Vibration Response of Rock Arches and Towers to Helicopter-Sourced Infrasound

In a December 2019 presentation to the American Geophysical Union, University of Utah researchers Riley Finnegan, Jeffrey R. Moore, and Paul R. Geimer discussed “Vibration Response of Rock Arches and Towers to Helicopter-Sourced Infrasound.”

Helicopters produce a variety of tones, the lowest of which is inaudible to the human ear, but is often the most powerful. Common civilian helicopters emit infrasound energy in the range of about 10-30 Hz, which can coincide with the natural frequencies of culturally significant rock landforms such as arches and towers. Sparse data from past studies suggest that close-proximity helicopter flight is capable of stimulating potentially damaging resonance of such landforms, but the structural response and long-term impact of this added energy remain poorly understood.

The researchers performed a series of controlled helicopter flights, using broadband seismometers and nodal geophones to measure the vibration response of five sandstone arches and six sandstone towers to infrasound emitted by a Bell 206 helicopter. They compared the measured vibration of the landforms, as well as nearby reference sites, to infrasound emitted during flight to distinguish periods of induced resonance. Alignment of the landform’s natural frequencies with those of the helicopter noise (including Doppler-shifted frequencies and overtones), the landform’s mode shape compared to the direction of infrasound propagation, and helicopter distance all contribute to the ability of helicopters to excite resonance and, in turn, the amplitude of induced vibrations.

They found that the range of landform size and geometry susceptible to helicopter-induced resonance is narrow (i.e. precise frequency alignment with the source energy is required), and that measured peak vibration amplitudes are generally below values thought to be instantaneously damaging. Nonetheless, the geographic reach of helicopters is virtually unlimited, and many well-known landforms experience regular overflights, validating concerns that sustained exposure to anthropogenic airborne energy may result in adverse structural degradation of culturally significant landforms over time.

Two of the arches they studied are Squint Arch (NABSQNO 12S-528380-4277606) and Arsenic Arch (12S-540406-4217628), both in Utah. Photos of their vibration sensors on these two arches are shown below.

Squint Arch

Arsenic Arch

A video clip of a drone flyover of Arsenic Arch is below, showing the instruments on the arch.

Presenter Riley Finnegan answered the following questions:

Can you give me an overview of this research in lay terms?

Rock arches and towers vibrate back and forth all the time—just like a manmade bridge or building—though, the vibrations are so small it’s hard for a human to notice. Certain elements in an arch’s natural environment can cause the arch to vibrate faster, like wind or earthquakes. Our study tested the ways helicopter noise causes arches and towers to vibrate in ways that they don’t necessarily experience in their natural environment. We found that the size, shape, and material of the rock, along with the type, speed, distance, and direction of the helicopter contribute to the vibration levels of each geologic feature. We found the range of rock arches and towers susceptible to strong vibrations from exposure to inaudible helicopter sound is narrow, and the vibration levels measured are generally below values considered immediately damaging. Nonetheless, helicopters can fly nearly anywhere, and many arches and towers experience regular helicopter flyovers, validating concerns that inaudible helicopter sound may cause damage to culturally significant rock features over time.

What is new about your findings compared to what is already known in this field?  How do your results compare to what is already known about this area of research?

We borrow techniques from a variety of fields, including seismology and structural engineering. Engineers have studied the vibrations of manmade structures for decades, and we use the same methods to understand the resonant frequencies of landforms like arches and towers. Few other groups study the resonant frequencies of arches and towers, so the natural frequencies of many of the features we’ve studied are shared here for the first time. Similarly, few past studies have recorded the vibrational response of natural landforms to helicopter noise, so we share those results here as well. In particular, we explore factors like the landform’s geometry and material properties, and helicopter speed, direction, and distance, and how they relate to the vibration of the landform.

Did you come across anything surprising or unusual in the course of your study?

The lowest frequency of noise emitted by the helicopter is the most powerful, so we predict that the strongest vibrational response of the arches and towers to helicopter noise would be from exposure to noise in that lower frequency range. However, where a landform’s natural frequencies didn’t fall in that lower frequency range of helicopter sound, but coincided with less powerful but higher overtones of helicopter noise, we saw a strong vibrational response.

We were also surprised that the propagation direction of the helicopter noise is extremely important in coupling with the mode shape of the landform exposed to the noise. If the noise is emitted in a horizontal direction, a landform that tends to vibrate horizontally will respond with greater vibrations than a landform that tends to vibrate vertically. In hindsight, this seems obvious, but we didn’t confirm this until our data showed an arch inclined towards vertical motion vibrating at the same velocities when the helicopter was thousands of meters away, banking with the noise directed downwards, as when the helicopter was only a few hundred meters away with the noise directed horizontally. This really showed to us that landform geometry and noise direction are important in describing the impact helicopter noise has on these landforms—blanket statements like, “If a helicopter is x meters away, then a feature will vibrate x meters per second” do not fully capture the nuances of landform vibration response to energy sources.

