Data Processing

DATA PROCESSING

Data Types and Formats || Processing of Digitially Recorded Data || BAP Software and Manuals

Data Types and Formats

Digital data available through the USGS NSMP Data Center are acquired from both analog and digital instruments. The various types of data served from the web site include:

Uncorrected Acceleration (Vol 1) - the term "uncorrected" indicates that a digitized or digitally recorded time series has received no processing that involves any hypotheses as to the character or frequency content of the ground motions or recording instruments. An "uncorrected" analog-recorded time series has been corrected only for uneven film transport with time, for transverse motion of the film as it moved longitudinally through the recorder, and for angular tilts between the baselines of adjacent scan segments of long records. Time series derived from both analog and digitally recorded data have been shifted to have zero mean; and have been converted from digitization units or volts to units of cm/sec/sec (ordinates) and seconds (abscissas). The sampling rate for all time series data, including acceleration, velocity, and displacement is typically 200/sec.

Corrected Acceleration data (Vol 2) - The "correction" process may apply baseline corrections, bandpass filters (removing noise contamination) and instrument correction (as necessary, to remove the effects of frequency-dependent instrument response) to a time series. Differences between the processing of analog and digitially recorded data are described below. Software, named BAP, that will correct the time series is also available from the NSMP web site.

Corrected Velocity (Vol 3) - For analog data and some of the digitally recorded data (see below), the velocity time series are derived by integrating the corrected acceleration time series. For some digitally recorded data, the velocity is derived by integrating the baseline-corrected but unfiltered acceleration time series and then filtering the velocity (see below).

Corrected Displacement (Vol 4) - For analog data and some of the digitally recorded data (see below), the displacement time series are derived by integrating the velocity time series. For some of the digitally recorded data, the displacement time series are derived by doubly integrating the baseline-corrected but unfiltered acceleration time series and then filtering the displacement (see below).

Response Spectra (Vol 5) - Response spectra of relative displacement, relative velocity, pseudo-velocity, and absolute acceleration are computed for damping values of 0, 2, 5, 10 and 20 percent. The periods at which response spectral values are computed range from from 0.04 sec to a long period limit that is determined by the corners of the filters applied to the corrected time series data.

Fourier Amplitude Spectra of Corrected Acceleration (Vol 6)

A general description of the SMC data format currently used for NSMP time series and spectral data can be viewed at /smcfmt.html or smcfmt.txt (text version).

Notes on the processing of digitally recorded data

The NSMP is currently engaged in a program to upgrade from analog to digital instruments. We have determined that the acceleration data being acquired even by these newer digital strong motion instruments are in some cases subject to small baseline instabilities, often appearing as step-like or transient offsets. There are indications that some instabilities occur when the ground shaking from earthquakes exceeds levels as low as about 10 cm/s/s. The cause of these offsets and their amplitude characteristics through time are not well understood at present, nor can these offsets be easily identified in the acceleration records from earthquakes in order to make appropriate corrections.

Thus far, we have found baseline instabilities to produce negligible effects (typically less than about 1 cm/s/s) in acceleration, but significant long-period signal distortions can be produced in velocities and displacements derived by integrating acceleration time series affected by such offsets. Nonetheless, experiments with a variety of processing procedures indicate that, even with such baseline instabilities present, response spectral values computed from unfiltered digital records are reliable to periods of at least 10 seconds.

In order to address these problems, we have developed a new scheme for semi-automated processing of the digital data. Due to variations in both event-triggering parameters and signal character, two classes of event signals are recorded: those with pre-event signal and those with no pre-event signal. Recognizing that the constraints on initial conditions are different for these two cases, two processing schemes are used as described below:

Constrained processing steps (used for events with pre-event signal, particularly those with very long pre-event signal of tens of seconds, but also applicable to events with no pre-event signal):

Remove a mean from the whole record, where the mean is determined from t = 0 to t = t_p, the approximate onset time of the earthquake signal. [if t_p <= 0.0, then the mean of the entire record is used.];
integrate the mean-corrected acceleration to velocity;
For ill-behaved velocity time series (e.g. significant linear trends), higher-order baseline corrections may be applied.
low-cut filter the baseline-corrected acceleration with an acausal, 8^th-order low-cut Butterworth filter that has a corner frequency determined by examining the Fourier Amplitude Spectrum, and a high-cut filter in the form of a cosine taper in frequency;
use BAP to integrate the baseline-corrected, filtered acceleration to obtain velocity and displacement, to compute Fourier amplitude spectra of corrected acceleration, and to compute response spectra.

