Near-surface Targets Research Articles

Detection of shallow underground targets using electrical resistivity tomography and the implications in civil/environmental engineering

Applying the electrical resistivity tomography (ERT) technique in detecting very near-surface targets is quite challenging in geophysical investigation, especially in civil and environmental engineering for adequate planning and designing of structural foundations, contributing to the overall safety and efficiency of construction projects. However, locating the exact position and depth of underground targets such as faults, underground utilities, and contaminants is more challenging. Therefore, this study is aimed at examining the geophysical response of various buried targets and evaluating the ability of ERT to detect buried targets in terms of locations and depths of occurrence in the context of engineering investigation and environmental studies. A laboratory test was conducted on the targets to determine their electrical conductivity and resistivity before burial. The two-dimensional (2D) ERT survey was performed on thirteen targets buried at the site using both Wenner and dipole–dipole (DD) arrays. Both arrays captured the metallic targets with a low electrical resistivity contrast (< 0.1 Ωm) corresponding to the laboratory results. In comparison, the positions of the non-metallic buried targets were found to have a high resistivity contrast greater than 3000 Ωm, matching the laboratory results. The modelled pipes and the car engine block captured by both DD and Wenner arrays on 1.0 m electrode spacing were relatively smeared and poorly resolved in shapes, sizes and geometries, while some were not captured. The electrode spacing of 0.25 m and 0.50 m was explored on undetected targets, which provide a better resolution with sizes and depths compared to 1.0 m spacing but did not produce satisfactory results in some cases. The success and failure of ERT to detect a few targets were discussed alongside the environmental and engineering implications. The effectiveness of both arrays was assessed by their sensitivity in mapping the change in subsurface resistivity values. The DD array shows sensitivity to horizontal variations in resistivity values with low signal. In contrast, the Wenner array shows a good signal strength with a good change in the horizontal and vertical resistivity values. In addition, both arrays show capacity in mapping the geophysical signature of the buried targets and subsurface structures, which has significant application in engineering and environmental investigations.

A Review on Multirotor UAV-based Magnetic Surveys for Near-Surface Targets

무인비행장치 기술의 발전과 함께, 다양한 무인비행장치 시스템을 이용한 자력탐사의 활용 가능성에 관한 연구가 진행되고 있다. 최근에는 무인멀티콥터 기반 자력탐사에 관한 연구가 활발히 이루어지고 있으며, 이 논문에서는 천부 대상 탐지연구를 중심으로 사례를 분석하였다. 특히, 자력계의 설치 위치와 배열의 다양화, 멀티콥터의 위치 정확도 향상을 위한 센서 활용에서 새로운 시도들을 확인하였다. 또한, 멀티콥터에서 발생하는 잡음을 고려하기 위한 자료 분석 및 처리 기술들을 소개하고, 자료 해석에 관한 다양한 연구들을 정리하였다. 제시된 연구사례들은 무인멀티콥터 자력탐사가 지표에 근접한 비행으로 소규모 이상체 탐지에 강점이 있으며, 탐사 영역에 대한 정밀한 비행을 통해 고해상도의 자료 획득이 가능함을 보여주었다. 이 기술은 향후 불발 병기나 지뢰와 같은 인공 구조물, 관로와 같은 기반 시설, 그리고 고고학적 조사 등에도 유용하게 적용될 것으로 기대된다.

Natural Field Airborne Electromagnetics—History of Development and Current Exploration Capabilities

The mineral resources exploration industry continuously expands the efficiency requirements for geophysical technologies. Due to their relatively inexpensive nature, coupled with the ability to rapidly acquire data over large areas, airborne electromagnetic technologies have been used for decades in subsurface exploration. Limitations on the depth of investigation of airborne platforms with controlled primary field sources is the main obstacle for using these systems in many geoelectrical conditions and geographical terrains. In addition, systems based on the time-domain principle are limited in applications requiring differentiations in a high resistivity range of the mapping parameter and suffer from parasitic electromagnetic non inductive natural effects in specific near surface conditions. Methods exploiting natural electromagnetic fields in the audio frequency range significantly increase depth of investigation and sensitivity to a wide range of resistivity contrasts including in the range of thousands of ohm-ms. A brief history of the development of the natural field airborne technology is provided accompanied by a comparison of the systems technical specifications. Field examples from the latest development in the airborne electromagnetic natural fields’ domain, MobileMT, demonstrate its exploration capabilities in both conductive and resistive environments, sensitivity to any direction of geoelectrical boundary, and detectability of near-surface discrete targets along with deeper structures.

