Data Services Newsletter

Volume 23 : No 2 : Summer 2021

MERMAID Data Now Available through the IRIS DMC

EarthScope-Oceans data are now fully archived, curated and available at the IRIS DMC. Data from the Mobile Earthquake Recording in Marine Areas by Independent Divers (MERMAIDs) from both the United States (Princeton) and France (GéoAzur) are now available. These deployments are in the South Pacific as shown in Figure 1, and they are part of a larger SPPIM (South Pacific Plume Imaging and Modeling) array collectively managed by the EarthScope-Oceans (ESO) consortium. This project was discussed in greater detail in a previous IRIS DS Newsletter (https://ds.iris.edu/ds/newsletter/vol19/no1/477/a-seismic-shift-in-the-oceans).

Figure 1. Initial Deployment Locations
The map above shows the initial deployment locations for the first 18 MERMAIDS from the Ifremer vessel R/V Alis. Starting from the left, the linear array of stations is numbered P0006—P0012, and those in the circular array are numbered (clockwise) P0013 and P0016—P0025. Access to data from floats P0006 and P0007 (the two most western MERMAIDs are contributed by GéoAzur and have small purple dots above them in Figure 1. P0008, P0010, and P0016 are contributed by Princeton (similarly marked by red dots) and are unrestricted. All Princeton and GéoAzur data are fully released after two years, and they are now available through the IRIS DMC. (Figure from https://ds.iris.edu/gmap/MH)

Moving Station Locations

MERMAIDs drift freely with the ocean currents, but they are not unique in this regard. Two other drifting data acquisition platforms include:

MERMAIDs use the following approach. At the surface, MERMAIDs determine their position using GPS. However, MERMAIDs record their hydroacoustic time series at depth—generally around 1500 m below the surface, out of contact with GPS—and so the location at the time of recording must be interpolated using measurements taken during the previous and subsequent dives. The technique used to determine these locations is discussed by Joubert et al. (2016; doi: 10.1785/0120140265) and codified in the open-source Python package Automaid (Simon et al., 2021; doi: 10.5281/zenodo.5057096).

ESO has submitted a proposal to the FDSN that specifies a simple approach to archiving changing metadata via the GeoCSV file format. Very much a spreadsheet-ready approach, it provides a flexible way to capture the location of a moving MERMAID. These GeoCSV files are available from the IRIS Metadata Aggregator (MDA) system. Examples of the files can be found at http://ds.iris.edu/data/reports/MH where relevant GeoCSV files can be found in subdirectories, one for each station.

Figure 2 shows some of the more recent GPS fixes reported by a single MERMAID, station P0010. The changing positions of MERMAID stations are not easily captured in current StationXML metadata.

Figure 2. Changing locations for station P0010
This map shows the most recent GPS fixes reported by Station P0010 (with older positions represented by increasingly transparent icons). These changing positional metadata are captured in the corresponding GeoCSV file in an easily accessible format. (Figure from http://earthscopeoceans.org)

We believe that other deployments where station position may change (e.g., on or near ice flows, glaciers, landslides) can also leverage the proposed GeoCSV format. Ultimately, the information in GeoCSV files could be incorporated into StationXML in an efficient manner once the GeoCSV proposal is approved by the FDSN.

Data Policy

The data policy established by ESO is quite progressive. Data from three Princeton and both GéoAzur stations are available without any delay other than the time required for their curation. Data from all other stations will be released after two years, or if permission is granted by ESO.

For more details concerning early access, refer to https://geoweb.princeton.edu/people/simons/earthscopeoceans/data/metadata.html. Please reference ESO data using DOI 10.7914/SN/MH.

Data Access and Distribution

Presently, ESO MH data are only available from the IRIS DMC. Access is possible via all tools supported by IRIS, including a rich set of web services (http://service.iris.edu) and web applications such as GMAP (https://ds.iris.edu/gmap), WILBER3 (https://ds.iris.edu/wilber3), and the MDA (https://ds.iris.edu/mda). IRIS supports all International Federation of Digital Seismograph Stations (FDSN) formats and tools in addition to services and access methods unique to IRIS.

We envision future distribution of MERMAID data being accomplished consistent with the Federated Data Center concept (https://www.fdsn.org/datacenters). Currently 19 centers on 5 continents operate FDSN web services that offer seamless user access regardless of where the actual data reside. Thus, users will have access to MH data held at any of the ESO nodes in the future. The French and American data at IRIS will hopefully be followed by data access via additional data centers that support FDSN standard formats and services. Until those federated services come on-line IRIS is willing to host data from other ESO partners.

An Example of MERMAID Data

We end with an example of the type of seismoacoustic signals that MERMAIDs record. One can use any of the IRIS DMC tools to access the unrestricted MH data including the IRIS web service called timeseriesplot used to generate the filtered waveform in Figure 3.

Figure 3. MERMAID Data Example
In this example, the onset around 31:10 is the P-wave associated with an Mw 5.8 earthquake south of the Fiji Islands, ~33 degrees southwest of the recording MERMAID. This figure was generated using the IRIS timeseriesplot web service which applied a bandpass filter from 1hz to 5hz.

by Tim Ahern (1) , Joel Simon (2) and Christoph Waldmann (3)

  • (1) IRIS Data Management Center
  • (2) Princeton University
  • (3) MARUM, University of Bremen
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