The goal of this program is to explore the use of offshore and onshore sensors for surveillance of activities on land. The context of this work is the surveillance and planning for an amphibious landing
During November 1996, the Dorman group of the Marine Physical Laboratory (MPL) conducted land and ocean based measurements of seismic and acoustic signals from explosive and vehicle sources. The experimental site was north of Oceanside, California at Red Beach, Camp Pendleton.
Thirteen Ocean Bottom Seismometers (OBS) were deployed and recovered in approximately 12 meters of water (OBS location plot ;OBS location table). The OBS's recorded continuously from November 1 to 23, 1996. Twelve of the OBS's sampled one vertical and two horizonal geophones (orthogonal to each other) and one differential presure gauge hydrophone at 128 Hz. The remaining OBS recorded 256 Hz data from a vertical geophone and OAS hydrophone ( Cruise Report/OBS performance ).
During the experiment, six seafloor ( Seafloor source locations & times ) and 14 terrestrial (see Onshore operations below) explosive sources were detonated .Travel time data from these sources provide velocity constraints for the geological substructure. A typical record section (range vs. time) of the water shots and one land shot for OBS frame 12 reveals some dispersion for the coastward water shot 6 (positive range). Due to the small signal-to-noise ratio created by unknown instrument-local events, only land shot point 4 (3 shots; detonation order: 3, 6, 11) was seen by most OBS's. However, the 256 Hz hydrophone on OBS frame 10 sampled all land shots well. In both the 8 second and 2 second record section plots (rotated 90 degrees) of the 256Hz hydrophone data, the abscissa is not distance but rather an index. Traces from shot point 1 are at 1, 2, and 3 km; shot point 2 is at 4, 5, and 6 km; .... ; shot point 5 is at 13 km; shot point 6 is at 14 km.
Using the OBS array, we are able to calculate wavenumber spectra and plot the ocean swell directionality between 0.0625 and 0.2 Hz and the associated array response. Note that the peaks correspond to the direction the energy is traveling to (not from). The primary peak shows the swell is coming from the west at approximately 150 s/km (6.7 m/s). Confirming that our process is correct, we also calculated the directionality for known energy sources: seafloor shot 4 at 3-4 Hz ( array response ) and seafloor shot 6 at 8-10 Hz ( array response ).
We conducted engineering tests of an implosive source.
In addition to the offshore seismic data, Katherine Kim conducted bathymetric surveying aboard the R/V Sproul.
While the OBS's recorded data continuously and autonomously on site, the land array seismic system had to be set up and taken down each day that it was used by teams of at least 3 people, recorded data for short durations (~ 1 minute), and needed an operator to run. The system used was a Geometrics 24-bit StrataView with 24 geophones sampling at 250 or 500 Hz. The system was set up in four primary configurations: NE to SW Line 1 down a hill to a sandy beach, NW to SE Line 2 down a hill through a swamp into chaparral, another NE to SW Line 3 parallel to the swamp onto the sandy beach, and a southern two-dimensional, minimally redundant, 4-ring array on a flat area landward of the sandy beach ( zoomed-in map of the land site & land geophone locations table ). The land seismic system recorded data during 6 primary days in November, 1996: 4th, 15th, 18th, 19th, 25th, and 26th; or Julian Days: 309-310, 320-321, 323, 324, 330-331, and 331.
Explosives were detonated on land ( locations & times ) to provide compressional, shear, and interface wave velocity constraints on the structure.
Source spectra and time series plots are presented for 3 of the large land shots recorded by nearby land geophones. (Note that the closest geophone was NOT used because the source signal was clipped; therefore, time series and source spectra will have some components of geological structure in them): SE shot point 1D recorded on geophone 22 (range ~ 25 m) of the NW to SE Line 2 SP1 Time Series & SP1 Source Spectra , N shot point 2C recorded on geophone 2 (range ~ 18 m) of the NE to SW Line 1 SP2 Time Series & SP2 Source Spectra , SW shot point 4C recorded on geophone 19 (range ~ 17 m) of the NE to SW Line 1 SP4 Time Series & SP4 Source Spectra . Notice the high frequency component of the arrival from SP4 is not present in the SP1 nor SP2 time series/ spectral plots. This is most likely due to coupling and structural differences amoung the 3 detonation sites. SP4 is on the sandy beach, close to the water table and hard-packed, wet sand; whereas, SP2 is located 15 meters higher on a hill composed of dry, air-filled soil and SP1 is in a chaparral covered, dried-up river bed where attenuation of high frequencies is reasonable.
The record sections for land shots recorded on the NW to SE Line 2 across the swamp show varying structure by their different range vs. time plots: Shots at the ends of the line: Line 2, Land Shot Point 1 & Line 2, Land Shot Point 3 ; Shots off the line: Line 2, Land Shot Point 2 & Line 2, Land Shot Point 4
The two-dimensional land geophone array recorded a track vehicle traveling from the east to the south on November 25, 1996. Spectral plot & Coherence plot for 12 geophones from the 3 inner rings (A, B, C) show similar frequency bands of energy across the array. The spectra are offset for clearity with the first geophone corresponding to the bottom most spectra. Wavenumber directionality plots: 8-18Hz , 18-22Hz , 22-38Hz , 38-48Hz , 48 and 65 Hz & 70-80Hz . Notice that in the 22-80Hz bands, the energy is traveling around 0.3 s/km (3.3 km/s) from the southeast which is faster than the measured compressional wave velocity for a small, shallow shot (~1.7 km/s). On the otherhand, the 8-22 Hz energy is coming out of the south much slower, approximately 6-7 s/km (143-167 m/s), which may be energy from the local swell.
On November 19, 1996, the two-dimensional land geophone array recorded freeway noise from Interstate 5. In the 10-20 Hz band traveling about 150 m/s, the energy direction is primarily from the north which corresponds to a freeway overpass/bridge (see land site ). The energy coming from the northeast may also be from the freeway.
Setting up the 2-D land array site on the beach, amphibian tracked vehicles in background.
Last revised: April 5, 1997.
This page created by Sean M. Wiggins, with Netscape Navigator Gold
