Marvin A. Speece

Analysis of temperatures in sedimentary basins: the Michigan Basin

We develop an analytical and numerical methodology for the analysis of large bottom‐hole temperature (BHT) data sets from sedimentary basins, and test the methodology using temperature, stratigraphic, and lithologic data from 411 boreholes in the Michigan Basin. Least‐squares estimates of temperature gradients in the formations and lithologies present are calculated as solutions to a large system of linear equations. At each borehole the temperature difference between the bottom and top of the hole is represented as a sum of temperature increments through the various formations or lithologies penetrated by the borehole. Quadratic programming techniques enable bounds to be placed on the gradient solutions in order to suppress or exclude improbable gradient estimates. Numerical experiments with synthetic data reveal that the estimates of temperature gradients for a given formation or lithology are sensitive to the degree of representation of that unit; well represented units have more stable gradient estima...

Precambrian basement structure and Laramide deformation revealed by seismic reflection profiling in the Laramie Mountains, Wyoming

The University of Wyoming conducted seismic reflection profiling in the Laramie Mountains of Wyoming to investigate the nature of Precambrian crustal structure and Laramide deformation in the region. Here the imaging of layering in the Laramie Anorthosite complex confirms that layered intrusions can be reflective and establishes an analog for interpreting events in other seismic reflection data sets where layered intrusions could be the cause of multicyclic events. Moreover, the presence of prominent primary layering in the Complex supports a conclusion that it formed in situ and not as the result of the emplacement of a diapiric crystal mush. Imaging of Laramide thrusts in the Laramie Mountains adds credence to the idea that brittle faults can be reflective in some situations. Migrated apparent dips on these faults are 30° westward. Basement-involved compressive structures, consisting of monoclinal folding and a fault triple junction, demonstrate that compression controlled the style of deformation during the Laramide orogen in Wyoming. West dipping Precambrian structures imaged in these data have a similar attitude to the Laramide structures, suggesting that Laramide deformation was influenced by Precambrian features. These Precambrian structures may be subthrust slices of exotic Proterozoic terranes.

Revised Velocity Structure of Western Montana

Using a one-dimensional (1D) layered earth approach, we determined a new crust and upper-mantle velocity model for western Montana to improve earthquake hypocenter locations. P -wave arrival times recorded on 280 stations from 1432 well-recorded earthquakes provided input for a sparse damped least-squares sensitivity matrix. We solved for 1D velocity structure, refractor depths, station corrections, and hypocentral positions by minimizing travel-time residuals. The new model has three layers with P -wave velocities of 5.70, 6.12, and 6.53 km/sec, with corresponding interface depths of 7.0, 19.8, and 39.7 km below the surface. The upper-mantle velocity is 8.00 km/sec. We determined station corrections and show that the Yellowstone caldera is the only regional geologic feature not adequately modeled by our new 1D velocity model. Use of the new model and station corrections reduced hypocenter location uncertainties and travel-time residuals, implying improved hypocenter locations for western Montana earthquakes. Online material : Table of seismographic stations.

An Over-sea-ice Seismic Reflection Survey In Antarctica Using a GI Air Gun And a Snowstreamer

During the austral spring of 2005, approximately 28 km of over-sea-ice seismic reflection data were recorded over McMurdo Sound, Antarctica, in support of the ANtarctic geological DRILLing program (ANDRILL). The 2005 ANDRILL Southern McMurdo Sound (SMS) seismic survey incorporated techniques that improved the quality of over-sea-ice seismic data. Previous over-sea-ice seismic experiments had limited success because of poor source coupling caused by thin sea ice and source bubble-pulse effects caused by explosive seismic sources placed in the water column. To mitigate these problems, a GeneratorInjector (GI) air gun was used as the seismic source. The GI air gun was lowered into the water column through holes drilled through the sea ice. The GI air gun minimized the source bubble effects that had plagued previous oversea-ice experiments in the Antarctic. Moreover, a 60channel seismic snowstreamer consisting of vertically oriented gimbaled geophones with 25-m takeout spacing was employed to aid rapid data collection. The 2005 SMS seismic survey produced data that accurately tied into existing single-channel marine seismic data and demonstrated the value of the air-gun/snowstreamer system for future over-sea-ice seismic surveys in the Antarctic.

