USGS
Hawaiian Volcano Observatory

State-of-the-Art Volcano-Monitoring Instruments Installed
on Mauna Loa and Kilauea volcanoes

View of Hilo and Mauna Loa Volcano, Hawai`i
Photograph by S.R. Brantley on 11 March 2000
Mauna Loa looms above the city of Hilo and Hilo Bay. This view is from Mokuola, commonly called Coconut Island. Mokuola and the rock in bottom of photograph formed from a lava flow erupted by Mauna Loa about 1,400 years ago. A flow from Mauna Loa entered what is now Hilo in 1881, and another neared the city in 1984. Improved geophysical monitoring of Mauna Loa is needed, and the following text describes the most recent advance.

In our latest effort to improve the volcano-monitoring instruments on Mauna Loa and Kilauea, we have just completed the installation of the most sensitive instruments available for monitoring the strain deep beneath the surface of a volcano. Three holes more than 100 m deep were drilled into Mauna Loa, and an existing deep hole in Kilauea's summit area was also used. In each hole were installed an ultra-sensitive strainmeter and a seismic package consisting of a three-component broadband seismometer and a strong-motion sensor. Currently the installations are being tested and modified where necessary. Later this year, a borehole tiltmeter may be added high in each hole. Once all of this is completed, the state-of-the-art instrumentation will radio data to the Hawaiian Volcano Observatory in real time and make possible the early detection and tracking of events occurring deep within Mauna Loa and Kilauea with unprecedented clarity.

The instruments were installed in a collaboration between USGS scientists from HVO and the Earthquake Hazards Program (Menlo Park, CA) and scientists from the Center for the Study of Active Volcanoes (CSAV), University of Hawai`i in Hilo. The Carnegie Institution of Washington - Department of Terrestrial Magnetism manufactured the instruments and advised us in their installation. Funding for the drilling came through a grant from the Department of Defense via NASA.

The project received invaluable assistance from Hawai`i Volcanoes National Park, the Mauna Loa Observatory, the Mauna Loa High Altitude Observatory, and Hokukano Ranch. The holes on Mauna Loa were drilled by DOSECC (Drilling, Observation, and Sampling of the Earth's Continental Crust, Inc.).

Map of drill holes
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Red stars show locations of deep holes on Mauna Loa and Kilauea containing the new instruments. From west to east, the locations are Hokukano Ranch, Mauna Loa Observatory, Strip Road, and Keller Well (southern Kilauea caldera).

The network of three dilatometers on Mauna Loa should allow detection and tracking of strain changes that occur before and during the next eruption, thereby enhancing our ability to anticipate when--and possibly where--it might occur. The fourth strainmeter, in the southern part of Kilauea's caldera and situated to monitor the deformation of that volcano, may also detect changes on Mauna Loa.

Strainmeters

Three single-component strainmeters (also called dilatometers) on Mauna Loa and one on Kilauea were cemented deep in the ground. Each strainmeter is a stainless steel pipe about 3 m long and 10 cm in diameter filled with silicon fluid. As moving magma or earthquakes causes the ground to change shape, the dilatometer is squeezed like a balloon. The amount of strain is precisely determined by measuring the flow of the silicon fluid into or out of the dilatometer into a secondary reservoir. Dilatometers are so sensitive that they can easily detect the small deformation of the Earth's crust caused by gravitational attractions of the sun and moon and by the loads applied to the Earth's surface by passing weather fronts. These types of changes are filtered out by analysis software, and what is left is a measure of the deformation of the ground. The strainmeters are sensitive to a few parts in 100,000,000! Already the Strip Road and Hokukano dilatometers have clearly detected a small, 7 microradian tilting event that took place at Kilauea's summit on September 24--not even on Mauna Loa, and as far as 60 km away!

Diagram showing installation site of straimeter and seismometer

NOTE: diagram is not to scale.
This simplified cross section shows a typical installation of a strainmeter and seismic package on Mauna Loa. The strainmeter is encased in expansive grout 100-120 m below the surface, and the seismometers are encased in cement about 10-20 m above the strainmeter. The strainmeter is installed below the steel casing, because it must be secured to the surrounding rock so that it can "sense" the pressure exerted on the rock.