Are the sounds emitted by helicopters unique, or could other sources of sound potentially cause damage to these rock features?

We are also investigating the impact that trains, road noise, and earthquakes can have on rock arches and towers, though those energy sources aren’t featured here.

What are the implications of this research, and why should people care?

I think too often we consider the desert landscape as still, and without much life. Our vibration measurements remind us that the rocks around us are moving all the time, and this particular study helps to remind us that we might be impacting the natural evolution of the landscape in ways we don’t even consider. I think people would generally oppose taking a jackhammer to Delicate Arch, but not everyone has the repercussions of helicopter flight near these culturally significant landforms at the top of their list of environmental concerns. Arches and towers are iconic in the desert southwest so it’s important that we work towards preserving these important features.

Have these results (or similar results by yourself or colleagues) been previously publicized by your institution or reported on by the press? If so, where and when?

None of our past-published works have focused on helicopter-induced resonance of natural landforms, but “Anthropogenic sources stimulate resonance of a natural rock bridge” (Jeffrey R. Moore, et al., 2016) received publicity by the University of Utah and other media outlets. That paper highlighted how an artificial reservoir, Lake Powell, excited resonance in Rainbow Bridge, one of the world’s largest natural bridges.

[Jeffrey Moore also published an article in the Fall 2015 issue of SPAN, the newsletter of the Natural Arch and Bridge Society, on “Monitoring the Structural Dynamics of Natural Arches” and reports on that work can be found at https://geohazards.earth.utah.edu/arch.html.]

How did this work come about?

This work came about by the Native American Consultation Committee (NACC, formed in the 1990s for indigenous groups to provide official stances on the regulation and management decisions for Rainbow Bridge National Monument) raising the question of how helicopter tours were impacting the structural integrity of Rainbow Bridge. The NACC is comprised of representatives from the Hopi, Kaibab Paiute, Navajo, San Juan Southern Paiute, Ute Mountain Ute, and Zuni tribes, all of which share a historical connection with Rainbow Bridge. While Rainbow Bridge resonates at such low frequencies that it is unlikely to be susceptible to damage from helicopter noise, the question opened up further exploration with smaller arches and towers that could resonate at the frequencies emitted by helicopters.

Red Butte Wilderness, Utah

Story and photos by Adam Elliott.

Red Butte Wilderness is adjacent to and south of the Kolob Section of Zion National Park. Red Butte Wilderness was created in 2009, contains 1,537 acres, and gets little visitation. There are no trails in the area and it’s very brushy.

Red Butte itself contains an arch (top and next two photos). The Butte has an elevation of 7,420 feet and rises 1,800 feet above the Kolob Reservoir road about 10 miles north of the Virgin River. The coordinates for the Butte are 12S-309870-4137310. It’s a technical climb to actually attain the arch.

I have (so far) found three other arches in Red Butte Wilderness. The first arch, which I’m referring to as Fisher Arch, is high above the canyon floor and is situated in a position where it doesn’t get sunlight for much of the day. Despite being a very large arch it blends in with the background and as a result is not easily noticed. The first time I saw it I was far away and only had a wide angle lens. Even looking at the pictures later, zoomed up on my computer, I could not tell for sure if it was an arch. So it nagged at me for almost a year before I went back to check it out and take the photos below.


Approaching Fisher Arch.

Closer view of Fisher Arch.

I recorded the coordinates with Gaia GPS on my phone (12S-309643-4136684) and took measurements as best I could with a laser meter (it’s approximately 65 feet high and around 30 feet at it’s widest). It’s not easy to move around the arch but it can be reached with a bit of a class 3+ scramble. The view from behind is shown below.

The second arch I thought this was just an interesting hole in the cliff until I got close and noticed it was separated from the wall behind it. I call it Pietila Arch and the coordinates are 12S 309935 4136444. the height is about 44 feet from below the boulders, and the span is about 20 feet. I tried to climb up the front entrance but the boulders in the photo below are about 10 feet high and I wasn’t able to get up them.

But the back side can be reached via some steep scrambling through brush and sand. Behind is is a nice little slot canyon shaped like a combination of a “V” vertically and a footprint like an “L” so water flows through here in storms. The photo below is looking through the arch from the back side.

In the photo above, you can see the arch in the center going through the rock to the left. The slot canyon goes off to the right. Another view of the arch can be seen below.

