David C. Nobes

Seismic traveltime inversion based on tomographic equation without integral terms

The Jacobian matrix in the seismic traveltime tomographic equations usually contains several integral terms. These integral expressions not only greatly increase the computational complexity of seismic traveltime tomography, but also increase difficulty for programming these expressions. Therefore, if these integral expressions of the Jacobian matrix can be eliminated, the program of seismic traveltime tomography can be greatly simplified. In order to solve the computational complexity of the traditional seismic traveltime tomography, we found an anisotropic seismic traveltime tomographic equation which does not contain integral expressions. Then, it is degenerated into an isotropic seismic traveltime tomographic equation. In order to verify the effectiveness of this seismic traveltime tomographic equation based on the node network, a program has been coded to execute seismic traveltime inversion. For a crosswell checkerboard velocity model, the same results are obtained by this proposed tomographic method and the traditional method (with integral terms). Besides, two undulating topography velocity models are used as testing models. Numerical simulation results show that this proposed tomographic method can achieve good tomograms. Finally, this proposed tomographic method is used to investigate near surface velocity distribution near a power plant. Tomogram indicates that contaminated liquid diffuses and aggregates along strata at a certain depth. And velocity is lower near pollutant source than that away from it. A tomographic equation without integral terms is deduced from anisotropic case.The equation is simpler and easier to program than equation with integral terms.The unit of velocity should keep in km/s for input and output velocity models.Numerical and field data test suggest that this proposed equation is effective.

Geophysical Imaging of an Early Nineteenth Century Colonial Defensive Blockhouse: Applications of EM Directionality and Multi-parameter Imaging

In 1845, the French navy built three blockhouses as part of their defence of French settlers in Akaroa, located on Banks Peninsula, near Christchurch, New Zealand. In the late 1860s, the blockhouses were removed and the timber used for other purposes. Two of the blockhouses were situated at either end of Akaroa township; their locations are well known and documented. The position of the third, in the village of Takamatua, near Akaroa, was not well known, but was thought to have been situated in what became a public reserve, first known as the Blockhouse Domain and more recently as the Takamatua Domain.

Ground Penetrating Radar Resolution in Archaeological Geophysics

Ground penetrating radar (GPR) is now a common tool for archaeological imaging. However, difficulties arise in choosing the right antenna. Do we choose high-frequency antennas to yield lots of detail? Or do we choose lower frequency antennas to see larger scale features that provide site context? Often we are tempted to use a high-frequency signal, in order to see all the detail. However, this can be counter-productive. Lots of detail may actually obscure the features that are the primary targets. Conversely, lower frequency antennas can locate the features of interest, but may not provide as much detail as desired. In general, choosing a lower frequency antenna yields better results. The optimum choice of antenna depends on the site conditions; using a range of antennas for initial tests helps establish the “best” signal frequency to use. The imaging may be best done in two stages: an initial stage using low frequency antennas; followed by high-frequency imaging to yield greater detail over the areas where it is useful.

Rock magnetic investigation and its geological significance for vein‐type uranium deposits in southern China

To characterize the metallogenic environment of a typical vein-type uranium deposit, samples from diabase dykes, alteration zones including metamorphic diabase and uranium ore, and granites were systematically investigated for six boreholes from southeastern China. Rock magnetic results indicate that coarse-grained magnetites (pseudosingle domain, PSD, and multidomain, MD) are dominant magnetic carriers in diabase. In contrast, the uranium ore is dominated by fine-grained magnetites (superparamagnetic, SP, and single-domain, SD). The concentration of magnetic particles in fresh granites is low. Magnetic properties of metamorphic diabases exhibit much greater variability of magnetic properties and higher degrees of sulfuration than unaltered diabase and granite, due to contact metasomatism and reduction effects close to the vein. Compared with diabase, magnetic remanence of the uranium ore is much lower, but displays much higher stability. The Koenigsberger ratio Q peaks in the uranium ore with a value of ∼1.00. Using the systematic rock magnetic results to constrain the interpretation, the contribution of the intersection zone of diabase dyke and silicified fault to magnetic anomalies was further modeled, and the effects of the ore body are significant for magnetic exploration. Overall, rock magnetic investigations of vein-type uranium deposit provide a better understanding of the interactions between different rock types, and further facilitate regional magnetic surveys on the ground.