People and jobs at HVO, Part 1: What on earth is a geodesist?

People and jobs at HVO, Part 1: What on earth is a geodesist?

_{Geophysicist Sarah Conway, a member of the USGS Hawaiian Volcano Observatory’s deformation team, readies a temporary GPS station during a campaign survey of benchmarks on Kīlauea Volcano to monitor changes in the ground surface. USGS photo by I. Johanson.}

The USGS Hawaiian Volcano Observatory (HVO) is responsible for monitoring active volcanoes in Hawaii, which involves assessing hazards and communicating with interagency partners and the public. Our mission also includes advancing our knowledge of volcanic processes through scientific research.

It takes many people in diverse roles to accomplish these goals. During January’s Volcano Awareness Month, our Volcano Watch articles will introduce readers to some of the people and jobs at HVO.

One role at HVO is that of “geodesist,” which is someone who studies the sub-field of geophysics called “geodesy.” I usually avoid calling myself a geodesist when meeting new people because it often results in puzzled looks. Instead, I use HVO’s terminology for our team, and say that I’m part of “the deformation group.”

This informal team name gets at the heart of what we do: we study how the surface of a volcano deforms as a way to determine what is happening underground.

Geodesy is primarily concerned with precise measurements of the earth, such as might happen during a geodetic survey. Results from surveys after the 1906 magnitude-7.9 San Francisco earthquake, which offset fence lines and property boundaries, had a profound impact on our understanding of how faults move—and ultimately brought geodesists into the earth sciences.

A geodesist’s tools are similar to those of a surveyor. In the past century, triangulation and leveling were popular techniques. Today, Global Positioning System (GPS) instruments form the backbone of our monitoring program, which also includes borehole tiltmeters and satellite radar (InSAR).

The general approach to using geodetic data on a volcano is to perform multiple surveys to determine how benchmark positions have changed. As magma moves into a volcano, the surrounding rock is pushed outward. When we measure positions of benchmarks on the surface of the volcano, we find that they have also been pushed away from the magma source. Today, permanently installed instruments constantly monitor benchmark positions so we can see ground motion within minutes.

Growing and maintaining HVO’s permanent geodetic instrument network is one of the deformation group’s most important jobs. This permanent network consists of over 60 GPS stations and 16 tiltmeters, and data from it are critical for hazard assessment. In particular, tiltmeters, which are incredibly sensitive to changes in ground slope, are often the first indicator of inflation as a volcano pressurizes.

While HVO’s deformation group is responsible for analyzing and interpreting the data, it takes many others to keep the network running. HVO’s field engineers build, install, and maintain our field instruments. IT staff ensure that our computers can communicate with remote sites from which data are transmitted and that we are equipped to analyze the data.

We supplement the permanent geodetic network with annual campaigns to collect additional benchmark data using temporary GPS stations. Around 80 benchmarks are surveyed each year for 2–3 days to determine yearly changes in position. These surveys provide a higher density of measurements in certain areas, enabling us to more precisely determine deformation patterns over many years.

To help interpret geodetic data, I use computer models that calculate the expected motion at the earth’s surface due to expansion or contraction of magma bodies with simplified shapes, such as spheres or ellipsoids. Simple shapes are used because they adequately match the data and are less time-consuming to calculate than irregularly shaped bodies. Time is important because I run many thousands of calculations to test different models to discover the position, length, width, depth and volume changes that best match the data we’ve collected.

The best-fitting model shows us the most likely place that magma is moving into or out of the volcano, as well as where magma is accumulating and how close it is to the surface. However, no single type of data gives the whole picture of a volcano, so we must interpret our geodetic data along with geologic, seismic and gas data. HVO’s different teams come together as a whole to develop sound hypotheses for current activity, hazard levels, and future scenarios.

Next week, another HVO team writes about its work. Until then, we hope to see you at one or more of our Volcano Awareness Month programs. The schedule is posted on HVO’s homepage (https://volcanoes.usgs.gov/hvo/).