The third arch I call Adams Arch. It is a small one (about 8 feet by 8 feet) but is quite pretty once you are up inside it. It’s up on a wall, again, on the east side of the hidden canyon behind Red Butte. The coordinates are 12S-309584-4136809. In the approach photo below it is on the very far right.

Here is a better view:

And the view from inside:

Wat Tham Mongkut, Thailand

By Roderick Wayland Bates
Associate Professor, Nanyang Technological University, Singapore

Ratchaburi is a large, mostly agricultural province to the West of Bangkok, Thailand. It is dotted with highly eroded limestone hills, many of which contain caves, and many of which host Buddhist temples. One of these hills is at Wat Tham Mongkut, surrounded by fields of sugar cane. The hill here has a small cave (tham = cave) used as a Buddhist shrine. Up at the top of the hill, overlooking the temple, is a natural arch, clearly visible driving into the grounds, and even visible if you come along the main road from the North. Although there is no easy access to the arch, Martin Ellis of the UK’s Shepton Mallet Caving Club, who maintains a database of caves in Thailand, gives a span of 10 m (32 feet) and a height of 30 m (98 feet), while giving the hill a height of 100 m (320 feet).

At the right time of year, the rising sun shines directly through the arch. You can see a video of this on YouTube (it is five minutes long and changes very little throughout). A still from the video is below.

 

Zapato de La Reina, Tenerife

Photos by Mark Berry

Zapato de La Reina (NABSQNO 28R-337392-3121540) is a natural arch in Teide National Park  on the island of Tenerife, the largest of Spain’s Canary Islands.  It is located  a short walk from a parking area at kilometer 50 of the TF-21 road. It has a span of about 14 feet and a height of about 19 feet.

Teide National Park features the Teide-Pico Viejo stratovolcano that, at 3,718 m, is the highest peak in Spain. Rising 7,500 m above the ocean floor, it is regarded as the world’s third-tallest volcanic structure.

At least two other of the Canary Islands harbor arches as well.  Los Arcos de Graziola is found on the island of Graciosa (photo here).  There are many more, primarily collapsed lava tubes, around the coast of the island of Lanzarote.

Large Remote Natural Arch in Jasper National Park

This feature known only as “Natural Arch” is carved out of Palliser limestone on the  southwest slope of Mount Perce in the Canadian Rockies. The arch is located along the upper reaches of Blue Creek in northern Jasper National Park. These photos and most of the information is courtesy of Brian Catto.

The  coordinates are 11U 375960 5922070 (or at least very close) and our best estimate of the span  from analysis of Brian’s photos is 150 plus or minus 15 feet. The photo below is taken near Caribou Inn Hiker Camp, the closest hiker campground to the arch.

Click image for larger version.

There is a horse camp called Natural Arch right at the base of the slope below the arch, but the arch cannot been seen from the horse camp. Brian climbed up near the arch and a closer photo is below. If you click for the larger version you can see two mountain goats on top of the arch (upper right of the opening).

Click image for larger version.

The shortest route to the arch involves 40 miles of trail one-way from Rock Lake using the Willow Creek, North Boundary, and Blue Creek trails. The trails are open to hikers and horseback riders. Brian hiked in but reported that he thought the horse camps were nicer than the hikers camps. A backcountry camping permit is required from the Jasper National Park trail office.

This area of the park is closed to all human use from November through February as part of Parks Canada’s woodland caribou recovery plan. The trail up Blue Creek receives little to no maintenance and a trail bridge washout requires a stream ford which would be difficult before late in the season (late August and September). Brian describes the trip as requiring a lot of “planning, route finding, and determination.”

Arch in Kazakhstan

The Natural Arch and Bridge Society is indebted to Niklaus Stöcklin for reporting a natural arch in Kazakhstan.  The arch is located at 39T 675930 4878720 and has a span of 40 feet.

The arch is near the city of Aktau, which is located on the east bank of the Caspian Sea. The name means “white mountain” in Kazakh, which may be due to the white cliffs that overlook the Caspian. The arch itself is in white cliffs about 40 km east of Aktau.

Arch Collapse in Arches National Park (Rainbow Arch)

Rainbow Arch, located just above the visitor center in Arches National Park, collapsed sometime this winter. A park ranger noticed it was no longer standing during a hike in February. The arch was cataloged by Stevens and McCarrick as SA-137 and had a reported span of 11.7 feet.

A research team from the University of Utah, including Jeff Moore and Paul Geimer, had actually been studying this arch not long before the collapse. The “before collapse” photos here were taken by Jeff Moore, and the “after collapse” photos were taken by Paul Geimer. The “before-after” comparison below was assembled by Holly Walker.

