Review article Caveats on tomographic images Gillian R. Foulger, 1 Giuliano F. Panza, 2,3 Irina M. Artemieva, 4 Ian D. Bastow, 5 Fabio Cammarano, 4 John R. Evans, 6 Warren B. Hamilton, 7 Bruce R. Julian, 1 Michele Lustrino, 8,9 Hans Thybo 4 and Tatiana B. Yanovskaya 10 1 Department of Earth Sciences, Durham University, Durham UK; 2 Department of Mathematics and Geosciences, University of Trieste, Italy and the Abdus Salam ICTP-SAND Group, Trieste Italy; 3 Institute of Geophysics, China Earthquake Administration, Beijing China; 4 Department of Geography and Geology, University of Copenhagen, Denmark; 5 Department of Earth Science and Engineering, Imperial College, London SW7 2AZ, UK; 6 U.S. Geological Survey, Menlo Park California, 94025, USA; 7 Department of Geophysics, Colorado School of Mines, Golden Colorado, 80401, USA; 8 Dipartimento di Scienze della Terra, Universit  a degli Studi di Roma La Sapienza, P.le A. Moro, 5, 00185, Rome Italy; 9 CNR – Istituto di Geologia Ambientale e Geoingegneria (IGAG) c/o, Dipartimento di Scienze della Terra, Universit  a degli Studi di Roma La Sapienza, Rome Italy; 10 Department of Physics of the Earth, Sankt-Petersburg State University, Sankt-Petersburg Russia ABSTRACT Geological and geodynamic models of the mantle often rely on joint interpretations of published seismic tomography images and petrological/geochemical data. This approach tends to neglect the fundamental limitations of, and uncer- tainties in, seismic tomography results. These limitations and uncertainties involve theory, correcting for the crust, the lack of rays throughout much of the mantle, the difficulty in obtaining the true strength of anomalies, choice of what background model to subtract to reveal anomalies, and what cross-sections to select for publication. The aim of this review is to provide a relatively non-technical summary of the most important of these problems, collected together in a single paper, and presented in a form accessible to non-seismolo- gists. Appreciation of these issues is essential if final geody- namic models are to be robust, and required by the scientific observations. Terra Nova, 25, 259–281, 2013 Introduction Seismic tomography is the only tool available to map the deep structure of the Earth, and it comprises a pivotal element of geochemical and dynamic models of the mantle. However, seis- mic tomography is limited by issues that are not widely appreciated, in particular by non-seismologists. Counterintuitively, teleseismic tomo- graphy cannot image the three-dimen- sional structure of the mantle. In all tomography methods, the strengths of calculated anomalies depend on subjective choices of inversion para- meters, but are still commonly trans- lated directly into critical geological parameters such as temperature and density. Tomography does not return thermal or geological information, but seismological parameters and assumptions have to be used to trans- late seismic results into other physical parameters, e.g. temperature or con- vective motion. Resolution- and error-assessment methods cannot encapsulate the true errors, and are insensitive to critical experimental limitations that invalidate parts of most derived structures. Petrology and geochemistry are indirect tools for probing the mantle. They can provide information on its composition, but with virtually no spatial resolution. Isotope geochemis- try can add a fourth dimension (time). The results from seismic tomography and petrology/geochemistry are fre- quently combined to develop geologi- cal models of the structure and dynamics of the mantle. This endeav- our is, however, fraught with difficul- ties. Few practitioners are equally expert in both disciplines, and often, the data from the more familiar disci- pline are interpreted jointly with pub- lished interpretations from the less familiar one. Tomography models are all too often assumed to provide essentially proof positive of things that the data physically cannot prove. Here, we provide an accessible overview, aimed primarily at non-seis- mologists, of the main problems that limit seismic tomography. This article is not intended to be an in-depth, technical review for theoretical seis- mologists. That material can be found elsewhere (e.g. Dahlen and Tromp, 1998; Aki and Richards, 2002; Nolet, 2008). It is essential to appreciate the main problems inherent in many tomographic results published over the last four decades, but not obvious prima facie, if cross-disciplinary inter- pretations are to be made that are both robust and required the data. Methodological problems Tomographic methods Tomographic methods used to image large-scale structures in the mantle may be grouped into teleseismic-, sur- face-wave- and whole-mantle tomo- graphy. The first two are most sensitive to shallow mantle structure. The latter is the only method that can provide spatial information about the lower mantle. Teleseismic tomography uses the relative arrival times of seismic waves from distant earthquakes. It has resolution on the scale typically of Correspondence: Professor Gillian R Foulger, Department of Earth Sciences, University of Durham, Science Labora- tories, South Road, Durham DH1 3LE, UK. Tel.: 0191 334 2314; fax: 0191 334 2301; e-mail: g.r.foulger@durham.ac.uk doi: 10.1111/ter.12041 © 2013 John Wiley & Sons Ltd 259