Norcal Geophysical Consultants INC


Case Studies

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Plans are underway to construct a thirty-meter telescope near the summit of Mauna Kea on the island of Hawaii. At 13,700-ft, this is the highest point in the Hawaiian Islands and the locale for many telescopes. When it takes in its first light around the turn of the decade the proposed thirty meter telescope (TMT) will be one of the most powerful, if not the most powerful, optical/infrared telescope in the world.

The primary objective of the GPR survey was to provide information that will aid the preliminary design of the foundations for the telescope, its enclosure, the support building and for crane locations during construction. Specific information of interest includes the delineation of conditions such as layers of rock created by the surface flows of lava; layers or pockets of cinders, ash, and clinkers; and voids such as lava tubes and cavities.

30 Meter Telescope GPR

NORCAL collected high resolution GPR data at 1-ft intervals along a total of six lines ranging in length from 120- to 240-ft. The lines were positioned to cover the footprint of the proposed telescope.

Three of the lines were oriented north-south and three were oriented east-west. We collected the GPR data using a Sensors & Software Pulse Ekko Pro equipped with bistatic 100-MHz antennae as shown in the photos above and to the right.

Sample GPR Profile

The processed GPR images provided information on the variation in subsurface conditions beneath each GPR Line to depths in excess of 40-ft as illustrated by the sample GPR record shown above. We interpreted variations in reflection patterns as variations in the fracturing, weathering, and flow characteristics of the vesicular basalt underlying the project site. The interfaces between the various rock types are depicted by dashed yellow lines on the interpreted GPR image shown below. The red dashed line indicates the top and sides of a suspected void or lava tube. The white band along the top of the section represents the ground surface.

GPR with Interpretation

The fence diagram shown below illustrates the layout of the six GPR profiles and how they correlate. The GPR anomalies labeled H, M and N (red dashed lines) appear to align along a southwest to northeast trend suggesting that they represent a lava tube.

Fence Diagram