Jacob R. Sheehan

An Evaluation of Methods and Available Software for Seismic Refraction Tomography Analysis

Seismic refraction tomography is an alternative to conventional seismic refraction analysis methods. While the limitations and potential pitfalls of conventional refraction methods are well-known the same is not true for refraction tomography. As refraction tomography becomes more widely used, the need to know and understand its capabilities as well as its limitations becomes more critical. In this study we created eight representative models for use in evaluating three commercially available codes as well as refraction tomography in general. These models range from simple two-layer or dipping-layer problems to more complicated models designed to represent features of karst terrains. We demonstrate quantitatively and qualitatively that all three codes perform at a similar level, although each has strengths and weaknesses. Refraction tomography performs well in many situations where conventional methods fail, e.g., where lateral or vertical gradients compose a significant component of the velocity structure.

Evaluation of refraction tomography codes for near‐surface applications

What are the advantages of tomographic methods over delay-time methods? Until recently seismic refraction analysis has been limited to generalized reciprocal, delay-time, or other techniques that require simplifying assumptions such as constant velocity layers and lateral homogeneity within (e.g. Lankston, 1990). The faster and more powerful computers available today have led to the development of various seismic tomography routines. Tomography methods do not make the limited assumptions mentioned above, although each has its own limitations. We compare the performance of three commercially available tomography codes and a delay-time code for representative near-surface models. We use a data set from a site on the Oak Ridge Reservation in East Tennessee (Watson et al., in press, Doll et al., 2002) to represent a typical situation. Results from the synthetic data for each tomography code are compared with results from conventional delay-time analysis. We examine the strengths and weaknesses of these tomographic routines and identify possible artifacts from the inversion. • What are the limitations and strengths of each code studied? • What kind of artifacts or other inaccuracies occur during tomographic analysis? • How does acquisition geometry affect the results of tomography?

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...

Blind Test of Methods for Obtaining 2-D Near-Surface Seismic Velocity Models from First-Arrival Traveltimes

ABSTRACT Seismic refraction methods are used in environmental and engineering studies to image the shallow subsurface. We present a blind test of inversion and tomographic refraction analysis methods using a synthetic first-arrival-time dataset that was made available to the community in 2010. The data are realistic in terms of the near-surface velocity model, shot-receiver geometry and the datas frequency and added noise. Fourteen estimated models were determined by ten participants using eight different inversion algorithms, with the true model unknown to the participants until it was revealed at a session at the 2011 SAGEEP meeting. The estimated models are generally consistent in terms of their large-scale features, demonstrating the robustness of refraction data inversion in general, and the eight inversion algorithms in particular. When compared to the true model, all of the estimated models contain a smooth expression of its two main features: a large offset in the bedrock and the top of a steeply...

Lightning-Induced Remanent Magnetic Anomalies in Low-Altitude Aeromagnetic Data

A low altitude helicopter magnetic survey for unexploded ordnance in New Mexico revealed several magnetic anomalies that were most likely induced by lightning strikes. Lightning-strike magnetic anomalies are not necessarily rare, but may be spaced so widely as to make their detection unlikely in a ground survey. Detailed examples are not often reported because ground geophysical surveys may not cover enough area to detect one, and traditional airborne surveys, which do cover large areas, are carried out at an altitude and line spacing which does not appropriately define the unusual shape of the lightning strike anomaly. However, very low-level (1–3m altitude) airborne magnetic surveys have data densities similar to ground geophysical surveys, yet cover much larger areas. Lightning anomalies appear in magnetic data as radial arms emanating from a central strike point. Each arm has distinct positive and negative lobes. Anomaly amplitudes in the New Mexico survey area ranged from roughly −30to+30nT∕m, and th...

Results of a high-resolution airborne TEM system demonstration for unexploded ordnance detection

Airborne geophysical sensor systems using boom-mounted configurations now play an important role in characterizing ordnance-contaminated defense sites. Most of the systems developed to date have been magnetometer systems. These have proven ineffective at sites where basalt or other magnetic geologic units or soils have caused unacceptable noise in the data. Electromagnetic (EM) systems have been developed as an alternative to magnetometer systems for such sites. Recent evaluation of New Mexico field results from the new TEM-8 time-domain EM system has shown successful detection of emplaced blind-seeded ordnance items. Overall, 109 of 110 items were detected, some as small as 81-mm mortars at an area with moderately magnetic geology. The TEM-8 system was also effective in mapping ordnance at a bombing target with severe geologic interference due to basalt, where a previous airborne magnetometer survey proved ineffective. Data and performance metrics for both survey areas are presented and evaluated.