Land Streamer Aided Geophysical Studies at Saqqara, Egypt

During December 2002 and January 2003, Montana Tech in collaboration with Ain Shams University, Cairo, collected Ground Penetrating Radar (GPR) and seismic data at Saqqara, Egypt. The purpose of this study was to see if GPR and seismic methods could detect manmade structures in the subsurface at Saqqara. In particular, land streamer aided, seismic diving-wave tomography was tested as a method to detect archaeological features. Saqqara was one of the principal necropolises of Memphis, an ancient capital of Egypt. The research site was near the 3rd Dynasty pharaoh Djoser’s Step Pyramid—the first monumental structure built entirely of stone. A preliminary GPR study of our site yielded numerous, possibly manmade features in the subsurface with a 4m depth of penetration using 100MHz antennas. A follow-up three-dimensional (3-D) GPR survey over one of the more interesting features showed a broad trench underneath the flat-lying sand that is seen at the surface. This feature is most likely manmade because the ho...

Latest advances in over‐sea‐ice seismic reflection surveys: offshore New Harbor, Antarctica

Approximately 48 km of multi-channel seismic reflection data were collected during the austral spring of 2008 on a sea-ice platform east of New Harbor, Antarctica. The Offshore New Harbor (ONH) survey is the latest in a series of three successful over-sea-ice seismic reflection surveys conducted in McMurdo Sound, Antarctica (Figure 1). Previous to the 2008 ONH survey were the 2005 Southern McMurdo Sound (SMS) and 2007 Mackay Sea Valley (MSV) surveys. Upgrades in the ONH project’s field equipment substantially increased the rate at which seismic data could be acquired in a sea-ice environment compared to all previous surveys. Moreover, despite the success of the SMS survey, timing errors were irreversibly left in SMS data, and static errors were unknowingly left within the SMS data. For all three surveys, timing errors were caused by miscommunications between the Generator Injector (GI) air gun (seismic source) and the acquisition software. Static errors—only on seismic traces coincident with source locations—were caused by GI air gun created air bubbles that were trapped beneath the sea ice. For MSV and ONH data, two methods of statics corrections greatly increased the quality of the final stacked seismic sections and mitigated the aforementioned timing and static errors.

Anisotropy Measurements and the Deep Structure of a Passive Margin: Southwestern Greenland

The non-uniqueness of the laboratory measured physical properties (P-wave velocities and densities) can be partly overcome by the inclusion of anisotropy. Seismic anisotropy estimation can constitute an additional constraint in the determination of the mineralogical composition of the deep crust from indirect seismic measurements. Resolution analysis studies reveal that p-and S-wave velocity measurements derived from wide-angle seismic reflection/refraction data are usually characterized by large error estimates (i.e., very low resolution ). These error estimates render physically meaningless Poisson’s ratio profiles. Also the duality in S-wave velocities due to anisotropy complicates the estimation of Poisson’s ratio. The almost general use of single component (vertical) seismic instruments and low resolution of refraction/wide-angle reflection experiments (mostly due to spatial under sampling) have prevented the use of a nisotropy as a constraint for the determination of lower crustal composition in favor of estimates on Poisson’s ratio. Therefore, we suggest that anisotropy estimates can place new and more relevant constraints on the different rock types present in the deep crust. In order to assess indirect seismic anisotropy measurements we employed three component, densely space large aperture (0-250 km offset, at 100-150 m spacing) seismic recordings acquired along the south west coast of Greenland utilizing REFTEK PASSCAL instruments deployed by the University of Wyoming. With the aid of an inversion scheme that uses reflected and converted energy we determined P- and S-wave velocity-depth functio ns for the passive margin of southwestern Greenland. The inversion suggests a P-wave velocity structure characterized by two gradient zones: a relatively high gradient from the surface to approximately 5 km depth where velocities exceed 6.0 km/s, followed by a low gradient to the base of the crust where velocities reach 7.0 km/s. A high P-wave velocity layer (7.2-7.4 km/s) can be identified between 6-8 km above the Moho. GraVity modeling suggests relatively high densities 3 .0-3.1 kg/m3 for this layer. Independent analysis of the radically and transversely polarized horizonta l components revealed average velocities of 4.9 ± 0.1 km/s and 4.5 ± 0.1 km/s respectively suggesting a seismically anisotropic crust. A time delay of 0.25 s between the radial and the transverse horizontal components of the SIS phase is observed at offsets of 70 km. The radically polarized S-wave is parallel to the southwest coast of Greenland. From the S-wave analysis. the ocean-continent transitional crust is clearly seismically anisotropic above a high velocity layer in the lower continental crust. The density and velocity values suggested for this high velocity structure above the Moho, and the anisotropy measured just above it seem to favor an accretion of hot. mafic mantle material (underplating) at the base of the crust during a rifting episode. Possibly. magmatic underplating during Late Cretaceous rifting of the Labrador Sea heated the preexisting lower crust promoting plastic flow and enabling alignment of anisotropic minerals to produce the seismic anisotropy.

