Alan J. Witten

Passive imaging of underground acoustic sources

The problem of imaging the intensity distribution of a subsurface acoustic source from passive measurements is considered. The source may be spatially extended, and is assumed to be random and spatially incoherent. Two passive imaging algorithms are examined and shown to be closely related. The first algorithm is based on time-domain migration in which signals recorded by an array of receivers are backpropagated to multiple source locations. The second algorithm is based on backprojecting cross-correlating signals computed between all pairs of receiving elements. When the latter algorithm is applied in two dimensions, the correlation peak can be shown to be the line-integral of the source intensity along a hyperbolic path whose foci are the receiving points. This suggests a filtered-backprojection procedure in which the backprojection is performed along multiple hyperbolic paths. The algorithm is easily generalized to three dimensions, in which the cross-correlated signals are backprojected over hyperbolo...

The Tulsa Race Riot of 1921: A geophysical study to locate a mass grave

On 30 May 1921, a black man was arrested in Tulsa, Oklahoma, U.S., for allegedly attempting to rape a white woman in an elevator in a downtown building. Many days later, the woman recanted her original story; however, by this time events had spun out of control. When the arrest was revealed to the public in a May 31 newspaper article, an armed mob convened outside the jail intent on lynching the suspect. A lynching was prevented by the arrival of armed black World War I veterans. The rage of the mob was redirected, resulting in a night of violence. Red Cross records state that 1115 residences were destroyed and historical photos document total destruction of the Greenwood section (the focal point of Tulsas black business community) of the city. The current estimate of deaths resulting from this event is approximately 300. As a result of the chaos that followed these violent events, there was little effort to document casualties. In fact, it was not until the definitive study by Elsworth in 1982 that there was any significant historical documentation. Now, 80 years later, not a single body has been recovered. Rumors of mass graves persist, but several recent excavations have yielded nothing. The only eyewitness account is a recent one by a man in his eighties who in June 1921 witnessed several crates containing the bodies of blacks being prepared for mass burial in a pit in a section of Tulsas Oaklawn Cemetery. In 1998, researchers with the Tulsa Race Riot Commission analyzed numerous accounts of mass graves and decided to study three locations: Newblock Park, Oaklawn Cemetery, and Booker T. Washington Cemetery. In July 1998 the three locations were investigated by ground penetrating radar (Maki and Jones, 1998). A pulseEKKO 1000 unit with 225-MHz and 450-MHz antenna tested …

Geophysical Surveys in the Jebel Hamrat Fidan, Jordan

The Jebel (Jebel is mountain in Arabic) Hamrat Fidan marks the “gateway” to the Feinan district of southern Jordan— one of the largest sources of copper during the prehistoric and Early Bronze Ages in the eastern Mediterranean. Preliminary excavations and surveys at sites along the Wadi Fidan have revealed a long history of settled occupation extending from the Pre-Pottery Neolithic (ca. 6,500 B.C.) to early medieval times. Because of this long history of occupation, and the fact that this area was a regional center for the production of copper, the study of this area is important for understanding early metallurgy, craft specialization, and social evolution. During the summer of 1997, geophysical investigations at a series of Neolithic and Bronze Age sites identified specific areas within Wadi Fidan for future intensive excavations. Three geophysical techniques (electromagnetic induction, ground-penetrating radar, and magnetometry) were used to help locate buried architectural and industrial features remaining from early mining and metallurgical operations, including copper ore bodies or voids. Geophysics was not used at the actual mining sites because of scheduling constraints; however, geophysics did delineate buried stone walls at three distinct Wadi Fidan sites. Magnetometry and ground penetrating radar provided little useful information. Buried stone walls were apparently “masked” by numerous magnetic stones on the ground surface making magnetometry useless. Reflections from known strata demonstrated that radar penetrated the ground adequately; however, known shallowly buried walls were not recognizable. Electromagnetic induction produced maps of linear and rectilinear features that suggested spatial distribution of widespread buried stone walls suitable for future excavation. A significant and unexpected finding was that electromagnetic induction proved capable of delineating buried stone walls. 2000 John Wiley & Sons, Inc.

Magnetic and Electromagnetic Induction Studies at Archaeological Sites in Southwestern Jordan

In support of ongoing archaeological excavations in southwestern Jordan, a number of geophysical studies have been performed in this region. The work reported was executed in three different years and, in the first year, electromagnetic induction, magnetometry, and ground penetrating radar studies were performed at three sites in the region known to have buried stone walls. While integrated geophysical methods were planned, this first field experience revealed that electromagnetic induction provided, by far, the most useful results and this became the mainstay of subsequent geophysical investigations. To date, five large sites and numerous smaller sites have been surveyed ranging in dates of occupation from the Pre-Pottery Neolithic to the Iron Age. Results from three of the larger sites are reported here. These sites are sufficiently large that they cannot be completely excavated in a single field season and the results of the geophysics are used to identify the most interesting areas at a particular sit...