The most significant departures of this new scheme from previous methods used by NSMP for digital data derived from analog recordings are the fitting of a quadratic instead of a linear function to velocity to obtain the baseline correction for acceleration, the use of an acausal instead of a causal Butterworth filter.

Like the data for analog recordings, the data being served from the NSMP web site for digital instruments include time series for uncorrected and corrected acceleration, time series for velocity and displacement obtained by integrating the corrected acceleration (bearing in mind that displacements derived by doubly-integrating filtered acceleration can differ significantly from actual ground displacement), Fourier amplitude spectra of corrected acceleration, and response spectra for relative displacement, relative velocity, pseudo-velocity, and absolute acceleration. The response spectral values for digital recordings are computed to periods as long as 15 s and damping of 0, 2, 5, 10 and 20 percent.

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Basic Strong-Motion Accelerogram Processing Software (BAP)

The Basic Strong-Motion Accelerogram Processing (BAP) computer program was developed at the U.S. Geological Survey (USGS) to process and plot digitized strong-motion earthquake records. BAP will calculate velocity and displacement from an input acceleration time series or it will calculate acceleration and displacement from an input velocity time series. The program will make linear baseline corrections, apply instrument correction, filter high frequency and/or low frequency content from the time series, calculate the Fourier amplitude spectrum, and calculate response spectra. It will also plot the results after each processing step. A general description of the SMC data format currently used for NSMP time series and spectral data can be viewed at /smcfmt.html or smcfmt.txt (text verson).

BAP can process time-series data files from the Strong-Motion CD-ROM that is also available from the USGS and should provide useful data processing functions to organizations outside the USGS that have acquired that CD-ROM (these same data are available online). BAP will also process the recently acquired time-series files posted on the NSMP web site (see "Data Sets" page).

You will need a PC program that decompresses and "unzips" the downloadable BAP archive. The archive includes BAP version 1.1 executables, BAP demos, and BAP version 1.1 source code. BAP is a DOS program and has been verified to run in DOS mode on Windows 95, 98, Me, and 2000 operating systems. BAP will NOT run on Windows XP (Microsoft no longer supports Fortran77 compiled code and attempts to recompile BAP with more recent Fortran compilers have not been successful).

Read "bapinfo.txt", included in the downloadable archive below, for more detailed information about installing and using BAP. Uncompressed versions of the BAP distribution files reside at https://ca.water.usgs.gov/nsmp/software/bap/.

Click here to download BAP version 1.1 software (2.4 Mb size zip file)

BAP version 1.0 Manual

Click here to download BAP Manual for Version 1.0 (PDF format, 1Mb size)

A paper copy of the BAP manual: "BAP: Basic Strong-Motion Accelerogram Processing Software; Version 1.0" by April M. Converse and A. Gerald Brady, 1992 (USGS Open-File Report 92-296A) is available upon request by contacting Kent Fogleman fogleman@usgs.gov (telephone: 650-329-4745) or by ordering a copy from the USGS Information Services (Internet: http:// mapping.usgs.gov/esic/to_order.html).

Changes from BAP version 1.0 to version 1.1

Version 1.1 of the BAP and related AGRAM programs has important changes that are listed in the document "whatsnew.txt" provided with the BAP zip archive in the "docs" folder. Because this documentation may be overlooked by users, it is now available here /bap/whatsnew.html or whatsnew.txt (text version).

This page was last updated on February 13, 2012.

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