Processing and Interpretation of UAV Magnetic Data: A Workflow Based on Improved Variational Mode Decomposition and Levenberg–Marquardt Algorithm

Unmanned aerial vehicles (UAVs) have become a research hotspot in the field of magnetic exploration because of their unique advantages, e.g., low cost, high safety, and easy to operate. However, the lack of effective data processing and interpretation method limits their further deployment. In view of this situation, a complete workflow of UAV magnetic data processing and interpretation is proposed in this paper, which can be divided into two steps: (1) the improved variational mode decomposition (VMD) is applied to the original data to improve its signal-to-noise ratio as much as possible, and the decomposition modes number K is determined adaptively according to the mode characteristics; (2) the parameters of target position and magnetic moment are obtained by Euler deconvolution first, and then used as the prior information of the Levenberg–Marquardt (LM) algorithm to further improve its accuracy. Experiments are carried out to verify the effectiveness of the proposed method. Results show that the proposed method can significantly improve the quality of the original data; by combining the Euler deconvolution and LM algorithm, the horizontal positioning error can be reduced from 15.31 cm to 4.05 cm, and the depth estimation error can be reduced from 16.2 cm to 5.4 cm. Moreover, the proposed method can be used not only for the detection and location of near-surface targets, but also for the follow-up work, such as the clearance of targets (e.g., the unexploded ordnance).

Developing new geophysical scanners for detection of near-surface buried objects

A new fusion of geophysical scanners is developed for detection of the near-surface buried objects, such as iron pipes, pieces of metal, and rocks. The new fusion was built up in a cart that integrates three kinds of geophysical instruments including an EM-61 metal detector, magnetometer, and OhmMapper. For the new system, we have innovate an electric control unit and adapted all the original connection cables to efficiently and simultaneously collect the electromagnetic field, magnetic intensity, and apparent resistivity on the surface of ground. We also modified and complement DOS-based computer programs to automatically measure the three kinds of data, record the surveying positions, and transfer the data from the system to a host computer for integration of the mapping images. The new system has been tested at a synthetic field site where five different objects are previously buried. The experimental results show successes in mapping these near-surface targets and prove that the new system archives saving surveying time, obtaining comprehensive data, reducing human interventions and number of human operators. It may become a new efficient and effective tool of geophysical exploration for detection of the near-surface buried objects.

The Applicability of an Inverse Schlumberger Array for Near-Surface Targets in Shallow Water Environments

We investigate the applicability of offshore geoelectrical profiling in the littoral zone, e.g., for archaeological prospection, sediment classification and investigations on coastal ground water upwelling. We performed field measurements with a 20 m long multi-electrode streamer in inverse Schlumberger configuration, which we used to statistically evaluate measurement uncertainty and the reproducibility of offshore electric resistivity tomography. We compared floating and submerged electrodes, as well as stationary and towed measurements. We found out that apparent resistivity values can be determined with an accuracy of 1% to 5% (1σ) depending on the measurement setup under field conditions. Based on these values and focusing on typical meter-scale targets, we used synthetic resistivity models to theoretically investigate the tomographic resolution and depth penetration achievable near-beach underneath a column of brackish water of about 1 m depth. From the analysis, we conclude that offshore geoelectric sounding allows the mapping of archaeological stone settings. The material differentiation of low-porosity rock masses &lt; 15% is critical. Submerged wooden objects show a significant resistivity contrast to sand and rocks. Distinguishing brine-saturated sandy sediments from cohesive silty-clayey sediments is difficult due to their equal or reversed resistivity contrasts. Submarine freshwater discharges in sandy aquifers can be localized well, though difficulties may occur if the seafloor encounters massive low-porosity rock masses. As to the measurement setups, submerged and floating electrodes differ in their spatial resolution. Whereas stone settings of 0.5 to 1 m can still be located with submerged electrodes within the uppermost 4 m underneath the seafloor, they have to be &gt;2 m if floating electrodes are used. Therefore, we recommend using submerged electrodes, especially in archaeological prospection. Littoral geological and hydrogeological mapping is also feasible with floating electrodes in a more time-saving way.