A MATLAB-based three-dimensional viewer

Rarely is subsurface structure one- or two-dimensional and interpretation of information derived from geologic mapping, well logs, and geophysical measurements can be enhanced when viewed in three dimensions with proper perspective. A MATLAB-based three-dimensional viewer is presented that allows the display of the three-dimensional volume as user-selected orthogonal slices, rendering of the entire volume with chair cuts, and the display of bounding surfaces in three dimensions.

Characterizing the Distribution of Near‐Surface Solution Channels Using Electromagnetic Induction and Ground Penetrating Radar

A series of four ponds were developed at a sewage treatment facility in southwestern Oklahoma. Three of these four ponds, when put into service, failed to hold water. The reason for this is that the hydraulic head produced by the water in these ponds caused sink holes to develop into a shallow gypsum formation allowing this water to drain. In order to evaluate potential strategies to mitigate this problem, it was first necessary to establish the horizontal extent and depth interval of the solution channels beneath the sewage treatment ponds. To reach this end, electromagnetic induction (EMI) and ground penetrating radar studies were performed at these ponds. The EMI data showed a distribution of apparent low electrical conductivity regions that correlated well with the pattern of open sink holes. Two‐ and three‐dimensional images reconstructed from the GPR data provided confirmation of the EMI results and established a depth to the voids of about 40 cm.

Imaging Underground Structures Using Broadband Electromagnetic Induction

Broadband electromagnetic induction is a promising technique for the detection and location of underground structures. Along with being simple to use and allowing rapid data acquisition, underground structures can be detected by this method either because the large volume of air necessary for human occupancy will be manifested as an electrical conductivity low relative to the host formation or because there could exist a large metallic mass associated with either structural support or the mission of the facility that would constitute a relative conductivity high. The use of broadband information literally offers an additional dimension, that being depth, to the level of information that can be gleaned from acquired data since, by varying the operating frequency, the probing depth changes. A three‐dimensional tomographic imaging algorithm is applied to broadband data acquired at two underground structure sites to reconstruct the relative spatial variations in electrical conductivity. A pair of parallel sub...

Three-dimensional imaging of a deep marine channel-levee/overbank sandstone behind outcrop with EMI and GPR

Deepwater channel-levee/overbank sandstones are important sources of oil and gas, but their internal stratigraphy is complex and not well understood. One means to better characterize this complex stratigraphy is through the study of outcrops, such as the Dad Sandstone member of the Lewis shale (Figure 1) in Carbon County, Wyoming, U.S. (Pyles and Slatt, 2000). Figure 1. (top) The outcrop of the Dad Sandstone member of the Lewis Shale. A series of channel-fill sandstones dip toward the lower right (west). The lowermost sandstone is the subject of this study. This sandstone trends along the outcrop face toward the lower right, but is juxtaposed against shale toward the west, at the same stratigraphic level. The GPR line on the lower left of the figure (modified from Young et al., 2003) shows the base of the channel-fill sandstone (red line) where it intersects the ground surface. Northwest of the base of the sandstone, at equivalent radar times, the lithology is shale. The lower right figure shows the EMI volume in plan view, with sandstone in yellow and shale in gray; note the bend in the sandstone at the location where the channel base intersects the ground surface. Channel-levee/overbank systems are a result of multiple sediment gravity flows. The channel itself is formed from aggradation of levees over long periods of time. As sediment flows within the channel, fine particles are expelled over the channel edges, thus forming the levees. Coarser particles not deposited on the levee are transported beyond the channel to form frontal splays or lobes. Sediment may slump off the levee walls onto the channel floor during this time interval. Channels are later backfilled with sand and/or mud as transport energy diminishes, probably during an early rise in sea level. The materials that fill the channel can be configured in different arrangements …

Target Parameter Estimation and Survey Design in Magnetometry

Magnetic targets can be detected from magnetic measurements because such targets possess permanent or remnant magnetization, or magnetization induced by the earths background magnetic field. Provided that these targets are of sufficient source strength relative to their depth, they can be detected from measurements of spatial variations in magnetic induction. In many cases, the identification of the existence of a magnetic anomaly represents insufficient information for addressing a particular problem. For example, consider the problem of remediation of buried unexploded ordnance (UXO). While it is obviously important to identify every UXO, it could be cost‐prohibitive to excavate every magnetic anomaly. There are many other applications where the capacity to discriminate various types of magnetic targets is of practical importance. A procedure for estimating the location and dipole moments of magnetic targets is presented here. Specifically, a Levenberg‐Marquardt non‐linear least squares parameter estim...

Seismic reflection processing by plane-wave synthesis

Seismic reflection is the preferred technique for subsurface stratigraphic and structural mapping and it is routinely used for hydrocarbon exploration and characterizing the structure of the Earths upper crust. Because of the time and skill typically required for processing seismic reflection data, the method is underutilized for near-surface applications. Here, a simple seismic reflection processing procedure is introduced that is computationally efficient and simple to apply. The method requires data from an array of geophones and a multiplicity of source location to synthesize the response to a plane wave propagating straight down in variable terrain. The signal processing method is demonstrated through its application to both synthetic and real data with encouraging results.