The Offshore New Harbour Project: deciphering the Middle Miocene through Late Eocene seismic stratigraphy of Offshore New Harbour, western Ross Sea, Antarctica

Abstract In the Austral spring of 2008, over 48 km of multi-channel seismic data were collected offshore of New Harbour, which is located in the western most Ross Sea, Antarctica. This project is part of the ANDRILL (Antarctic Geological Drilling) Programmes Offshore New Harbour Project, with its aim to investigate the stratigraphic and tectonic history of the inner shelf of southern McMurdo Sound. Correlating the seismic data to CIROS-1 and ANDRILL AND-2A drillholes provided age dates ranging from Late Miocene to at least Late Eocene for the reflectors and lithological descriptions for the seismic units. This permitted development of a glacial history of this area, resulting in dividing the seismic data into three units: an Upper Eocene–Lower Oligocene unit; an Upper Oligocene–Lower Miocene unit; and a Middle Miocene and younger unit. The seismic section below 700 mbsf has two possible interpretations: (1) substantial Eocene strata are present downdip of CIROS-1, which would represent a potential future-drilling objective for the ANDRILL Programme; or (2) these reflectors may be equivalent to Devonian strata recovered at the Cape Roberts 3 as the two-way traveltime of Eocene and Devonian strata are nearly identical, making it difficult to discriminate between them in seismic data.

A 3D seismic land-streamer system

The 3D seismic reflection method has not been extensively utilized for shallow subsurface investigations because of the relatively high cost of performing 3D surveys. We have designed and constructed a system that could make 3D seismic reflection an affordable option for shallow subsurface exploration by significantly reducing time and labour. In a fashion similar to marine work, a vehicle tows an array of four parallel seismic cables, or land streamers. Each streamer consists of 24 gimballed geophones. The vehicle drags the array from station to station and shots are taken while the array is stationary. We tested our system near Belt, Montana, in an attempt to image an abandoned subsurface coal seam and associated mine workings at a depth of approximately 88 m. For this survey, receiver, receiver line, source and source line spacings were all 1 m. In total, we surveyed a surface area of 100 m x 34 m, achieving a nominal fold of 24. Typical combined advance and occupation times for each station were less than 30 s, using a crew of three people. We were able to image the layered geology in the area and the coal seam; although we were not able to view the individual mine workings because of limited resolution. This testing shows that our system has the potential to expand the application of 3D seismic reflection to shallow subsurface exploration by greatly increasing efficiency.

Using New Laboratory Equipment for Geoscience Technical Outreach

Every year the National Science Foundation (NSF) funds new laboratory equipment for research and education. New laboratory equipment can be used to actively engage the public in new science innovation through hands-on demonstrations and workshops. Recently, an NSF Instrumentation and Laboratory Improvement (ILI) grant was used to acquire a ground-penetrating radar (GPR) system at Montana Tech. In general, ILI grants are used for enhancing undergraduate programs at colleges and universities. In this case, the grant was leveraged to provide technical outreach to K-12 students and teachers, a public service club, local industries, a Native American reservation, and Yellowstone National Park. These outreach activities have introduced future college students and the general public to geoscience as well as educated local industry about the use of GPR. Long-term federal support for the geosciences depends on an educated American public.