Experiment Overview

The data described here were collected during a series of explosion tests conducted at the Glendora Lake Facility, of the Naval Surface Warfare Center Crane (NSWCC) on September 10-11, 1998.  NSWCC is a U.S. Navy facility located in southern Indiana that maintains a stockpile of approximately 50% of military ordinance in the U.S.  (The other 50% is mainly at China Lake in California).  The Navy purchased an abandoned strip mine near Sullivan, Indiana  (Figure 1)  in the early 1990s before the open pit was filled.  They allowed the pit to fill with water that forms the present landmark now called Glendora Lake.  NSWCC has been developing facilities at Glendora for the past several years with the primary focus to date being testing of hydrophones.  Due to increasing environmental regulation they have had problems testing underwater ordinance at sea.  As a result, they were interested in the potential of utilizing Glendora Lake to test ordinance.  The primary purpose of this experiment from the Navy's perspective was to develop empirical ground peak ground motion curves to evaluate potential seismic regulation limits on blasting in Glendora Lake.  That is, they needed to know how large an explosive could be detonated in Glendora Lake before they exceeded peak ground motion limits defined in state and federal blasting regulation.   This applied problem shaped the design of the experiment, but the availability of 100+ instruments gave us enough flexibility to also do some very small aperture array experiments.

The near surface geology strongly shapes these data in ways it is important to understand.   Figure 2 shows an enlarged view of the area along with a cross-section of the known near-surface geology.  The point to recognize is that all the area east of the main part of the lake (the part oriented north-south) is covered by approximately 30 m of mining spoil formed when the strip mined blasted and moved overburden rocks above the coals they were after.  To the west of the lake and in the southernmost corner of the 200 line stations were located on the natural surface.

For the experiment we utilized 102 digital recorders equipped with 4.5 Hz natural period, triaxial seismometers. These instruments were deployed on a total of 174 points on the ground within a 3000 meter radius of the shot points   (Figure 1) . 63 of these stations were operated in fixed, linear profiles directed radially away from the shot points in three different directions. These stations had a nominal station spacing of 75 m. Four additional instruments were equipped with strong-motion accelerometers. These were deployed near the foundations of buildings 8000, 8001, and 8002 and in the southwest corner of the floating building I will refer to in this report as the "Barge".
The remaining 105 points shown in  Figure 1   form three small clusters of stations labeled "Array 1", "Array 2", and "Array 3". These arrays were formed by utilizing 35 instruments that were moved during the course of the experiment. The numbers indicate the deployment sequence. Array 1 recorded the first four shots, array 2 recorded the final two shots on September 10, and array 3 ran for all of September 11. These arrays were designed to examine wave propagation effects in the Glendora region in an attempt to better quantify the variability of seismic wave propagation in different directions at the site. Arrays 1 and 2 were designed to understand surface wave interactions with the edges of the mined out region and array 3 was designed to investigate possible site amplification at the top of the highwall pit on which building 8000 is situated and is shown in more detail in  Figure 2

We had two different model REFTEK instruments: (a) newer units that were equipped with 24 bit digitizers, and (b) older units that used 16 bit digitizers. The 24 bit units were placed in the linear profiles at the points closest to the shot point. However, the more limited dynamic range of the 16 bit units (96 dB) led to clipping on some of the shots when they were  located within 1000 m of the shot point. This could have been prevented by lowering the fixed gain on these instruments, but time constraints and access restrictions imposed for safety reasons allowed us to make a gain change only on units in the moving arrays during the second day of the experiment.   For this reason potential users of this data need to beware of clipped data.   Almost all the data from array 1 are clipped and several of the 100 line stations in the same area are clipped on almost every shot.

The location precision of these data is as good as is technologically feasible today.  All station points  were measured to a precision of approximately 1 cm using a Real-time Kinematic Global Positioning System owned by the Indiana University Department of Geological Sciences. We also used this equipment to precisely locate the shot point, the relative positions of survey points on most of the structures currently present at the Glendora Lake Facility, and a digital outline of the lake itself. The later was accomplished by putting the GPS system in a continuous acquisition mode and driving a boat around the shoreline and were used in preparing the maps in Figures 1 and 2.

Timing is not as good as would have been desirable for an experiment at this scale because we had only enough GPS units to equip about 1/4 of the stations.  The rest had to utilize external clocks and a pulse test.  The original log files are included in this tape under the directory data/logfiles to allow potential future users to evaluate the reliability of timing on any station.  To correlate serial numbers of the log files with station names the best source is the stage table of the css3.0 database found in data/css3.0_format on the data tape for this experiment.