Three-component broadband seismometer

The strainmeter systems are complemented with packages of seismic sensors. Two sets of seismic instruments are placed in each hole. One set, referred to as a broadband system, features uniform, well-characterized sensitivity to a wide range of frequencies of ground motion. The other set is a strong-motion system that allows us to record faithfully the ground motions produced by nearby large earthquakes; standard seismometers go off scale during such earthquakes. Unlike typical seismic sensors placed at the ground surface, the systems must be somewhat miniaturized to fit in the drill holes, and a customized electronic package is required to provide appropriate sensitivity and response. The data from these drill holes will be used, in conjunction with the data from our more traditional seismic systems, to monitor and catalog seismic activity, both regionally and from Mauna Loa, in unprecedented ways.

Three component seismometer before being sealed and installed in drill hole
Three-component broadband seismometer ready for installation in a drill hole. The person's right hand holds the part of the instrument housing the vertical sensor. The person's left hand cradles the part with both horizontal sensors. The size and cylindrical shape of the package are governed by the shape and diameter of the drill hole and are not typical of broadband seismometers in general.

Installation steps

Coring methods were chosen over conventional rotary drilling methods for two reasons. A straight hole is required to emplace the long delicate instruments, and core drilling is best suited to such a task. In addition, we wanted to retrieve the rock cores for geologic study. When the drilling operation reached at least 90 m below the surface, we examined the cores carefully to find solid basalt sections in which to install the strainmeter. A minimum depth of 90 m was required in order to shield the instruments from local effects such as daily temperature fluctuations. Once cemented into place, the electrical components were connected and the collection of radio-telemetered data was initiated.  

Installation site for monitoring instruments, Mauna Loa Volcano, Hawai`i
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Top: The DOSECC drilling rig on the northern flank of Mauna Loa at 3,350 m elevation. Mauna Kea rises above clouds in the background.

Bottom: Drillers attach a new pipe before coring another section of basalt. 

Close view of drillers working on drill rig, Mauna Loa Volcano, Hawai`i
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The rock cores pulled up from the hole were described in the field and painstakingly archived for later study by HVO and other scientists. Several ash layers were intersected by two of the holes. These layers are of particular interest, because they may be able to be dated. Ages of these ash layers would help us to constrain the ages of some lava flows from Mauna Loa. In addition, the layers tell us more about the recent explosive histories of Mauna Loa and Kilauea.

Geologist inspects rock from drill core, Mauna Loa Volcano, Hawai`i
The cored rocks were examined and logged by scientists from HVO and CSAV. These cores give scientists an unprecedented glimpse into the eruptive history of Mauna Loa volcano relatively near its rift zones.  

Drill core at depth of stainmeter installation, Mauna Loa Volcano, Hawai`i
A solid section of rock was required to firmly anchor the strainmeters. The core was examined with expectation once the minimum depth of 90 m was reached. At left, the long piece of unbroken basalt core at a depth from about 100 m at the Strip Road site indicated to scientists that this depth was suitable for the strainmeter.

Install instruments and cement in place

Pouring special cement mixture into hole to secure strainmeter in place, Mauna Loa Volcano, Hawai`i
The strainmeter was lowered into the hole once a suitable depth and rock interval were found. Then expansive grout was poured into the hole (left) to anchor the instrument to the surrounding rock. After the grout  hardened, the seismic instruments were lowered into the hole. Then cement was added to secure the seismometers in place (see diagram below).

Install electronics and telemetry system

Three component seismometer before being sealed and installed in drill hole
Finally the electronics packages for the strainmeter and seismic package were readied on the surface and connected to the instruments, power source, and telemetry system. The photo shows a completed installation at Hokukano Ranch on the west flank of Mauna Loa.


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Updated: 17 October 2000 (SRB, DAS, FSC)