The team made vibration measurements four times at the site in 2017, focusing on a prominent crack working through the center of the span that appeared to be putting the structure in jeopardy. The crack is circled in the photo below.

Close-up photo of the crack before collapse:

However, the team observed no changes in the crack or vibration characteristics over 12 months, and backed off on their monitoring program, believing the arch to be more stable that it appeared.

The collapse is bittersweet for the team, as it highlights and validates the fragility of these features, but unfortunately they were unable to record its last few weeks and months and identify accumulating damage.

Below is a photo after collapse.

While they can’t say for certain what caused the collapse, they believe the most likely explanation is that fatigue caused by daily and annual heating cycles finally stressed the tip of the crack enough to cause a runaway failure sometime this past winter.

Rainbow Arch lives on in the virtual world as an interactive 3D model made by the research team that can be found at https://skfb.ly/VMvB.

 

Rope Swinging Permanently Banned at Corona Arch and Gemini Bridges

Rope swinging at Corona Arch is now a thing of the past (photo courtesy Brian Mosbaugh at Slacklinemedia.com, Instagram: @Moabmonkeys).

BLM public information officer Lisa Bryant has supplied NABS with the following statement:

BLM-Moab has restricted roped activities, including swinging, for about 37 acres of BLM administered public lands, including Corona Arch (and nearby Bowtie Arch) and Gemini Bridges. The rest of the Moab Field Office area is still open for roped activities and includes several focus areas specifically for climbing and activities such as base jumping and highlining.

Gemini Bridges and Corona Arch are two outstanding geologic formations located northeast of Moab, Utah, in spectacularly scenic settings reached by short hiking trails. Corona Arch was acquired May 8, 2014 through the Utah Recreational Land Exchange, although the hiking trail leading to the arch has always crossed BLM managed lands.

Both features have been very popular destinations for hikers, sightseers and photographers for many years. It is estimated that 40,000 people visit Corona Arch and 50,000 people visit Gemini Bridges each year. Both geological features, but especially Corona Arch, are among the most often photographed sites on BLM lands.

In recent years Gemini Bridges and Corona Arch have become popular areas for a small number of visitors engaging in roped activities, such as highlining and swinging. This had led to a number of complaints from the public about the roped activities diminishing the experience of hikers and sightseers. Recently damage to the arch has also been noted from the rigging structures and ropes. On January 6, 2016 BLM issued a decision to temporarily restrict roped activities for two years, while it looked at appropriate management for the area. Following several public comment periods and environmental reviews, and the federal rule making process, that restriction became permanent on August 17, 2017.

Hawaii: A Few Big Island Arches

By Nick Terzakis

Pat and I visited the Big Island of Hawaii to do some arch hunting and snorkeling. The first day I drove from the Kailua-Kona International Airport: 2.2 miles north on State Hwy 19 there is a lava tube on the right which contains a natural tunnel (4Q-812026-2188066, estimated 50 feet);

and a lava bridge (4Q-811905-2188044, estimated 30 feet);

and another lava bridge (4Q-811901-2188039, estimated 20 feet):

On the way to Hilo we visited some beautiful waterfalls such as Kamaee Falls, Umauma Falls and Akaka Falls.

The second day I drove west to some more beautiful waterfalls such as Rainbow Falls, Peepee Falls and Wailuku Falls. Next, I drove south on State Hwy 11 to Hawaii Volcanoes National Park and then down the Chain of Craters Road to the end of the road and walked to the cliffline to see Holei Arch (5Q-279568-2134773, estimated 50 feet):

On the way back up on the Chain of Craters Road I found a natural arch at a pullout on the south side of the road and call this “Pele’s Eye” (5Q-266481-2142225, 4 feet). This arch is 14.6 mile from the end of the Chain of Craters Road or 0.4 miles past  a road to Mauna Ulu:

We then visited the Kilauea caldera which is amazing.

The third day I drove State Hwy 19 to Hapuna Beach and hiked south to M’s Arches (5Q-203756-2212281, estimated 4 feet and 8 feet), a nice double sea arch:

You can also reach this arch from Beach 69 (Waialea Beach) or a 4WD. The next area we visited was Kuki’o Beach which has two sea arches, each 4 feet in size (4Q-814140-2194122):

From State Hwy 19 turn west onto Kaupulehu Road and then turn left on a road (Aina Kaha Pl.), turn right before a gate up ahead and get a visitors pass from a guard. Go back to the gate and show the pass and the road will soon end at a parking area. Walk the trail to the beach. The last area was Noio Point which has two arches (5Q-911379-276932, estimated 5 feet and 20 feet). There are a dozen sea arches near Honaunau Bay that can be seen if you take a snorkel boat tour. Aloha!