Historical Development and Performance of Airborne Magnetic and Electromagnetic Systems for Mapping and Detection of Unexploded Ordnance

Over the past fifteen years, notable progress has been made in the performance of airborne geophysical systems for mapping and detection of unexploded ordnance in terrestrial and shallow marine environments. For magnetometer systems, the most significant improvements include development of boom-mounted platforms, and implementation of higher sample rates, denser magnetometer arrays, and vertical gradient configurations. Nine magnetometer-based systems are described and their performance summarized. In prototype analyses and recent U.S. Department of Defense Environmental Security Technology Certification Program (ESTCP) assessments using new production systems, the best performance has been achieved with a vertical gradient configuration. As effective as magnetometer systems have proven to be at many sites, they are inadequate at sites where basalts and other ferrous geologic formations or soils produce anomalies that approach or exceed those of target ordnance items. Additionally, magnetometer systems ar...

Comparison of Performance of Airborne Magnetic and Transient Electromagnetic Systems for Ordnance Detection and Mapping

Magnetic and electromagnetic data collected by helicopter boom-mounted systems at three different sites permit direct comparison of the systems as to their suitability for buried ordnance detection, mapping, and discrimination. Airborne boom-mounted magnetic systems are at a more advanced stage of development than their electromagnetic counterpart. However, in basaltic terrain, transient electromagnetic systems have proved capable of detecting buried ordnance, whereas magnetic systems may fail to detect ordnance altogether. Magnetic systems use passive sensors and these can be distributed along the boom structure such that dense data can be collected with sensors spaced 1–2 m apart over a broad swath, up to 12-m wide. The ORAGS-TEM electromagnetic system, having only two receivers, must rely on interleaved flight lines to obtain data of a spatial density approaching that of the airborne magnetic systems. The total magnetic fields from unexploded ordnance decays at 1/R3 for total field and its gradient at 1/R4. This permits adequate signal-to-noise levels to be easily attained for larger ordnance types at survey heights up to seven meters. Active electromagnetic fields decay at between1/R4 and 1/R6, depending on ordnance type and sensor geometry, and this constrains current electromagnetic systems to practical survey altitudes of less than three meters. Tests at the Badlands Bombing Range indicate that, in some circumstances, the signal-to-noise for the airborne electromagnetic system exceeds that of airborne magnetic systems, and even ground electromagnetic systems. Because time must be allowed for transmitter current buildup and decay, ORAGS-TEM is not capable of sampling along line at the same spatial density as can magnetic systems. However, the temporal signal decay permits greater opportunity for ordnance discrimination than magnetic measurements.

Advances in airborne geophysical systems for ordnance mapping and detection

Over the past 10 years, several advances have been made in helicopter geophysical systems for mapping and detection of unexploded ordnance (UXO). Earlier total-field magnetometer systems for this application invoked a novel boom-mounted sensor design, which en-abled safe operation at altitudes of a few meters above ground level and permitted detection of individual small ferrous objects. The first such system, the Aerodat HM-3, had three cesium vapor magnetometers at 6-m spacing, mounted at the tips of one forward and two lateral booms (Gamey and Mahler, 1999). Subsequent improvements incorporated as many as eight magnetometers at 1.7-m spacing (Doll et al., 2003), as in the ORAGS-Hammerhead and ORAGS-Arrowhead systems. Since that time, improved magnetometer systems have been developed, and a production time-domain electromagnetic system has been introduced.

UXO detection and prioritization using combined airborne vertical magnetic gradient and time‐domain electromagnetic methods

In January 2008, low-altitude (~1-3 meters above ground level) airborne geophysical surveys were carried out at the Marine Corps Air Ground Combat Center (MCAGCC), near Twentynine Palms, California. The primary goal of the surveys was to assess the viability of airborne magnetic or electromagnetic geophysical surveys at MCAGCC for detection and mapping of unexploded munitions. Due to high magnetic content in the rocks and soils at MCAGCC magnetic methods had been shown in the past to have limited usefulness. The background magnetic conditions made this site a good candidate for the use of a new 8channel Time-domain Electromagnetic system (TEM8) developed by Battelle. The first phase of the project was an assessment of the efficacy of TEM8 and vertical magnetic gradient (VG-22) technologies, based on results from surveys of two 8 hectare areas and a 2 hectare geophysical prove out (GPO) area. This demonstration showed that both VG-22 and TEM8 were useful at this site, but the combination of the two datasets is more effective than either dataset used singularly for target prioritization.