James A. Doolittle

Use of soil information to determine application of ground penetrating radar

Abstract Ground penetrating radar (GPR) has been used as a pedologic tool in the United States since 1978. The National Cooperative Soil Survey Program has used GPR to assess properties of soils which affect their use, management, and classification. Principal uses have been to estimate the variability and taxonomic composition of soils, chart the lateral extent, and estimate the depth and thickness of soil horizons and geologic layers and to map and interpret soils. All soils are not equally suited to GPR techniques. Soil maps and taxonomic descriptions contained in published soil survey reports can be used to assess the suitability of soils and sites for GPR applications In addition, the criteria used to define and classify the soil can be used to aid interpretations of radar imagery.

Contributions of water supply from the weathered bedrock zone to forest soil quality

One measure of forest soil quality is the ability of the soil to support tree growth. In mediterranean-type ecosystems, such as most of Californias forests, there is virtually no rainfall during the summer growing season, so trees must rely on water stored within the substrate. Water is the primary limitation to productivity in these forests. Many forest soils in California are relatively thin, but are underlain by thick zones of weathered bedrock. Weathered granitic bedrock, the most prevalent lithology, has available water capacities (0.12-cm water/cm rock) that approach those of soils (0.2-cm water/cm soil) and, because the weathered rock zone is usually so much thicker (several meters) than the soil (<1 m), it almost always constitutes the larger storage reservoir for plant-available water. The weathered bedrock retains the original rock fabric and is friable and easily excavated, but the primary minerals are not thoroughly altered to clay minerals, so it is not considered saprolite. Roots of ponderosa pine (Pinus ponderosa) seedlings penetrate through the soil and encounter weathered bedrock within the first 2 years on many sites. Thus, the influence of the weathered bedrock zone on plant growth begins early. Root access to the weathered bedrock is restricted to fractures, which are often spaced about 50 cm apart. Water is extracted from the intervening rock matrix through unsaturated flow toward the root mat in the fractures and by mycorrhizal fungal hyphae that penetrate the rock matrix. At one site in the Sierra Nevada, 30-year-old Jeffrey pine (P. jeffreyi) depleted the soil-held water by mid-June and relied on water stored in the weathered bedrock until the rainy season began in October. In this case, the weathered bedrock supplied at least 70% of the water used by the trees during the growing season. In the same area, we found that thin soils are not a detriment to pine productivity when they are underlain by a thick zone of weathered bedrock. In mediterranean-type ecosystems, the weathered bedrock zone is an essential component of the plant water supply system and is thus an important contributor to forest soil quality.

Ground Penetrating Radar Soil Suitability Maps

The performance of ground-penetrating radar (GPR) is dependent upon the electrical conductivity of soils. Soils having high electrical conductivity rapidly attenuate radar energy, restrict penetration depths, and severely limit the effectiveness of GPR. Factors influencing the electrical conductivity of soils include the amount and type of salts in solution and the clay content. Data on the clay and soluble salt contents of soils were used to develop thematic maps showing at different scales the relative suitability of soils for GPR applications. The United States Department of Agriculture-Natural Resources Conservation Service (USDA-NRCS) bases these maps on field experience and soil attribute data contained in the State Soil Geographic (STATSGO) and the Soil Survey Geographic (SSURGO) databases. Attribute data used to determine the suitability of soils include clay content, electrical conductivity, sodium absorption ratio, and calcium carbonate content. Each soil attribute was rated and assigned a value...

