The Surprise Valley field expedition is an expedition in every sense. On the one hand, the team is trying to map underground faults and fractures beyond a region whose subsurface they’ve already studied in great detail. But analyzing the data in these new maps has itself proven to be an expedition. Although data analysis is one step in the pursuit of specific research questions, today it led them off course, toward new, unanticipated paths of inquiry. Finally, after a long night of troubleshooting, the team had begun the adventure of research. It’s what beckons them to the field year after year, in spite of the inevitable technical hurdles. With preliminary data, the team not only made headway in answering one of their main research questions–they even encountered a few surprises along the way.
Yesterday marked the first day of the team’s expedition to map the underground faults and fractures in Surprise Valley, California using SIERRA, a small aircraft capable of flying without a pilot onboard, or unmanned aerial system (UAS). Faults and fractures generate distinct magnetic patterns,or anomalies. Geophysicists can look at a set of magnetic readings for a region and readily distinguish those representing these subsurface features from those that don’t. But SIERRA and its maneuvers also produce magnetic anomalies. As you might recall from our previous post, to compensate for anomalies associated with SIERRA, the aircraft is fitted with an instrument called a fluxgate magnetometer, which allows the team to subtract the appropriate magnetic field values based on SIERRA’s position at any given point in time. This removes the effects of the aircraft’s magnetization, which could obscure the magnetic readings from the subsurface features the researchers are interested in studying.
During the flight, the fluxgate yielded compensation data values that deviated far from what the team had expected. After removing the instrument from the UAS and spending hours troubleshooting it, NASA engineers Corey Ippolito and Ritchie Lee, Geometrics engineer Misha Tchernychev, and lead USGS scientist Jonathan Glen removed the instrument from SIERRA and spent hours troubleshooting it, eventually tracing the problem to an error in its calibration.
The team hoped that a technician at Applied Physics, the manufacturer of the fluxgate, would happen to be in the office on Labor Day so that he or she could provide them with the proper calibration. While they weren’t able to contact Applied Physics, they were able to get in touch with another engineer from Geometrics, the company that installed the cesium vapor magnetometer, at their headquarters in the San Francisco Bay Area. The engineer said that he could provide the team with a new, properly calibrated fluxgate, which he agreed to drop off with them at a halfway point, in the town of Redding. Just before 8:00 AM, two USGS researchers hit the road to Redding. To allow themselves time to retrieve the new fluxgate and correct the previous day’s data, the team called off the flight for the day.
Misha met Jonathan at the house where USGS researchers are staying this field season. They planned to determine the correct the calibration themselves by recording the magnetic field strength and direction that the fluxgate magnetometer measured as they moved it through a series of maneuvers. This would yield the mathematical relationship between the raw and improperly calibrated versions of the data, allowing them to convert yesterday’s data to raw data. The fluxgate’s measurements of the magnetic fields produced by the maneuvers would also enable Jonathan and Misha to determine the correct calibration, which they could then apply to the raw data from the first flight day. Once the two USGS researchers returned from Redding with the new fluxgate, NASA engineers could mount it on SIERRA for use in future flights.
Jonathan and Misha spent the entire morning on the front lawn, one standing and maneuvering the instrument while the other kneeled across from him, jotting down the instrument’s readings on a sheet of paper. Misha then fed the readings into a computer program to generate an equation reflecting the relationship between the raw and incorrectly calibrated versions of the data. He then applied the equation to the data from the first flight.
The resulting raw data appeared as a magnetic map, crisscrossed with grid lines showing yesterday’s flight path, the bright green lines representing magnetic anomalies, indicating faults and fractures below the surface. This field season, the team plans to do three central detailed surveys, with the UAS flying along tightly spaced, east-west lines in three polygonal sections running north to south near the center of the valley. SIERRA will also fly the perimeter of the valley and in a broad zigzag pattern across the entire valley to map its large subsurface features, particularly in the unexplored southern region. Next year, the experimental UAS will be able to perform a more detailed, accurate survey over the entire valley since it can fly closer to the ground, and its wider wingspan enables the cesium vapor magnetometer to be mounted further from the center of the aircraft, where electronics and instruments could produce magnetic noise. The experimental system is also electronic, meaning it lacks the numerous magnetic parts that comprise a combustion engine. The team will even be able to turn off the electric mode and switch the aircraft to glide mode.
