Optimizing Borehole Station and Array Performance, Enabled by the Trillium Slim Borehole 120 Seismometer

Date: 4/24/2019

Time: 03:00 PM

Room: Grand Crescent

Downhole installation offers the potential for best seismic performance but also greater uncertainties compared to a near-surface direct bury or vault installation. This talk surveys some typical pitfalls, solutions, and future directions for cased borehole stations based on Nanometrics’ experience.

A heuristic model is presented of the installation as a series of material interfaces where each presents a potential source of mechanical noise and affects the system transfer function. The single most important factor is the location of the seismometer relative to the interface of loose soil to solid rock. There are also a series of connections from the seismometer to a holelock or sand, to the casing, to cement, then to soil or rock, and also up to the wellhead via the cables and casing. Each of these interfaces can affect performance, which we illustrate with data and recommendation of best practices.

We also present Nanometrics’ new borehole instrument: the Trillium Slim Borehole 120 seismometer, a 4.1 inch (104 mm) diameter instrument with Trillium 120QA/PH class performance having significantly improved SWaP (size, weight and power). It is designed for smaller holes down to 4.5” or 115 mm diameter, in shallow or deep deployment, using a simple passive holelock or sand installation. The small diameter permits deployment in existing small boreholes, and facilitates construction of new lower cost boreholes, minimizing disturbance of the surrounding rock and improving instrument coupling.

High performance and simpler logistics makes the Trillium Slim Borehole well suited for many applications, including new higher-density arrays for full waveform analysis and detection of earthquake gravity signals. In conclusion we present a proposed array design for optimal measurement of earthquake gravity signals on a regional scale.

Presenting Author: Geoffrey Bainbridge

Authors