Performance of ground-penetrating radar on granitic regoliths with different mineral composition

Although ground-penetrating radar (GPR) is extensively used to characterize the regolith, few studies have addressed the effects of chemical and mineralogical compositions of soils and bedrock on its performance. This investigation evaluated the performance of GPR on two different granitic regoliths of somewhat different mineralogical composition in the San Jacinto Mountains of southern California. Radar records collected at a site where soils are Alfisols were more depth restricted than the radar record obtained at a site where soils are Entisols. Although the Alfisols contain an argillic horizon, and the Entisols have no such horizon of clay accumulation, the main impact on GPR effectiveness is related to mineralogy. The bedrock at the Alfisol site, which contains more mafic minerals (5% hornblende and 20% biotite), is more attenuating to GPR than the bedrock at the Entisol site, where mafic mineral content is less (<1% hornblende and 10% biotite). Thus, a relatively minor variation in bedrock mineralogy, specifically the increased biotite content, severely restricts the performance of GPR.

Revised Ground-Penetrating Radar Soil Suitability Maps

The United States Department of Agriculture-Natural Resources Conservation Service (USDA-NRCS) recently revised its ground-penetrating radar (GPR) soil suitability maps (GPRSSM). These maps, which have been prepared for most areas of the USA at different scales and levels of resolution, show the relative suitability of soils for GPR soil investigations. These digital maps are based on physical and chemical properties of approximately 22,000 different soils. The smaller scale (1:250,000) Ground-Penetrating Radar Soil Suitability Map of the Conterminous United States shows the relative suitability of soils to GPR within major soil and physiographic areas. The larger scale (1:12,000 to 1:63,360) state ground-penetrating radar soil suitability maps duplicate the scale and level of detail of the original soil survey maps. GPR soil suitability maps have been used to evaluate the relative appropriateness of using GPR, select the most suitable antennas, and assess the need and level of data processing. Limitation...

Improved radar interpretations of water table depths and groundwater flow patterns with predictive equations

Recent interest in soils and hydrologic modeling has increased the need for information concerning the depth and movement of ground water. Ground-penetrating radar (GPR) was used eight times over a two-year period to chart water table depths and ground-water flow patterns within a 32-ha forested site in northwestern Indiana, USA. Radar imagery was correlated with depths to the water table in 16 observation wells. The velocity of propagation ranged from 0.0508 m/ns to 0.1606 m/ns at these wells. Propagation velocities were generally slower during the spring and early summer months when depths to the water table were relatively shallow. Propagation velocities were faster through dunes than through the more poorly drained interdunes. Because of the spatiotemporal variability in propagation velocities and the known complexity of soil and landform patterns, a predictive equation based on water table depths and two-way travel times was developed for each GPR survey. In this setting, the use of a predictive equation based on multiple GPR measurements over a known reflector substantially improved the accuracy of radar depth interpretations over single or averaged measurements.

Using Remotely Sensed Soil Conductivity to Monitor Restoration Activities on Vernal Pools, Northern Great Basin, USA

The sagebrush-steppe of the northern Great Basin, USA, receives 120–500 mm of precipitation per year. Clay horizon formation in these semiarid/arid Pleistocene-lake landscapes allows development of seasonal wetlands (vernal pools) that are recognized critical habitat for several native animal species. Most pools were dug out to create livestock water holes in the early and mid-1900s. Restoration efforts are underway to restore these ecosystems. This study was undertaken to evaluate the pre- and post-restoration hydrology of several regraded vernal pools. Five total sites, one undisturbed and four dugouts, were mapped for apparent electrical conductivity (ECa) using electromagnetic induction to evaluate hydrologic flow patterns. Two sites were subsequently regraded to fill in dugouts and redistribute excavated piles. ECa for the restored sites was remeasured 1 year later. ECa patterns of the dugout pools indicate that the hydrology is directed toward the low areas concentrating soluble salts in the dugout through evaporation. Patterns of the undisturbed site suggest a broader distribution of water and salts. Conductivity patterns of post-restoration show a marked difference in conductivity with a broadening out of the high-concentration area and a decreased difference between the former dugout area and surrounding area. The pattern after 1 year of hydrologic activity suggests that regrading allows the water to spread across a larger area and begins to develop hydrologic patterns similar to undisturbed pools, suggesting potential to restore desired ecological function. Results of this study will refine understanding of vernal pool hydrology and ecology for the region.