Yesterday, SIERRA surveyed the middle and southern central detailed regions. When Jonathan saw the data, he grinned broadly. Even the raw airborne magnetic data closely coincided with the ground-based data his group had gathered from the same region in previous years. After further sifting through the data, Jonathan observed that the pattern of structures buried below the regions mapped during yesterday’s survey looked very similar—essentially analogous—to that of structures on the surface along Summer Lake to the north of the valley. This pattern consists of a low-lying area, called a basin, which was formed when the Earth’s crust thinned and cracked as forces originating in the underlying mantle pulled it apart. This stretching resulted in one part of the crust thrusting upward, forming a mountain range, and an adjacent region sinking downward, forming a basin. The two structures collectively comprise a basin-range region. In Surprise Valley, the Warner Mountain Range meets a dry lakebed, or playa, and the Winter Rim Mountain Range meets Summer Lake. In both regions, numerous faults run parallel to the edge of the basin where it meets the range. Sheets of magma injected from the Earth’s core into the surrounding rock, a type of geologic structure known as a dike, run parallel to the faults, lying snugly alongside them.
Both Summer Lake and Surprise Valley share a similar structural pattern, the major difference being that while this pattern is visible on the surface of Summer Lake, it remains hidden underground in Surprise Valley, revealed only by magnetic mapping. This hints that similar patterns may be found throughout the Basin and Range region, which spans much of the western United States. It may be possible that the magmatic intrusions that form dikes may play a role in the development of the basins in this area. Right now, the team doesn’t know whether the magma creates faults as it rises, or whether the crumbling of rocks along faults makes it easier for magma to flow through. This knowledge could point the team to how magmatic intrusions play a role in basin development in the area.
Jonathan pored over the map displayed on laptop throughout the afternoon, every now and then looking up, exhilarated, and gathering the other USGS researchers around his screen. Meanwhile, Misha holed up at Cedarville Airport, calculating the correct calibration from the maneuvers earlier today. He returned a few hours later with the correct calibration applied to the raw data. While the magnetic map based on the raw data was mostly clear, the edges appeared blurry. When Misha applied the correct calibration, the resolution of the map image improved significantly. The image he showed Jonathan centered on a feature that previous ground-based data had indicated to be two diagonally parallel segments arranged in a stairstep, or an echelon pattern. Now, the magnetic data revealed these actually represented only one fault. The two veins of the fault branched out from a single fault, which scientists wouldn’t have known if they hadn’t mapped the region below.
The feature that the team is most curious about appears on their maps as a magnetic anomaly stretching over 30 kilometers through the valley, which they detected during earlier ground-based surveys of Surprise Valley. Unlike the corrugated Surprise Valley Fault, which meanders along the base of the Warner Mountain Range, this feature runs an almost perfectly straight course. It also coincides with a number of major hot springs, suggesting that it plays an important role in the system of channels and pores the circulate fluid through the hot springs, or the geothermal system, of the valley.
In previous years, the team had collected ground-based magnetic data on the northern and southern sections of the feature. But with this data alone, they couldn’t tell whether the feature represented a single through-going structure or multiple en echelon segments. They hope that aerial surveys of the central detailed regions between the northern and southern parts will yield the missing pieces that will allow them to answer this question.
Looking at the raw data from the yesterday’s survey of the middle and southern central detailed regions, Jonathan spotted a magnetic anomaly continuing along the same direction as the anomaly he and Anne had mapped when they performed a ground-based survey of the area surrounding the southern part of the feature. So far, the data hints that the feature may in fact represent a single structure, but the team can’t draw any conclusions before SIERRA completes the map, at least of the feature of interest, by surveying the north central detailed region, which the team has planned for it to do tomorrow.
Knowing whether or not the feature is continuous is important, since the magnitude of an earthquake that can occur along a fault is determined primarily by the length of the fault. The longer the fault, the larger the earthquake it causes when it ruptures. In other words, if the feature the researchers are interested in is long and continuous, it will cause a much larger earthquake than if it were partitioned into segments. A continuous fault also means a continuous channel for geothermal fluids, a dangerous scenario, since a hazardous groundwater zone high in mineral content sits in the middle of the feature. Since continuous and en echelon faults differ in the type of earthquakes they produce, knowing the feature’s structure will also help refine predictions of how likely and how damaging earthquakes could be in the region.
As Jonathan continued to navigate through the data, the two USGS researchers returned with the new fluxgate, which they handed off to the NASA engineers to install in SIERRA tonight so that the aircraft can fly first thing tomorrow morning. The team excitedly awaits the results of tomorrow’s survey. While no one can guarantee that they won’t run into any more technical mishaps, the alternative possibilities—the glimmer of an answer to a research question, a surprise discovery–make the adventure worthwhile.