C. R. Hutt

Laboratory and Field Testing of Commercial Rotational Seismometers

Abstract There are a small number of commercially available sensors to measure rotational motion in the frequency and amplitude ranges appropriate for earthquake motions on the ground and in structures. However, the performance of these rotational seismometers has not been rigorously and independently tested and characterized for earthquake monitoring purposes as is done for translational strong- and weak-motion seismometers. Quantities such as sensitivity, frequency response, resolution, and linearity are needed for the understanding of recorded rotational data. To address this need, we, with assistance from colleagues in the United States and Taiwan, have been developing performance test methodologies and equipment for rotational seismometers. In this article the performance testing methodologies are applied to samples of a commonly used commercial rotational seismometer, the eentec model R-1. Several examples were obtained for various test sequences in 2006, 2007, and 2008. Performance testing of these sensors consisted of measuring: (1) sensitivity and frequency response; (2) clip level; (3) self noise and resolution; and (4) cross-axis sensitivity, both rotational and translational. These sensor-specific results will assist in understanding the performance envelope of the R-1 rotational seismometer, and the test methodologies can be applied to other rotational seismometers.

20 – US Contribution to Digital Global Seismograph Networks

This chapter summarizes the contribution to digital global seismograph networks by the United States and provides information about station installation practices. The global effort is coordinated by the Federation of Digital Broad-Band Seismograph Networks (FDSN). Its membership comprises groups responsible for the installation and maintenance of broadband seismographs either within their geographic borders or globally. Electronic digital computers have resulted in many significant advantages by digitizing the analog records. Some of the advantages of digital seismograph systems include large dynamic range, fast process, easy to store, and high information yield. The digital networks developed and installed by the United States include High-Gain Long-Period (HGLP), Seismic Research Observatories (SR0), and Digital World Wide Standardized Seismograph Stations (DWWSSN). Seismic data acquisition systems exhibit several relevant features such as continuous recording of broadband data at 20 sps, accurate time tagging without human dependence, and real-time telemetry of data to data centers. These capabilities are achieved through the use of low-noise very broadband (VBB) primary seismometers in conjunction with high-frequency broadband or short-period seismometers and low-gain accelerometers.

Seismic noise on Rarotonga: Surface versus downhole

Seismic noise data are presented from the new Global Seismographic Network station, RAR, on the Island of Rarotonga in the South Pacific. Data from the first new borehole site in the GSN are compared with a surface vault installation. Initial indications from the data show that borehole siting on a small island significantly reduces long-period (>20 s) horizontal seismic noise levels during the daytime, but little or no improvement is evident at periods shorter than 20 s or on the vertical component. The goal of the Incorporated Research Institutions for Seismology (IRIS) GSN program is broad, uniform coverage of the Earth with a 128-station network. To achieve this goal and provide coverage in oceanic areas, many stations will be sited on islands. A major siting consideration for these new stations is whether to build a surface vault or drill a borehole. Neither option is inexpensive. The costs for drilling a cased hole and a borehole sensor are large, but the benefit of a borehole site is that seismic noise is reduced during certain periods when a surface installation may be subject to wind, weather, and thermal effects. This benefit translates into recording greater numbers of smaller earthquakes and higher signal-to-noise ratio.

Broadband Seismic Noise Attenuation versus Depth at the Albuquerque Seismological Laboratory

Abstract Seismic noise induced by atmospheric processes such as wind and pressure changes can be a major contributor to the background noise observed in many seismograph stations, especially those installed at or near the surface. Cultural noise such as vehicle traffic or nearby buildings with air handling equipment also contributes to seismic background noise. Such noise sources fundamentally limit our ability to resolve earthquake‐generated signals. Many previous seismic noise versus depth studies focused separately on either high‐frequency (>1  Hz) or low‐frequency (

Seismometer Self-Noise and Measuring Methods

Seismometer self-noise is usually not considered when selecting and using seismic waveform data in scientific research as it is typically assumed that the self-noise is negligibly small compared to seismic signals. However, instrumental noise is part of the noise in any seismic record, and in particular, at frequencies below a few mHz, the instrumental noise has a frequency-dependent character and may dominate the noise. When seismic noise itself is considered as a carrier of information, as in seismic interferometry (e.g., Chaput et al. 2012), it becomes extremely important to estimate the contribution of instrumental noise to the recordings. Noise in seismic recordings, commonly called seismic background noise or ambient Earth noise, usually refers to the sum of the individual noise sources in a seismic recording in the absence of any earthquake signal. Site noise (e.g., cultural sources, nearby tilt signals, etc.) and noise introduced by the sensitivity of an instrument to non-seismic signals (e.g., temperature and pressure variations, magnetic field changes, etc.) both contribute to the ambient seismic noise levels. The background noise ultimately defines a lower limit for the ability to detect and characterize various seismic signals of interest. Background noise levels have also been found to introduce a systematic bias in arrival times because the amplitude of the seismic phase must rise above the station’s noise levels (Rӧhm et al. 1999). The upper limit of useful signals is governed by the clip level of the recording system (the point at which a recording system’s output is no longer a linearly time-invariant representation of the input). Site noise can be reduced by careful site selection (e.g., hard rock far from strong noise sources) and by emplacing instruments in good vaults or boreholes. It is also possible to reduce sensitivity to non-seismic signals by thermal insulation and appropriate shielding such as pressure chambers (Hanka 2000). At quiet sites with well-installed instrumentation, instrument noise may be the dominant noise source (Berger et al. 2004); this is especially true for long-period seismic data (>100 s period) on very broadband instruments (e.g., Streckeisen STS-1 seismometer). The interpretation of such data only makes sense if the instrumental noise level is known. Also, research on noise levels in seismic recordings, the effect of noise reduction by the installation technique, and the nature and contribution of different noise sources to the recordings require knowledge of instrumental self-noise to rule out bias from the instrumentation self-noise.

Detection and Characterization of Pulses in Broadband Seismometers

Abstract Pulsing—caused either by mechanical or electrical glitches, or by microtilt local to a seismometer—can significantly compromise the long‐period noise performance of broadband seismometers. High‐fidelity long‐period recordings are needed for accurate calculation of quantities such as moment tensors, fault‐slip models, and normal‐mode measurements. Such pulses have long been recognized in accelerometers, and methods have been developed to correct these acceleration steps, but considerable work remains to be done in order to detect and correct similar pulses in broadband seismic data. We present a method for detecting and characterizing the pulses using data from a range of broadband sensor types installed in the Global Seismographic Network. The technique relies on accurate instrument response removal and employs a moving‐window approach looking for acceleration baseline shifts. We find that pulses are present at varying levels in all sensor types studied. Pulse‐detection results compared with average daily station noise values are consistent with predicted noise levels of acceleration steps. This indicates that we can calculate maximum pulse amplitude allowed per time window that would be acceptable without compromising long‐period data analysis.

Obtaining Changes in Calibration‐Coil to Seismometer Output Constants Using Sine Waves

The midband sensitivity of a broadband seismometer is one of the most commonly used parameters from station metadata. Thus, it is critical for station operators to robustly estimate this quantity with a high degree of accuracy. We develop an in situ method for estimating changes in sensitivity using sine‐wave calibrations, assuming the calibration coil and its drive are stable over time and temperature. This approach has been used in the past for passive instruments (e.g., geophones) but has not been applied, to our knowledge, to derive sensitivities of modern force‐feedback broadband seismometers. We are able to detect changes in sensitivity to well within 1%, and our method is capable of detecting these sensitivity changes using any frequency of sine calibration within the passband of the instrument.