Category Archives: 1. ARCH INFO

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]

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.

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


Azure Window Collapses

Famous Azure Window in Malta collapsed during a storm on March 8, 2017. Not only did the lintel itself collapse, but the entire outer column fell into the sea.

The arch was considered at risk of collapse and walking across the arch was prohibited only last December. And in January, a slab of rock fell off a lateral face of the outer column during high seas.

The spectacular arch was featured in many movies, including Game of Thrones. Some history of partial collapse can be seen in these movies. In the scene below from the 1981 movie Clash of the Titans, the underside of the arch is flat.

In the scene below from the 2002 movie The Count of Monte Christo, it can be seen that the underside of the lintel has fallen to create a more arced shape.

Valley of Fire Arch Rally

The October 2016 NABS Valley of Fire Arch Rally in Nevada was, as usual, filled with fun and camaraderie. Presented here is a slide show of photos taken by NABS President Larry Beck. Click on any image to start the slide show. The title of each image in the slide show provides the UTM WGS84 coordinates (the NABSQNO number) in zone 11S, and where present the name and catalog number of the arch. Catalog numbers refer to the Arch Hunter Books Valley of Fire series (available to NABS members).

Portland Head Lighthouse Arch, Maine

By Nick Terzakis

Maine is not known for natural arches but there were four small ones reported in the Journal of Natural Arch Discoveries.

This October, Pat and I  drove up the Maine coast stopping at some nice lighthouses. At the Portland Head Lighthouse in South Portland, ME you can see Portland Head Lighthouse Arch (ME-4), which is just south of the lighthouse.



Directions: In Portland, drive State Street south across the bridge into South Portland and turn left onto Cottage Road. Turn left into Ft. Williams State park and head to the lighthouse parking lot. You can see the arch to the south. Walk the trail south along the fence then go uphill to the top of the cliffs. Go left on a faint trail which goes on a pebble beach, and go right around the rocks to see the arch (only if it is low tide). The arch has a span of three feet.

Unfortunately nearby Portland Head Arch (ME-2) has fallen. It had a span of four feet. A scan of my old Polaroid photo is below.



Arch collapse at Legzira Beach, Morocco

The photo below shows two large natural arches at Legzira Beach, Morocco, in the Province D’Agadir about 10 kilometers north of Sidi Infi.

Legzira Beach - Victor Kaposi

The arch in the foreground, NABSQNO 29R 391178 3257234 with an estimated span of 60 feet, collapsed on September 23, 2016. The remains are shown in the photo below.

Legzira collapse

The remaining arch, with an estimated span of 90 feet, still stands. Our own Guilain Debossens stands by the arch in the photo below.

Legzira 2 - Guilain