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This paper presents a comparison of ray-theoretical and finite-frequency traveltime tomography for compressional waves. Our data set consists of 86 405 long-period P and PP–P traveltimes measured by cross-correlation. The traveltime of a finite-frequency wave is sensitive to anomalies in a hollow banana-shaped region surrounding the unperturbed ray path, with the sensitivity being zero on the ray. Because of the minimax nature of the surface-reflected PP wave, its sensitivity is more complicated. We compute the 3-D traveltime sensitivity efficiently by using the paraxial approximation in conjunction with ray theory and the Born approximation. We compare tomographic models with the same χ2 fit for both ray theory and finite-frequency analysis. Depending on the depth and size of the anomaly, the amplitudes of the velocity perturbations in the finite-frequency tomographic images are 30–50 per cent larger than in the corresponding ray-theoretical images, demonstrating that wave front healing cannot be neglected when interpreting long-period seismic waves. The images obtained provide clear evidence that a limited number of hotspots are fed by plumes originating in the lower mantle. [ABSTRACT FROM AUTHOR]

Comments on the query on the accuracy of the algorithm for the approximate estimation of the resolution matrix of tomographic systems. Suggestion of a Newtonian scheme; Question of the usefulness of the one-step backprojection; Similarity between A[sup T]A and the estimated resolution matrix.

We present tomographic evidence for the existence of deep-mantle thermal convection plumes. P-wave velocity images show at least six well-resolved plumes that extend into the lowermost mantle: Ascension, Azores, Canary, Easter, Samoa, and Tahiti. Other less well-resolved plumes, including Hawaii, may also reach the lowermost mantle. We also see several plumes that are mostly confined to the upper mantle, suggesting that convection may be partially separated into two depth regimes. All of the observed plumes have diameters of several hundred kilometers, indicating that plumes convey a substantial fraction of the internal heat escaping from Earth.

The diarrhea associated with malabsorption of bile salts such as the secondary hydrophobic taurodeoxycholate (TDC) may be partly explained by the TDC-induced increase in colon Cl(-) secretion. We, therefore, investigated the effects of TDC (0.5-8 mM) on electrical parameters and electrolyte transport of rat proximal colon mucosa mounted in Ussing chambers. Colonic secretion, measured as short circuit current (I(SC)), progressively increased on mucosal incubation with TDC ranging 0.5-2 mM; up to TDC 2 mM, a spontaneous recovery toward control values with no changes in epithelial resistance (Rt), and lactate dehydrogenase (LDH) release was observed. In contrast, for TDC > 2 mM, I(SC) increased further and the effect was progressive and associated with a significant decrease in the Rt and increased LDH release, implying a cytolytic effect. Mucosal preincubation with the Cl(-) channel inhibitor 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB), fully prevented the precytolytic effect of TDC on I(SC). Serosal preincubation with furosemide, a Na(+)/K(+)/2Cl(-) cotransporter inhibitor, significantly reduced TDC-induced increase in I(SC). Inhibition of the basolateral Ca(2+)-dependent K(+) channel-rSK4-with serosal clotrimazole or incubation with mucosal Ca(2+)-free (EGTA) buffer completely prevented precytolytic TDC-induced increase in I(SC). In conclusion, Cl(-) secretion is activated in colon mucosa by TDC low concentrations; while at higher concentrations, a detergent cytotoxic effect intervenes. Activation of the Ca(2+)-dependent basolateral K(+) pathway, through TDC-induced apical Ca(2+) influx, provides the Na(+)/K(+)/2Cl(-) basolateral activation, thereby the driving force for the apical exit of Cl(-) ions. These findings further enhance the knowledge of the pathogenic mechanisms of diarrhea associated with bile salt malabsorption.