Automatic Detection of Near-Surface Targets for Unmanned Aerial Vehicle (UAV) Magnetic Survey

Great progress has been made in the integration of Unmanned Aerial Vehicle (UAV) magnetic measurement systems, but the interpretation of UAV magnetic data is facing serious challenges. This paper presents a complete workflow for the detection of the subsurface objects, like Unexploded Ordnance (UXO), by the UAV-borne magnetic survey. The elimination of interference field generated by the drone and an improved Euler deconvolution are emphasized. The quality of UAV magnetic data is limited by the UAV interference field. A compensation method based on the signal correlation is proposed to remove the UAV interference field, which lays the foundation for the subsequent interpretation of UAV magnetic data. An improved Euler deconvolution is developed to estimate the location of underground targets automatically, which is the combination of YOLOv3 (You Only Look Once version 3) and Euler deconvolution. YOLOv3 is a deep convolutional neural network (DCNN)-based image and video detector and it is applied in the context of magnetic survey for the first time, replacing the traditional sliding window. The improved algorithm is more satisfactory for the large-scale UAV-borne magnetic survey because of the simpler and faster workflow, compared with the traditional sliding window (SW)-based Euler method. The field test is conducted and the experimental results show that all procedures in the designed routine is reasonable and effective. The UAV interference field is suppressed significantly with root mean square error 0.5391 nT and the improved Euler deconvolution outperforms the SW Euler deconvolution in terms of positioning accuracy and reducing false targets.

An Unbalanced Sinuous Antenna for Near-Surface Polarimetric Ground-Penetrating Radar

Sinuous antennas are capable of producing ultra-wideband radiation with polarization diversity. This capability makes the sinuous antenna an attractive candidate for UWB polarimetric radar applications. Additionally, the ability of the sinuous antenna to be implemented as a planar structure makes it a good fit for close-in sensing applications such as ground-penetrating radar (GPR). In this work, each arm of a four-port sinuous antenna is operated independently to achieve a quasi-monostatic antenna system capable of polarimetry while separating transmit and receive channels-which is often desirable in GPR systems. The quasi-monostatic configuration of the sinuous antenna reduces system size as well as prevents extreme bistatic angles, which may significantly reduce sensitivity when attempting to detect near-surface targets. A prototype four-port sinuous antenna is fabricated and integrated into a GPR testbed. The polarimetric data obtained with the antenna is then used to distinguish between buried target symmetries.

Integrated geophysical investigations to reconstruct the archaeological features in the episcopal district of Side (Antalya, Southern Turkey)

Electrical resistivity tomography (ERT) and ground-penetrating radar (GPR) are frequently used geophysical methods in the exploration of near-surface targets such as archaeological features. These methods were therefore applied to investigate a wide area in the Side ancient city located nearby the Manavgat district of Antalya, Turkey. The main purpose of the geophysical survey was to answer several questions about the design or usage of a huge area located nearby the episcopal residence of Side that belongs to the late antique/early Byzantine times (5th/6th century A.D.). The investigation area is thought to be the garden (hortus) of the residence. The survey site covering an area of approximately 6000m2 was investigated in detail by two geophysical survey campaigns carried out in 2016 and 2017. During the two survey campaigns, 102 ERT and 127 GPR lines were measured. ERT data were collected by dipole-dipole electrode array, and a 500 MHz shielded antenna was used to measure the GPR data. By evaluating both data sets with appropriate processing steps, 3D geophysical models of the survey area were produced. These models provided obvious traces of buried anthropogenic structures, characterized by relatively high resistivity and high EM wave reflection amplitude. The identified structures lay on the NS-EW axis, which is consistent with the major layout of the city; however, they do not correctly align with the late period structures currently standing on the ground. Therefore, it is interpreted that these buildings belong to a predecessor period (i.e. Roman Imperial Period). Further information such as the purpose of the use of these buildings can only be revealed by an archaeological excavation.

Quantitative Examination of Piezoelectric/Seismoelectric Anomalies from Near-Surface Targets

The piezoelectric and seismo-electrokinetic phenomena are manifested by electrical and electromagnetic processes that occur in rocks under the influence of elastic oscillations triggered by shots or mechanical impacts. Differences in piezoelectric properties between the studied targets and host media determine the possibilities of the piezoelectric/seismoelectric method application. Over a long time, an interpretation of obtained data is carried out by the use of methods developed in seismic prospecting. Examination of nature of piezoelectric/seismoelectric anomalies observed in subsurface indicates that these may be related (mainly) to electric potential field. In this paper, it is shown that quantitative analysis of piezoelectric/seismoelectric anomalies may be performed by the advanced and reliable methodologies developed in magnetic prospecting. Some examples from mining geophysics (Russia) and ancient metallurgical site (Israel) confirm applicability of the suggested approach.

Field Tests of the Vertical-Array Differential Target Antenna Coupling (DTAC) System

We have tested the Vertical-Array Differential Target Antenna Coupling (DTAC) system over a variety of well-characterized, buried targets. The DTAC system produces a primary field in the frequency range of 10 Hz to 10 kHz and records secondary magnetic fields from subsurface targets, relative to a reference frequency. The current DTAC system has higher power, compared with our earlier tests, and this system is suitable for either ground or airborne mapping of a wide variety of near-surface targets, which might be encountered in civil engineering, water resources, environmental characterization, mining, and other natural-resource exploration. Profiles over three well-characterized targets show a close agreement between the measured DTAC response and model simulations. Conventional field measurements using induced magnetic fields are also displayed and they do not show nearly as effective mapping of the targets. Another profile shows that the DTAC method has greatly reduced sensitivity to surface clutter, compared to conventional measurements.

Detecting near-surface buried targets by a geophysical cluster of electromagnetic, magnetic and resistivity scanners

We recently built up a geophysical cluster that includes three kinds of instruments EM61-MK2, Magnetometer and OhmMapper, which can simultaneously measure the secondary electromagnetic, magnetic and apparent resistivity data. This paper will demonstrate field experiments of using this cluster and development of integrated data proceeding schemes that can individually and simultaneously process the three kinds of data measured by the cluster, and yield integrated images of near-surface buried targets. We conducted mapping experiments on a 20m×20m synthetic field site, beneath which five targets: a cross iron pipe, two small iron cylinder and block, a piece of limestone and PVP pipe are buried in different depths. The results show that the integrated cluster and data-processing schemes are successful in detecting these near-surface buried targets.

A solution to unexploded ordnance detection problem from its magnetic anomaly using Kaczmarz regularization

Old military events pose a critical and severe problem for many countries, including the Egyptian northwestern coast. These result in extensive areas of surface/subsurface landmines that affect the economic development plans of many countries. Detection of these landmines becomes a target for many geophysical research teams. Currently, unconventional near-surface flight technologies, such as quad-hexacopters instead of regular land surveys, are used for safety reasons in the acquisition phase. We have introduced a new processing and modeling technique of magnetic data conducted over mines or near-surface geophysical targets for accurate and precise determination of location and depth. The technique is based on the application of the Kaczmarz regularization method to the ill-posed magnetic inverse problem. The advantage of this method is the optimum transformation of regularized normal equations to an equivalent augmented regularized normal system of equations. The condition number of the updated system, which determines the degree of ill posedness, is greatly lower than the original one; this improves and guarantees a good solution to the system. The method is applied to an unexploded ordnance (UXO) test site in the United Kingdom. Our results have determined that the technique is appropriate and promising in efficiently addressing a wide number of problems that are important to near-surface geophysicists, including UXO detection.

Numerical Studies and Potential Applications of the Vertical Array-Differential Target Antenna Coupling (DTAC) Method for Rapid Sensing and Imaging of Subsurface Targets

A new electromagnetic geophysical system has been developed that significantly improves target sensing and imaging capabilities compared to conventional measurements. The Differential Target Antenna Coupling (DTAC) method uses at least two frequencies. One frequency is used as a reference signal to establish the null direction in the magnetic field at that frequency, and then a different frequency is used to measure the change in the null at the new frequency. We can also use an arbitrary number of frequencies with just one reference frequency for wide-bandwidth spectral measurements. Using numerical simulations, we have demonstrated six key aspects of the vertical array-DTAC method: 1) Errors in the receiver coil orientation have little effect on the measurements; 2) anomalies caused by variations in the background resistivity have little effect on the measurements; 3) surface interference, e.g., fences, buildings, and vehicles, have little effect on the measurements; 4) high resolution mapping of targets has been demonstrated through a series of model simulations; 5) frequency-domain measurements, or off-time transient measurements, can be used to provide the required information. Current airborne electromagnetic (AEM) technology can be adapted to this method; and 6) the method can be adapted to a variety of rapidly moving survey applications, including near-surface targets (civil engineering, water resource, and environmental restoration) as well as deep targets (mining and other natural-resource exploration).

Experimental Studies and Verification of the Vertical Array-Differential Target Antenna Coupling (DTAC) Method for Rapid Sensing and Imaging of Subsurface Targets

We have developed a new electromagnetic geophysical system that has significantly improved target sensing and imaging capabilities compared to conventional measurements. In a previous series of papers, we described a new measurement technology called the Differential Target Antenna Coupling (DTAC) method, which used a magnetic-field transmitter (TX) and a magnetic-field receiver (RX), oriented in a horizontal array, with separation distances of the order of tens of meters, and up to tens of kilometers. In the present paper, we use the DTAC method in a vertical-array configuration, with the TX coil directly above two null-coupled RX coils. This vertical array provides a number of important advantages over the large-offset horizontal array for some applications. For example, the vertical array-DTAC method can be adapted to rapidly moving measurements, such as from a helicopter. In addition, the vertical array-DTAC method also suppresses surface clutter, such as fences, steel buildings, and parked vehicles. The DTAC method uses at least two frequencies. One frequency is used as a reference signal to establish the null direction at that frequency, and then a different frequency is used to measure the change in the null at the new frequency. We can also use an arbitrary number of frequencies with just one reference frequency for wide-bandwidth spectral measurements. We first describe the vertical array-DTAC measurement system and discuss the calibration procedure for the instrumentation. Next, we discuss the methods employed for scale-model testing of the system. We validate the vertical array-DTAC method using small-scale controlled tests and demonstrate the enhanced resolution and sensitivity of the vertical array-DTAC method. Suitable TX moments and frequency ranges can be selected for effective study of near-surface targets (civil engineering, water resource, and environmental characterization) as well as deep targets (mining and other natural-resource exploration).

Multistatic detection and tracking using linear maximal sequences

A simulation using linear maximal sequence (m-sequence) transmissions demonstrated the potential for detecting and tracking near-surface targets. In this simulation a low power, omnidirectional source and four omnidirectional hydrophones were arbitrarily located in water approximately 4 m deep. The simulated -15 dB target traveled at 1.5 kn. Using "channel digit response" processing and "block zeroing," the direct arrival, multipaths, clutter and reverberation were rejected. (The block zeroing process is very different from other methods typically labeled as "cancellation.") With the improved signal-to-interference ratio, a simple probability based algorithm demonstrated tracking at source-target distances of 250 m, the maximum range investigated.

Overview of multimethod geophysical system development for enhanced near-surface target detection, discrimination, and characterization

Near-surface geophysical “targets” include unexploded ordnance (UXO), landmines, cavities (including tunnels and underground facilities), contaminant plumes, utilities (including underground storage tanks, pipelines, etc.), archaeological artifacts, graves, forensic evidence, structural foundation investigations for new and existing structures, and assessing the condition of engineered structures (e.g., bridges, dams, levees, roads, airfields, buildings). Application of near-surface geophysics to detect and characterize any of these “targets” is in the public interest, and many applications are clearly and directly related to public safety. While the detection of these targets in a geologic background can often be challenging, the discrimination of the desired target signatures or expressions from “false alarm” target signatures can be an even greater challenge. The discrimination challenge can be as simple as a “go/no-go” decision on the target, or the properties of the target may need to be further characterized after the decision. There is near unanimity among geophysicists (a rare thing) that multimethod, collocated complementary geophysical data enhance not only target detection but also the capability for discrimination and characterization. The U.S. Army Engineer Research and Development Center (ERDC) conducted a multiyear research and development effort that resulted in complementary, collocated simultaneous geophysical survey capabilities for near-surface targets.

