Tutorial : 2-D and 3-D electrical imaging surveys

At the watershed scale, soil moisture is the major control for rainfall–runoff response, especially where saturation excess runoff processes dominate. From the ecological point of view, the pools of soil moisture are fundamental ecosystem resources providing the transpirable water for plants. In drylands particularly, soil moisture is one of the major controls on the structure, function, and diversity in ecosystems. In terms of the global hydrological cycle, the overall quantity of soil moisture is small, ∼0.05%; however, its importance to the global energy balance and the distribution of precipitation far outweighs its physical amount. In soils it governs microbial activity that affects important biogeochemical processes such as nitrification and CO2 production via respiration. During the past 20 years, technology has advanced considerably, with the development of different electrical sensors for determining soil moisture at a point. However, modeling of watersheds requires areal averages. As a result, point measurements and modeling grid cell data requirements are generally incommensurate. We review advances in sensor technology, particularly emerging geophysical methods and distributed sensors, aimed at bridging this gap. We consider some of the data analysis methods for upscaling from a point to give an areal average. Finally, we conclude by offering a vision for future research, listing many of the current scientific and technical challenges.

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Soil Moisture Measurement for Ecological and Hydrological Watershed-Scale Observatories: A Review

Direct current (DC) resistivity (here referred to as resistivity) and induced polarization (IP) methods allow, respectively, the determination of the spatial distribution of the low-frequency resistive and capacitive characteristics of soil. Since both properties are affected by lithology, pore fluid chemistry, and water content (see Chapter 4 of this volume), these methods have significant potential for hydrogeophysical applications. The methods can be applied at a wide range of laboratory and field scales, and surveys may be made in arbitrary geometrical configurations (e.g., on the soil surface and down boreholes). In fact, resistivity methods are one of the most widely used sets of geophysical techniques in hydrogeophysics. These surveys are relatively easy to carry out, instrumentation is inexpensive, data processing tools are widely available, and the relationships between resistivity and hydrological properties, such as porosity and moisture content, are reasonably well established. In contrast, applications of induced polarization methods in hydrogeophysics have been limited. As noted by Slater and Lesmes (2002), this is partly because of the more complex procedure for data acquisition, but also because the physicochemical interpretation of induced polarization parameters is not fully understood.

DC Resistivity and Induced Polarization Methods

This enduringly popular undergraduate textbook has been thoroughly reworked and updated, and now comprises twelve chapters covering the same breadth of topics as earlier editions, but in a substantially modernized fashion to facilitate classroom teaching. Covering both theoretical and applied aspects of geophysics, clear explanations of the physical principles are blended with step-by-step derivations of the key equations and over 400 explanatory figures to explain the internal structure and properties of the planet, including its petroleum and mineral resources. New topics include the latest data acquisition technologies, such as satellite geophysics, planetary landers, ocean bottom seismometers, and fibre optic methods, as well as recent research developments in ambient noise interferometry, seismic hazard analysis, rheology, and numerical modelling - all illustrated with examples from the scientific literature. Student-friendly features include separate text boxes with auxiliary explanations and advanced topics of interest; reading lists of foundational, alternative, or more detailed resources; end-of-chapter review questions and an increased number of quantitative exercises. Completely new to this edition is the addition of computational exercises in Python, designed to help students acquire important programming skills and develop a more profound understanding of geophysics.

Fundamentals of Geophysics

Due in large measure to the prodigious research efforts of Rhoades and his colleagues at the George E. Brown, Jr., Salinity Laboratory over the past two decades, soil electrical conductivity (EC), measured using electrical resistivity and electromagnetic induction (EM), is among the most useful and easily obtained spatial properties of soil that influences crop productivity. As a result, soil EC has become one of the most frequently used measurements to characterize field variability for application to precision agriculture. The value of spatial measurements of soil EC to precision agriculture is widely acknowledged, but soil EC is still often misunderstood and misinterpreted. To help clarify misconceptions, a general overview of the application of soil EC to precision agriculture is presented. The following areas are discussed with particular emphasis on spatial EC measurements: a brief history of the measurement of soil salinity with EC, the basic theories and principles of the soil EC measurement and what it actually measures, an overview of the measurement of soil salinity with various EC measurement techniques and equipment (specifically, electrical resistivity with the Wenner array and EM), examples of spatial EC surveys and their interpretation, applications and value of spatial measurements of soil EC to precision agriculture, and current and future developments. Precision agriculture is an outgrowth of technological developments, such as the soil EC measurement, which facilitate a spatial understanding of soil-water-plant relationships. The future of precision agriculture rests on the reliability, reproducibility, and understanding of these technologies.

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Application of soil electrical conductivity to precision agriculture: theory, principles, and guidelines

Applied Geophysics: Frontmatter

Ini adalah versi yang sepenuhnya direvisi dan diperbarui dari buku teks populer dan sangat dihormati, Geofisika Terapan. Ini menjelaskan metode fisik yang terlibat dalam eksplorasi hidrokarbon dan mineral, yang meliputi metode gravitasi, magnet, seismik, listrik, elektromagnetik, radioaktivitas, dan well-logging. Semua aspek dari metode ini dijelaskan, termasuk teori dasar, peralatan lapangan, teknik akuisisi data, pemrosesan dan interpretasi data, dengan tujuan untuk menemukan endapan komersial mineral, minyak, dan gas serta menentukan luasnya. Dalam empat belas tahun atau lebih sejak edisi pertama Geofisika Terapan, banyak perubahan telah terjadi di bidang ini, terutama sebagai hasil dari teknik baru, instrumentasi yang lebih baik, dan peningkatan penggunaan komputer di lapangan dan dalam interpretasi data. Para penulis menjelaskan perubahan ini dengan sangat rinci, termasuk metode yang lebih baik untuk memecahkan masalah terbalik, metode seismik khusus, magnetotelurik sebagai metode eksplorasi praktis, metode elektromagnetik domain waktu, peningkatan penggunaan spektrometer sinar gamma, dan peningkatan metode well-logging dan penafsiran.

Applied Geophysics second edition

DIRECTION DETERMINATION The three degrees of freedom, latitude, longitude, and elevation (or equivalents), constitute a rectangular coordinate system (neglecting earth curvature effects), and clearly determination of the orientations of the axes is important. The vertical direction is usually defined by means of a level or plumb line. Horizontal orientation can be determined with respect to the position of a celestial body, the Earth's rotation, magnetic north, or a previously known orientation. Although the most obvious celestial orientation is by sighting on Polaris (which is not exactly at the celestial pole), any star can be used. Sighting on the Sun ( sun shots ) permits orientation during daylight. Nightime observations can establish orientation to a fraction of an arc second (arcsec). Gyrocompass orientation is extensively used in marine and airborne surveys. A gyrocompass gives orientation with respect to true north within about 20 arcsec. A magnetic compass is often used to determine direction with respect to magnetic north, correction for the magnetic declination giving true north. Diurnal variations may create 10 min of error, more during magnetic storms. Care has to be exercised that no local iron or electric currents distort the magnetic field. Benchmarks (identified points of known location) often have nearby reference points (azimuth bars) that can be used to establish orientation. DISTANCE MEASUREMENT Direct measurement of distance is done by seeing how many times a standard measure will fit between points whose separation is to be determined, a technique called chaining , the unit of measure usually being a steel tape (a chain ) of precise length.

L. P. Geldart

Papers

Applied Geophysics: Frontmatter

Applied Geophysics second edition

Applied Geophysics: Appendix B. Location Determination

Applied Geophysics: Preface to the Second Edition

Applied Geophysics: Electrical Properties of Rocks and Minerals