As a follow-up to a 2006 airborne geophysical survey of the Alaska Highway corridor, a team of 10 to 12 geologists from Alaska’s Division of Geological and Geophysical Surveys spent approximately a month during the summer of 2007 traversing the forests east of Delta Junction in Alaska’s Interior, mapping the geology of the area.
DGGS is conducting a multi-year survey of the highway corridor, to identify geologic hazards such as active faults and permafrost zones, and to identify possible sources of construction materials, in anticipation of the potential construction of a gas pipeline, railroad or other infrastructure. Relatively little is known about the geology and geologic hazards of the area — publicly available current knowledge is based on early reconnaissance mapping and regional scale investigations.
The highway corridor sits between two major fault systems, the Denali fault system and the Tintina fault system. And although the area generally experiences only low levels of seismic activity, movement on the Denali fault, which is only about 25 miles from the corridor, caused a magnitude 7.9 earthquake in 2002.
2006 geophysicsThe 2006 geophysical survey gathered magnetic and geomagnetic data along a 16-mile-wide strip straddling the Alaska Highway, between Delta Junction and the Canadian border. The geophysical data showed, among other things, some areas of possible permafrost and many linear structures interpreted as geologic faults. In fact the data suggested that many more faults exist in the region than geologists had previously thought.
DGGS initiated its highway-corridor study with airborne geophysics because river sediment and other surface deposits largely obscure the bedrock in terrain that is also heavily forested.
“Some of that area is just really thick with trees and very covered by unconsolidated soils and things, so especially for the bedrock geology, the geophysics really helps tie things across the valleys where we can’t see,” Diana Solie, geologist in the Engineering Geology Section of DGGS, told Petroleum News Jan. 15.
But, with geophysical data subject to interpretation, observation of what is actually in-situ on the ground is essential.
“Our next phase was the follow-up on that, to do the ground work, geology, geohazards and resources for that corridor,” Solie said.
FieldworkSo, starting in 2006 and continuing in the summer of 2007, a DGGS team deployed to the field. Armed with GPS receivers for navigation, geologists landed by helicopter at remote sites to work their way through the dense forest, looking for bedrock, digging mole holes to identify what’s under the ground surface and searching for evidence of active faulting.
Although there are isolated outcrops of bedrock in the region, the bedrock mappers tended to follow ridge tops and river cuts, where rock exposures are most likely to occur, Solie explained.
“Surprisingly there really are outcrops poking up through the moss here and there. … You just have to go out and walk it, to find what’s there,” she said.
The geologists mapped at a scale of 1 inch to 1 mile and covered the section of the highway corridor east from Delta Junction to Dot Lake, at the edge of the Mount Hayes quadrangle. In the summer of 2008 the team plans to continue the fieldwork, moving east from Dot Lake toward the Canadian border, Solie said.
“We’re working in segments. We can’t do the whole corridor at once,” she said.
And in the interests of completing the work in a timely manner the team mapped a zone 12 miles wide, centered on the Alaska Highway — a narrower zone than the area covered by the 2006 geophysical survey.
Aerial photographsTo assist with the mapping of surface materials such as soil, gravel, sand dunes, glacial deposits or river deposited material, retired DGGS geologist Dick Reger interpreted aerial photographs of the region during the winter of 2006-07, ahead of the summer field season. Stereographic pairs of aerial photographs provided 3-D images of the surface, and variations in vegetation types gave clues regarding where different types of surface material might exist. The field geologists were then able to spot check the photo interpretations on the ground and thus use the interpretations to extrapolate maps of the deposits. The geologists took field samples for analysis.
The geologists are also re-examining the geophysical data, using the field results to learn how to better use the data to identify surface deposits.
“We’re still working on that part of it,” Solie said.
The geophysical data collected in 2006 particularly focused on the material lying immediately beneath the ground surface and, so, the DGGS team wants to determine how data of this type can be used to identify the location of permafrost — the presence of permafrost has a significant impact on construction projects. A student at the University of Alaska Fairbanks is also doing a Ph.D. research project involving the use of satellite remote sensing imagery in conjunction with the geophysical data to locate areas of permafrost, Solie said.
“He was out there with us doing field checking as well,” Solie said. “… You can’t do remote sensing without some field checking.”
The field team tested for frozen ground by pushing a probe into the ground, and installed thermistor strings in representative areas, which will be monitored for several seasons.
Locating faultsThe 2006 geophysical data proved particularly helpful in mapping the bedrock and locating geologic faults. Faults also often show up as linear scarps or other surface features, Solie said. Knowing the locations of active faults is especially critical to the engineering of new infrastructure such as a pipeline or railroad.
By cutting trenches across a suspected active fault line, geologists could verify the existence of the fault and examine the layering in the soil for evidence of occasions when the fault had moved — displacement of the soil layers indicates fault movement after the soil has formed. Then, by using radiocarbon dating to determine the ages of the soil layers, the geologists could determine the approximate times at which fault movements had occurred.
“We have seen in some places up to three (movement) events, so these things are repeatedly activated,” Solie said. “… In terms of engineering that’s something that would need to be considered in design.”
It turned out that four of the faults investigated had moved in about the past 10,000 years, in one case as recently as 1,200 years ago, thus indicating that some faults may still be active. However, there was no evidence for current activity on any of the faults.
And some of the more major discontinuities seen in the geophysics turned out to be old faults that ceased activity in the distant geologic past.
“It was quite interesting. Some of the more spectacular breaks in the geophysics don’t appear to be anything recently active. They’re old faults in the bedrock,” Solie said.
However, Solie stressed that the DGGS fault investigation was preliminary in nature.
“If anything gets built through there, there will certainly need to be more detailed work to get the parameters for design,” Solie said, “But the first step is to know that they’re there.”
Geohazards and materials resourcesThe potential for earthquakes arising from fault movements also gives rise to concerns about the potential for the liquefaction of surface material. The mapping of surface deposits coupled with information about past liquefaction incidents should help identify areas where there is potential for future liquefaction, Solie said.
The identification of areas of potential flooding involves a similar approach.
“In the surficial geology mapping we can see features that are characteristic of flooding, remnant channels and that kind of thing,” Solie said.
The team also plans to create a materials map depicting where construction materials such as gravels are likely to be found, based on an assessment of where the materials are likely to occur in mapped surface deposits.
“I think that’s our best approach and it gives those who are going to be looking for the specific material sites target areas where the materials could be expected to be found,” Solie said.
And the new geologic mapping is adding much detail to previous knowledge of the highway corridor.
“At inch-to-the-mile mapping we’re able to add a lot of detail that simply wasn’t there,” Solie said. “… We have really done what we set out to do and we will be able to publish maps that are an improvement over what was previously available. … It’s a publicly available baseline of geologic information.”
The summer 2008 fieldwork should extend the mapping east as far as Tetlin Junction.
“Hopefully we’ll be able to do from there to the (Canadian) border in … 2009,” Solie said.
Meantime DGGS expects to publish the results of the 2007 field season at some time in 2008.