Chapter 3
Performing a Surface Vault Installation
Trillium Compact User Guide
16889R5
2014-03-04
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3.2.1 Choosing the Right Concrete
The concrete used in a seismic pier should be as homogeneous as possible to avoid
inducing tilts from differing thermal coefficients of expansion. To create a homogeneous
concrete mixture do not use any aggregates and ensure the concrete is free of air
bubbles. Steel reinforcement is not necessary as strength is not a concern in seismic piers.
The recommended concrete mixture is 50 percent Portland cement and 50 percent sieved
sand.
1
After pouring the concrete, shake it to allow trapped bubbles to escape. Allow
24-hours for the concrete to harden before positioning the seismometer on the pier.
3.2.2 Decoupling the Pier and Vault Walls
When setting up the concrete forms for the pier, include a gap between the edge of the
concrete and the walls of the vault. Decoupling the pier and the vault walls prevents the
transfer of non-seismic forces, such as wind, from the vault walls to the pier. Such forces can
cause the pier to tilt or twist and obscure the desired seismic signal. These signals are mostly
long period, so vault wall decoupling is critical for quiet site long period studies.
3.3 Common Types of Installations
Two common types of installations for the standard Trillium Compact are long-term vault
installations and temporary installations on rock. The following sections describe each of
these installation options.
3.3.1 Vault Installations
Vault installations can be at or below the surface and usually include a pier that provides
a level platform for the seismometer to sit on and good coupling to the ground (see
Section 3.2 “Recommendations for Pier Construction”
14). Insulation of the pier,
vault roof, and most importantly the seismometer, is required.
The pier must be insulated from air currents to prevent tilt noise caused by the thermal
expansion or contraction of its surface. For a pier solidly connected to the ground (such as
a poured cement pad on top of bedrock), a useful technique is to place a thick quilt over
the surface of the pier. Cutting a hole out of the quilt allows it to drop over the insulating
cover of the seismometer and cover the pier.
Thoroughly insulate the roof of the vault and any exposed sides. Seal the door and any
other openings. Do not use a thermostat-controlled heating or cooling system in the vault
because the temperature cycling will show up as periodic noise in the seismic signal.
Insulate the seismometer to protect it from drafts and temperature change. To achieve
optimal insulation of the seismometer, the following optional equipment is recommended:
a Carrying Case / Insulating Cover (Nanometrics part number 16862). In addition, if you are
1. Bob Uhrhammer and Bill Karavas,
Guidelines for Installing Broadband Seismic Instrumentation
The Regents of the University of California, 1997),
http://seismo.berkeley.edu/bdsn/instrumentation/guidelines.html
.
The pier may generate spurious signals as the concrete cures, which can take two to
four weeks.
