Hawaiian GPR studies:

In December, 2005 I worked with Sam Connell and the Joint POW/MIA Accounting Command at Hickam Air Force Base developing GPR techniques for mapping graves and other shallow archaeological sites.  Sam now teaches at Foothills College in Los Altos, California.

Our goals were to test the GPR method’s effectiveness in mapping buried archaeological sites and to prospect for modern and ancient human burials at a number of test areas on Oahu and Hawaii .  These tests were carried out in December 2005 during a period of very dry ground conditions, which are generally assumed to be best for GPR energy penetration in the ground.  In all these tests buried cultural materials and human remains were either known or suspected to exist, which could be used as targets.  We used the GSSI SIR-3000 system with the 400 and 270 MHz antennas.

Here are some of the sites we worked on, with some images:

Weathered coral soils

            A number of grids of data were collected at Hickam Air Force Base in Honolulu on ground that is composed of a very thin surface soil underlain by weathered coral rubble with admixtures of coral and shell sand.  This weathered substrate is mostly fill, originally dredged from nearby Pearl Harbor .  The tests were performed to test the resolvability of buried bones and objects placed in the ground to mimic human remains and to search for the possible remains of the country retreat of Queen Emma (1836-1885) who lived here until her death.

This is a reflection profile across what we think is a part of her house:

And when you look at all the profiles in a slice-map, this is the foundation reflection in map view:

            Most of our tests in the Hickam area were located 100 meters or more inland from the bay, and good GPR reflection data indicated that the ground water in this area was mostly fresh.  There was little energy attenuation within the time window that data were collected, with good reflections recorded to at least 2 meters depth.

            In a similar near-shore carbonate setting a test was conducted in a vacant lot in the Waikiki District that was slated for construction The GPR reflection profiles in this location showed good radar reflections in profile, discovering a possible house floor or other historic feature at about 1 meter depth, which is about 12 nanoseconds in two-way radar travel time. Below that level the brackish groundwater appears to have attenuated all radar energy.

Carbonate dunes

            The numerous coral and shell fragments that make up dunes in low lying areas of the coast, as well as most of the modern Hawaiian beaches were also tested to determine resolution and depth penetration.  A 20x30 meter grid of GPR reflection data was collected using the 400 MHz antennas with 50 centimeter profile spacing at Bellows Air Force Base on Oahu ’s windward side. In this data set excellent reflections were recorded to about 2 meters depth and a number of buried features were imaged including the midden that had previously been tested, and a second previously-unknown midden in a different part of the grid.  This is a profile in the area:

And here are amplitude slice-maps of all the profiles in this grid, showing the pipe, but also burials and the midden deposits:

            As active carbonate dunes can often contain archaeological materials along the coast, test profiles were collected along the modern beach at the Marine Corps Base Hawaii, also on the windward side of Oahu , just west of the Klipper Golf Course.  Although the stratigraphy there is quite complex, reflections from a number of buried features are visible in reflection profiles, which are cross-beds of the dunes, and buried surfaces of inter-dune compacted surfaces that were later covered by the advancing dunes:

            Ancient Hawaiian burials were often placed in sand dunes both along the coast and in dune deposits farther inland.  Just south of the runway at Dillingham Air Field on Oahu ’s northwest coast, one such burial ground was tested in carbonate sand where both historic and ancient burials were known to be present.  Some of this area had been disturbed by earth moving equipment in the past, and it was unknown if or where intact burials might still be present.   Using the 400 MHz antennas good reflections were recorded to between 2 and 2.5 meters (30 nanoseconds) and a number of possible burials discovered:

Weathered basaltic soils

            As most of the Hawaiian island chain’s bedrock is composed of basalt it is important to understand GPR energy penetration in soils that have formed on this material, as well as energy propagation within the volcanic rock.  An understanding of the electrical properties of the weathering products formed from basalt, which can produce the thick reddish-brown clay soils visible throughout the islands is therefore crucial.  Hawaiian basalt is composed of silica, plagioclase feldspar (sodium and calcium aluminum silicates) and iron-magnesium minerals such as magnetite, pyroxene and olivine.   The silica in the basalts is mostly stable over time, and its chemical composition affects radar energy very little.  The feldspar minerals in basalt, however, will readily weather to a variety of clay minerals depending on the environment conditions, with kaolonite clays formed in the more humid windward areas and allophane, spectite and montmorillonite clays in the drier leeward areas.   The most common way to measure the electrical conductivity of soils is by measuring cation exchange capacity (CEC), a common measurement used in determining soil fertility for agriculture.  The higher the CEC, the greater the electrical conductivity, as measured in units of centimeters of cation change per kilogram.  Kaolinite has the lowest CEC of common Hawaiian soils, in the range of 2 to 15 cm/kg, while montmorillonite has the highest (ranging from 80 to 150).  The calcium carbonate found in soils in leeward areas will also increase the electrical conductivity when wet, as this mineral constituent can mobilize cations.  Therefore in a basic way the clay soils formed on basalt in leeward locations will have poorer radar energy propagation, as these electrically conductive soils will destroy most radar energy readily in the ground.  As the soils become progressively more weathered because of greater rainfall toward the windward areas, radar energy depth penetration will improve. As a test of basalt soils on the leeward side of the island GPR reflection profiles were collected at the National Memorial Cemetery of the Pacific, also called Punchbowl Cemetery where metal caskets were known to be located at standard depths in the ground.  No coherent reflections were obtained below 20 nanoseconds (80 centimeters) in any of the profiles, and none of the caskets, whose tops were about 120-150 centimeters below ground surface were visible.
            Tests were conducted soils formed on basalt parent material were performed at the Schofield Barracks Post Cemetery .  Reflection profiles from the cemetery showed good reflections from the caskets to about 35 nanoseconds, which is about 1.5 meters below ground surface after correcting for velocity:

Amplitude slice-maps of a portion of the cemetery where child burials were located (at various depths) were constructed in order to map individual graves and other buried features:

Basalt rock: lava tubes

          In tests using the 270 MHz antennas in 5,000 year old basalt flows on the Island of Hawaii excellent energy penetration occurred to at least 4 meters or more.  In similar tests in Hawaiian Volcanoes National Park on Hawaii , Doolittle describes as much as 9 meters depth penetration in older lava flows and 3-4 meters in the very recent flows at Kilauea .  He suggests that the shallower penetration at Kilauea is perhaps due to the “hot” nature of these lavas.  Our tests were performed at the O’oma Phupua’a lava tube just northwest of the Kona airport.  In numerous reflection profiles using both 270 and 400 MHz antennas both the top and floor of a lava tube were recorded in reflection profiles.  Below the profile shows the top of the tube is visible at 40 nanoseconds, which is 2.8 meters below the ground surface. Volcanic stratigraphy along the margins of the tube is well defined to a depth of about 5 meters (60 nanoseconds).  The floor of the lava tube is also visible as an upward bowing reflection at about 60 nanoseconds: