Les P. Beard

Simultaneous inversion of airborne electromagnetic data for resistivity and magnetic permeability

Where the magnetic permeability of rock or soil exceeds that of free space, the effect on airborne electromagnetic systems is to produce a frequency‐independent shift in the in‐phase response of the system while altering the quadrature response only slightly. The magnitude of the in‐phase shift increases as (1) the relative magnetic permeability is increased, (2) the amount of magnetic material is increased, and (3) the airborne sensor gets nearer the earth’s surface. Over resistive, magnetic ground, the shift may be evinced by negative in‐phase measurements at low frequencies; but over more conductive ground, the same shift may go unnoticed because of the large positive in‐phase response. If the airborne sensor is flown at low levels, the magnitude of the shift may be large enough to affect automatic inversion routines that do not take this shift into account, producing inaccurate estimated resistivities, usually overestimates. However, layered‐earth inversion algorithms that incorporate magnetic permeab...

Fast resistivity/IP inversion using a low‐contrast approximation

By computing only the diagonal terms of the volume integral equation forward solution of the 3-D DC resistivity problem, we have achieved a fast forward solution accurate at low to moderate resistivity contrasts. The speed and accuracy of the solution make it practical for use in 2-D or 3-D inversion algorithms. The low‐contrast approximation is particularly well‐suited to the smooth nature of minimum structure inversion, since complete forward solutions may be computationally expensive. By using this approximate 3-D solution as the forward model in an inversion algorithm, and by constraining the resistivities and polarizabilities along any row of cells in the strike direction to be held constant, we effect a fast 2-D resistivity inversion that contains end corrections. Because the low‐contrast solution is inaccurate for cells near the electrodes, we employ a full solution to compute the response of the near‐surface when the near‐surface environment is substantially different from the host rock. This resp...

Airborne geophysical surveying for hazardous waste site characterization on the Oak Ridge Reservation, Tennessee

Airborne geophysical methods that were developed for mineral and petroleum exploration can, with some modification, be applied to environmental problems where large areas must be characterized. A helicopter survey that deployed magnetic, electromagnetic, and radiometric sensors carried out one of the first large‐scale airborne environmental surveys at a U.S. government facility at Oak Ridge, Tennessee in 1993–1994. The survey included testing of a new airborne electromagnetic system designed specifically for environmental applications and for controlled field tests of magnetic systems. Helicopter‐borne magnetic measurements were capable of discriminating groups of as few as ten metallic 208-liter (55-gallon) storage drums under representative field conditions. Magnetic and electromagnetic sensors were able to distinguish groups of metal‐filled waste disposal trenches within disposal sites, but were unable to resolve individual trenches. Electromagnetic data proved to be the most effective airborne techniq...

Recent advances in airborne survey technology yield performance approaching ground-based surveys

Airborne magnetic and electromagnetic systems have been very effective over the years for mineral prospecting and in support of petroleum exploration. More recently, these towed-bird systems, operating at sensor altitudes of 30–50 m, have supported environmental investigations. The towed-bird systems can provide regional data for site investigations, such as locating or delimiting the boundaries of waste areas, identifying geologic contacts that influence environmental issues, or mapping saline intrusion. However, these conventional systems cannot provide the resolution required in many environmental and engineering problems because the distance between sensors and target objects is too great. Ground-based surveys are often suitable for addressing these problems but, for many sites, the area can be too large to be expediently addressed with surface geophysics. Contamination of government land with unexploded ordnance (UXO) is one such large-scale problem. As military bases are being closed and returned to civilian use, the removal of UXO has become a critical problem. By one estimate, about 11 million acres within the United States, roughly the sum of the areas of the states of New Hampshire and Vermont, are contaminated with UXO. This includes Department of Defense (DOD) sites, Department of Energy (DOE) sites, Native American sites, and National Park lands. At some DOD sites, large areas are currently off limits for training pending removal of UXO. In other parts of the world, UXO threatens peoples lives every day. In the 1990s, two coauthors of this article, Holladay and Gamey, began to address the UXO problem while working at Aerodat in Toronto. They devised a three-magnetometer system, the HM-3, in which the sensors were mounted in booms attached directly to the helicopter. This architecture provided an opportunity for the pilot to safely fly much closer to the surface. During subsequent joint projects with Oak Ridge National Laboratory (ORNL) researchers, both …

Investigating the resolution of IP arrays using inverse theory

Using a fast 2-D inverse solution, we examined the resolution of different resistivity/IP arrays using noisy synthetic data subject to minimum structure inversion. We compared estimated models from inversions of data from the dipole-dipole, pole-dipole, and pole-pole arrays over (1) a dipping, polarizable conductor, (2) two proximate conductive, polarizable bodies, (3) a polarizable conductor beneath conductive overburden, and (4) a thin, resistive, polarizable dike. The estimated resistivity and polarizability models obtained from inversion of the dipole-dipole data were usually similar to the pole-dipole estimated models. In the cases examined, the estimated models from the pole-pole data were more poorly resolved than the models from the other arrays. If pole-pole resistivity data contain even a fraction of a percent of Gaussian noise, the transformation of such data through superposition to equivalent data of other array types may be considerably distorted, and significant information can be lost using the pole-pole array. Though the gradient array is reputed to be more sensitive to dip than other arrays, it evidently contains little information on dip that does not also appear in dipole-dipole data, for joint inversion of dipole-dipole and gradient array data yields models virtually identical to those obtained from inversion of dipole-dipole data alone.

Comparison of methods for estimating earth resistivity from airborne electromagnetic measurements

Abstract Earth resistivity estimates from frequency domain airborne electromagnetic data can vary over more than two orders of magnitude depending on the half-space estimation method used. Lookup tables are fast methods for estimating half-space resistivities, and can be based on in-phase and quadrature measurements for a specified frequency, or on quadrature and sensor height. Inverse methods are slower, but allow sensor height to be incorporated more directly. Extreme topographic relief can affect estimates from each of the methods, particularly if the portion of the line over the topographic feature is not at a constant height above ground level. Quadrature–sensor height lookup table estimates are generally too low over narrow valleys. The other methods are also affected, but behave less predictably. Over very good conductors, quadrature–sensor height tables can yield resistivity estimates that are too high. In-phase–quadrature tables and inverse methods yield resistivity estimates that are too high when the earth has high magnetic susceptibility, whereas quadrature–sensor height methods are unaffected. However, it is possible to incorporate magnetic susceptibility into the in-phase–quadrature lookup table. In-phase–quadrature lookup tables can give different results according to whether the tables are ordered according to the in-phase component or the quadrature component. The rules for handling negative in-phase measurements are particularly critical. Although resistivity maps produced from the different methods tend to be similar, details can vary considerably, calling into question the ability to make detailed interpretations based on half-space models.

Field tests of an experimental helicopter time-domain electromagnetic system for unexploded ordnance detection

Field trials of a low-flying time-domain helicopter electromagnetic system designed for detection of unexploded ordnance have yielded positive and encouraging results. The system is able to detect ordnance as small as 60-mm rounds at 1-m sensor height. We examined several transmitter and receiver configurations. Small loop receivers gave superior signal-to-noise ratios in comparison to larger receiver loops at low heights. Base frequencies of 90 Hz and 270 Hz were less affected than other base frequencies by noise produced by proximity to the helicopter and by vibration of the support structure. For small ordnance, a two-lobed, antisymmetric transmitter loop geometry produced a modest signal-to-noise enhancement compared with a large single rectangular loop, presumably because the antisymmetric transmitter produces smaller eddy currents in the helicopter body, thereby reducing this source of noise. In most cases, differencing of vertically offset receivers did not substantially improve signal-to-noise ratios at very low sensor altitudes. Signal attenuation from transmitter to target and from target to receiver causes signals from smaller ordnance to quickly become indistinguishable from geological background variations, so that above a sensor height of about 3 m only large ordnance items (e.g., bombs and large caliber artillery rounds) were consistently detected.

Airborne vertical magnetic gradient for near-surface applications

Many specialized applications in near-surface geophysics require greater spatial and amplitude sensitivity than conventional geophysical systems can provide. Recently developed boom-mounted airborne magnetometer and electromagnetic systems are designed to map unexploded ordnance or other small metallic objects over large tracts of government land. These systems operate at altitudes of 1.5–2.0 m to detect ferrous objects with a mass as small as 2 kg. In this low-altitude environment, factors controlling the applicability of a vertical gradient configuration must be reassessed. Our results demonstrate the superiority of measured vertical gradient over calculated gradient (from total field configurations) for data acquired at these low altitudes. We believe the effectiveness is related to reduction of secondary gridding effects associated with several types of positioning errors, and reduction of correlated rotor and compensation noise.

Results of an Airborne Vertical Magnetic Gradient Demonstration, New Mexico

In April 2007, Battelle demonstrated two new airborne vertical magnetic gradiometer systems for unexploded ordnance (UXO) mapping and detection at the Former Kirtland Precision Bombing Range, New Mexico. The primary benefit of vertical gradient is that it reduces helicopter noise, improving signal-to-noise by about a factor of 5 relative to the ORAGS-Arrowhead total field system (Gamey et al., 2004). The two systems are called VG-16 and VG-22. VG-22 was designed for high-resolution detection of small ordnance under good field conditions, while VG-16 was designed with a wider swath for better production rates on wide-area assessment surveys or where conditions require slightly higher altitudes. Performance of the systems was assessed in a 500‐acre test area in which site conditions were well known from previous surveys. This area was deemed relatively quiet magnetically, and was prepared by an unaffiliated contractor which buried 88 small ordnance items at locations that were unknown to Battelle. This “bli...

UXO Time-Constant Estimation from Helicopter-Borne TEM Data

We have developed a new helicopter-borne transient electromagnetic system, known as ORAGS-TEM, which was designed for the detection of unexploded ordnance (UXO) through very low altitude measurements. This system has already achieved considerable success in demonstrations over prepared test grids and a bombing site at the former Badlands Bombing Range (BBR) in South Dakota. UXO ranging in size from 113 kg (250 lb) bombs to 60 mm illumination shells and 7 cm (2.75 in) rocket components were detected by both magnetometer and transient electromagnetic technologies during these trials, conducted in September, 2002. The signal/noise ratio (SNR) observed in TEM measurements during these trials was high, prompting us to ask, “What degree of UXO discrimination can be achieved through detailed analysis of this airborne TEM dataset?” Given the degree of spatial averaging and sampling limitations imposed by the systems flight height and speed, we felt that a very detailed analysis of the type performed by Pasion and Oldenburg would not be practical. Instead, we developed an improved transient analysis technique based on the Matrix Pencil Method to improve the accuracy of exponential decomposition of the observed transients. Where target SNR values of 10 or higher were present this method yielded repeatable results that reliably distinguished compact, long-time-constant targets such as bombs and artillery shells from short-time-constant targets such as thin-walled scrap from practice bombs. As system sensitivity and resolution continues to improve, we expect that similar target discrimination methods will become standard data analysis tools.