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March 29, 2006

HarryTho 3/29 Natalee Holloway Commentary

Topics: Natalee Holloway

With the Dutch search team on the island of Aruba, I thought I would discuss some of their equipment. The first is ground penetrating radar (Readers will find a good overview of the operating principles here).

"Ground penetrating radar is a nondestructive geophysical method that produces a continuous cross-sectional profile or record of subsurface features, without drilling, probing, or digging. Ground penetrating radar (GPR) profiles are used for evaluating the location and depth of buried objects and to investigate the presence and continuity of natural subsurface conditions and features.

Ground penetrating radar operates by transmitting pulses of ultra high frequency radio waves (microwave electromagnetic energy) down into the ground through a transducer or antenna. The transmitted energy is reflected from various buried objects or distinct contacts between different earth materials. The antenna then receives the reflected waves and stores them in the digital control unit.

When the transmitted signal enters the ground, it contacts objects or subsurface strata with different electrical conductivities and dielectric constants. Part of the ground penetrating radar waves reflect off of the object or interface; while the rest of the waves pass through to the next interface.

The reflected signals return to the antenna, pass through the antenna, and are received by the digital control unit. The control unit registers the reflections against two-way travel time in nanoseconds and then amplifies the signals. The output signal voltage peaks are plotted on the ground penetrating radar profile as different color bands by the digital control unit.

For each reflected wave, the radar signal changes polarity twice. These polarity changes produce three bands on the radar profile for each interface contacted by the radar wave.

Ground penetrating radar waves can reach depths up to 100 feet (30 meters) in low conductivity materials such as dry sand or granite. Clays, shale, and other high conductivity materials, may attenuate or absorb GPR signals, greatly decreasing the depth of penetration to 3 feet (1 meter) or less.

The depth of penetration is also determined by the GPR antenna used. Antennas with low frequencies of from 25 to 200 MHz obtain subsurface reflections from deeper depths (about 30 to 100 feet or more), but have low resolution. These low frequency antennas are used for investigating the geology of a site, such as for locating sinkholes or fractures, and to locate large, deep buried objects."

gprblock2.gif

Comment: Clearly, the Dutch ground penetrating radar system will be able to detect a drum down to 30 feet in depth with ease in the sand dune area. I doubt anyone would bother to dig beyond six (6) feet into the sand dunes. Any deeper depth would attract attention and leave some obvious, soil disturbance indications.

I should mention that ground penetrating radar (GPR) is no mystery test. It is a common nondestructive test (NDT) conducted by engineering firms all over the world. Nondestructive means that they do not break anything in order to conduct the test.

Engineering firms are those corporations that conduct studies of various kinds: civil, mechanical, geological, etc on behalf of construction companies. The constructions companies (E&Cs) include their own engineering services.

One such E&C is Fluor Corporation (30,000+ employees) which is considered America's number 1 E&C. All engineering and design is contained within Fluor's Engineering Department. Engineering costs comprise some 10% of any project. An international construction project, project at US $300 Million, would contain some US $30 Million in Engineering costs. The ground penetrating radar (GPR) would be included within the engineering costs.

Smaller construction firms would ally with small engineering firms in order to accomplish the same goals as someone like the Fluor Corporation (such as Brown and Caldwell and STATS).

As you can read from the links, the field of civil engineering is quite developed. Entire companies, with employments exceeding 400 engineers, thrive on merely conducting engineering studies like GPR. Accordingly, any reasonably sized engineering firm (not necessarily and E&C) could have provided this GPR service to Aruba.

Any of the engineering firms contracted to perform the required, geophysical surveys for the hotel construction in Aruba would have the capability to perform a GPR survey of the sand dune area for Aruba. GPR services were readily available. So, why did it take so long to conduct such a survey?

With Aloha,
Posted for HarryTho

Posted by Richard at March 29, 2006 8:46 PM



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