,” Robin says with a rueful smile. “That’s the challenge. … You have to go with the model that looks the most realistic, and you have to end up smoothing or averaging or ignoring or totally throwing out data. Especially when you have a lot, you can do that.”
Robin and Don will give a presentation about their mapping process at the Digital Mapping Techniques (DMT) workshop in Anchorage, Alaska, later this month.