A new methodology for the acquisition and processing of audio-magnetotelluric (AMT) data in the AMT dead band

Distant lightning activity, the natural energy source for the audio-magnetotelluric (AMT) method, has a signal minimum between 1 and 5 kHz, the so-called AMT dead band. The energy in this band exhibits both diurnal and annual variation; magnetic-field amplitudes during the daytime are often well below the noise levels of existing sensors (coil magnetometers), thus reducing the effectiveness of the method for quantitative high-resolution studies of near-surface targets. To overcome this deficiency, we propose a hybrid acquisition and processing methodology based on combining the telluric-telluric (T-T) and telluric-magnetotelluric (T-MT) methods in this frequency range. Our method records the telluric channels at several sites and at base and remote reference stations during the day and records the full magnetotelluric (MT) components at the base and remote stations only during the night. Applying a tensor multiplicative relationship between these responses, we obtain the T-MT AMT transfer functions for the sites; these transfer functions can represent a reasonable approximation of the real AMT impedance tensors. To test the approach, a T-MT experiment was carried out in Sudbury, northern Ontario, during summer 2000. We compare the processed daytime data using the conventional MT approach to those obtained from our T-MT approach. The results demonstrate that our method can determine high-quality estimates in the dead band, although the estimates can be severely affected by noise.

Measurements of near-surface bubble plumes in the open ocean with implications for high-frequency sonar performance.

This study examines near-surface bubble data obtained with a self-contained 200-kHz inverted echo-sounder deployed at Ocean Station Papa (NE Pacific, 1400 km west of Vancouver Is.) over an 81-day period in the spring of 1996. The instrument operated continuously, recording calibrated volume scattering profiles from near-surface bubbles with 3-s and 30-cm resolution. The data show the frequent occurrence of bubbles organized into vertical, plume-like structures, presumably drawn downwards by turbulence and other near-surface circulations. Average bubble plume penetrations of up to 15 m were observed, with maximum penetrations up to 25 m. Within the plumes, the backscatter cross section exhibited an exponential decay with depth, with e-folding scale in the range 0.5 to 3 m, increasing proportionally to the square of average plume depth. Using standard models for bubble scattering, and incorporating recent acoustic resonator measurements of bubble-size distributions along with actual bubble plume data, high-frequency near-surface sonar performance models were developed. These models show that on a ping-to-ping basis the bubble plume structures can induce significant spatial variations in the reverberation level and path-integrated extinction losses to near-surface targets.

Integrated signal enhancements in magnetic investigation in archaeology

The magnetic method can be efficiently applied to archaeological investigation especially when the S/N ratio is enhanced appropriately. This study presents a model experiment and field examples of magnetic exploration in archaeology. By using appropriate measuring processes and filtering methods, the conventional and more recent magnetic prospecting techniques are successfully applied to the very shallow, small-scale investigations, which are used to locate and map archaeological targets. We focus on mapping the buried slate caskets in the alluvial environment, which is the most commonly encountered and readily preserved ones at the archaeological sites of Taiwan. The gradiometry and the inferred derivatives may resolve individual anomalies. Locations of the maxima determined by the 3-D analytic signals can be used to describe the outlines of the bodies that cause the anomalies. Furthermore, the susceptibility was very successful in mapping near-surface targets at the Chubin site. The magnetic results of the example at the Hutzushan site, compared to the GPR survey carried out by previous investigators, indicate that the GPR anomalies may be caused by other sources. The magnetic response of a casket can be displayed in various ways, depending on the pole distribution of the casket, the geometry, the magnetization direction and the orientation of buried casket. The processing methods may improve the data resolution, but precautions must be taken for the artifacts.