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Looking for hazards on the AK Highway route DGGS airborne geophysical survey pinpoints locations of geologic faults, areas of permafrost in an important transportation corridor Alan Bailey Petroleum News
Spurred by the possibility of future pipeline, railroad or other infrastructure along the route of the Alaska Highway, Alaska’s Division of Geological and Geophysical Surveys is conducting a study into geologic hazards and possible sources of construction materials in a land corridor bordering the highway. Geologic hazards to be investigated include active faults, areas of permafrost and areas of slope instability. Construction materials might include sand and gravel from near-surface deposits.
Faced with an area where river deposits and other surface sediment largely obscure the bedrock, DGGS initiated its study with an airborne magnetic and electromagnetic geophysical survey. Through a contract with DGGS, Stevens Exploration Management Co. subcontracted Fugro Airborne Surveys to conduct the survey along a 16-mile-wide highway corridor, covering about 3,000 square miles in the almost 200-mile distance between Delta Junction and the Canadian border.
In a presentation to the Alaska Geological Society on Dec. 7 Laurel Burns of DGGS described some of the results of the survey and the plans for the second phase of the Alaska Highway corridor study.
“Most geologic mapping in the area is reconnaissance from the 1960s and 1970s,” Burns said. “… There’s also a very poor knowledge of the material resources and geologic hazards along this route, particularly permafrost and recent faulting in this area.”
Burns pointed out that the Alaska Highway corridor sits between two major fault systems, the Denali fault system and the Tintina fault system. And although in general the area only experiences low levels of seismic activity, movement on the Denali fault caused a magnitude 7.9 earthquake in 2002. The highway corridor is only about 12 to 25 miles from the Denali fault zone, Burns said.
Helicopter survey The helicopter-borne magnetic and electromagnetic geophysical survey used transmitting and receiving equipment housed in what is known as a “bird,” a torpedo-shaped device hung below the helicopter.
A magnetometer in the bird recorded the magnetic values of a relatively wide and deep area below the instrument. This type of data generally shows variations in the amount of iron in the rocks and can be used to help distinguish and delineate geologic features such as rock types and faults.
The electromagnetic data were collected using electromagnetic transmitting and receiving coils in the bird, with different coils operating at different electromagnetic frequencies. Each transmitting coil emits a primary magnetic field at the frequency at which the coil operates. The magnetic field induces an alternating current in any material in the ground that can conduct electricity. That alternating current produces a secondary magnetic field that a receiving electrical coil in the bird can detect.
The time taken for the secondary magnetic signal to reach the receiving coil and the strength of that signal provide information about the depth of a subsurface electrical conductor and how well that subsurface feature conducts electricity.
Different types of rock tend to have different electrical conductivity, so that an electromagnetic survey can reveal information about the near surface geology. An electromagnetic survey can also detect subsurface conducting material such as water or metal pipelines. The technique has been used for applications such as detecting old river channels (potential sources of gravel), leaking water and buried pipelines.
Depth of penetration A key factor in determining the depth of penetration below the land surface of a survey is the frequency of the signal that is used — the higher the frequency the shallower the depth of the electromagnetic measurement. The Alaska Highway corridor survey used a frequency range from 140,000 hertz to 400 hertz to map features from about 5 meters to about 150 meters below the surface.
And, because the survey was targeting sediment and rock strata, the electromagnetic coils were configured to emphasize horizontal subsurface features.
Acquiring airborne magnetic and electromagnetic data together provides more criteria to aid in geologic mapping than can be obtained from the individual types of data — some features show up well from magnetic data, while other features can be detected better from electromagnetic data.
The helicopter flew a series of parallel survey lines aligned slightly west of north and one quarter of a mile apart. Fugro Airborne Surveys then used the survey data to derive an electromagnetic profile for every third survey line.
Burns showed some subsurface cross-sections that demonstrated the results of the survey — cross-sections of the subsurface resistance to electrical currents depicted features such as presumed permafrost. A typical section near the town of Tok showed a shallow, electrically resistive layer of presumed sediments lying over a more conductive layer that probably represents the water table. Another resistive layer underlies the conductive layer.
Fault patterns A complex pattern of faults proved to be a particularly interesting finding from the survey — because of the lack of surface rock outcrops in the area these features were previously unknown. In fact the survey showed a much higher density of faults than existing geologic maps of the surrounding area have depicted. And the vast majority of the interpreted faults appear to slope downwards at steep angles.
A key issue from the point of view of the construction of a transportation infrastructure is the question of which faults are currently active.
“A lot of what we are observing is likely related to older faulting and it will take detailed geologic mapping and geophysical modeling to determine what’s recent and considered a potential geologic hazard,” Burns said.
Surface field mapping In phase two of the survey project, DGGS plans to do field mapping in the highway corridor to verify features identified from the geophysical survey and from air photo interpretations of the area — surface geologic mapping is essential for the refinement and verification of the geophysical analysis and interpretation. The ground mapping will target features identified from the geophysics and then use the geophysical data to extrapolate surface features below the sedimentary cover.
The geologists will also use handheld meters to measure the magnetic susceptibility of all rocks seen in surface outcrop.
An investigation of potentially active faults will involve trenching across each fault and assessing any evidence for the direction and timing of fault movement. Where possible the geologists will verify the actual existence of permafrost in potential permafrost areas identified from the geophysics and air photo interpretations. The field program will also provide a means of calibrating the use of remote sensing data from satellites, for example, to locate areas of permafrost.
Laboratory data, test hole data and records from the 2002 Denali fault earthquake will provide insights into areas where there is potential for the liquefaction of the ground during an earthquake. Air photo interpretations and field observations will provide information about potential slope instability. And air photo interpretations, field observations and historical records will provide insights into areas of potential flooding.
Geophysical data available Meantime, the data from the geophysical survey are becoming available to the public.
Maps containing the basic data can be downloaded from the DGGS web site or purchased as paper copies from the division. The original survey data and gridded data used to make the DGGS maps are available from the division on CD or DVD. DGGS hopes to soon make available electrical resistivity profiles and additional gridded data produced from some of the profiles, Burns said.
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