Geodesy through Time: A History of Measuring the Shape of Hawaiian Volcanoes

Geodesy is the science of measuring and understanding the exact geometry of the Earth, the gravitational field, and orientation in space – and how these change over time. Many geodesy scientists today map shorelines, define land boundaries, and improve transportation and navigation.

In the last century, geodeticists have used various surveying tools and methods to help measure the shape of the Earth. While these scientists can wear many different hats, geodesy scientists at the US Geological Survey Volcano Observatory in Hawaii are interested in studying how the surface of a volcano is deformed to determine what is happening underground.

This article provides a brief history of the different volcanic geodesic methods used over time.

Triangulation is one of the most appreciated and earliest methods, and it gained momentum by the nineteenth century. Triangulation is the process of locating a point by measuring the angles (of a triangle) between control points at a known, widely spaced distance. This allows the measurement of long distances, which are only controlled with the sight of the beacon. Each calculated distance can then be used to form another triangle that forms a complex chain. The first triangulation occurred in Kilauea in 1896.

While similar to the geometry you learned in school, the formulas used in triangulation involve the curvature of the Earth due to the miles length of lines. Triangulation has been used sporadically to monitor the volcanoes at Kilauea, although it was replaced in the mid-20th century with a more accurate electronic measurement of distance.

Triangulation is a similar scanning technique but instead of measuring angles, the side lengths of the triangle are measured. Any distortion in the existing grid area can then be measured by scanning at a later time.

Leveling, a technique used to measure elevation differences, has been performed at Kilauea since 1921 and has been an integral part of HVO monitoring from the early 1960s through a decade. During leveling, differences in height are measured along a series of parameters and then compared with previous measurements to determine the amount of rise or fall.

Leveling remains one of the most accurate method for measuring vertical elevation over long distances. Unfortunately, leveling is rarely done at Kilauea today due to the number of personnel required and the expansion of modern satellite-based technologies, such as the Global Positioning System and Interferential Modular Aperture Radar (InSAR).

In the mid-1960s, the electronic distance meter (EDM) was introduced to Kilauea. With EDM, distances can be measured electronically quickly and accurately. Most EDMs use lasers to measure the time of light passing through the atmosphere of reflective targets, which are placed around the volcano.

Using travel time, speed of light, air temperature and pressure, the distance can be calculated with an accuracy of 0.04 inch over a distance of 0.6 miles. If the distance between surveys changes, then distortion occurs! Most of the EDMs in use today are of short range and are a component of an integrated instrument known as the “total station”. Since the late 1990s in HVO, EDM devices have given way to satellite technologies.

Microgravity surveys have been conducted on the island of Hawaii since the early 1970s. The force of gravity depends on the mass underneath your feet and your distance from that mass, so gravity is not static as magma moves and changes the shape of the Earth’s surface. For volcanologists, microgravity surveys are conducted to measure the amount of magma that may be moving underground. Gravimeters are extremely accurate (and expensive) instruments that use either a highly sensitive spring and arm sensor or a quartz sensor.

Inclinometers have been used to monitor volcanoes for decades. Just like a carpenter’s level, an electronic inclinometer uses a small container filled with a conductive liquid with a bubble to measure the change in slope. There are a variety of tilt meters in use but the majority of Hawaiian tilt meters are installed in wells no less than a few meters (5-20 feet) deep to control surface noise.

The most common geodetic tools used by HVO today are the Global Navigation Satellite System (GNSS), which includes GPS and InSAR; These are described in two recent articles from “Volcano Watch” (see here and here).

Geodeticists from all over the world come to the island of Hawaii to study the ever-changing shape of the Earth, making Kilauea and other Hawaiian volcanoes a “wonderful natural laboratory!”

Visit https://volcanoes.usgs.gov/observatories/hvo/ to see past Volcano Watch articles, previous Volcano Watch articles, Kilauea and Mauna Loa updates, volcano photos, maps, recent earthquake information, and more. Questions emailed to [email protected].

Volcano Watch is a weekly article and activity update written by the scientists and affiliates of the Hawaiian Volcano Observatory of the US Geological Survey.

Volcano activity updates

Kilauea volcano does not erupt. The USGS Volcano alert level remains at NORMAL (https://volcanoes.usgs.gov/vhp/about_alerts.html). Kilauea updates are released monthly.

Kilauea monitoring data for last month show variable but typical rates of earthquakes and ground deformation, low rates of sulfur dioxide emissions, and only slight geological changes since the end of volcanic activity in September 2018. The lake of water at the bottom of Halema’uma continues to expand and deepen slowly. For the most recent information on the lake, see https://volcanoes.usgs.gov/volcanoes/Kilauea/summit_water_resources.html.

Mauna Loa does not erupt and is still at Volcano Alert Level. This alert level does not imply that an eruption is imminent or that progression to an outbreak from the current level of disturbance is certain. Mauna Loa updates are released weekly.

Last week, about 90 small-scale earthquakes were recorded under the upper elevations of Mauna Loa; Most of these occurred at shallow depths of less than about 5 miles. GPS measurements show a slow rise in long-term summit magnification, consistent with the supply of magma to the volcano’s shallow storage system. The gas concentrations and fumarole temperatures as measured at both the sulfur cone and the summit remain stable. Web cameras do not show any changes to the landscape. For more information on the current monitoring of Mauna Loa volcano, see: https://volcanoes.usgs.gov/volcanoes/mauna_loa/monitoring_summary.html.

There have been two events with three or more reports on the Hawaiian Islands over the past week: a 2.7-magnitude earthquake 6 miles east and northeast of Pahala on August 25 at 10:41 p.m. and a 3.0-magnitude earthquake less than a mile away. East – Southeast Pahala on August 23 at 10:50 am

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