Geophysical Prospecting, 2013, 61, 701–711 doi: 10.1111/1365-2478.12010 The use of low frequencies in a full-waveform inversion and impedance inversion land seismic case study Guido Baeten 1 , Jan Willem de Maag 1 , Ren´ e-Edouard Plessix 1∗ , Rini Klaassen 1 , Tahira Qureshi 1 , Maren Kleemeyer 1 , Fons ten Kroode 1 and Zhang Rujie 2 1 Shell Global Solutions International and 2 BGP International Received February 2012, revision accepted July 2012 ABSTRACT Velocity model building and impedance inversion generally suffer from a lack of intermediate wavenumber content in seismic data. Intermediate wavenumbers may be retrieved directly from seismic data sets if enough low frequencies are recorded. Over the past years, improvements in acquisition have allowed us to obtain seismic data with a broader frequency spectrum. To illustrate the benefits of broadband acquisition, notably the recording of low frequencies, we discuss the inversion of land seismic data acquired in Inner Mongolia, China. This data set contains frequencies from 1.5–80 Hz. We show that the velocity estimate based on an acoustic full- waveform inversion approach is superior to one obtained from reflection traveltime inversion because after full-waveform inversion the background velocity conforms to geology. We also illustrate the added value of low frequencies in an impedance estimate. Key words: Full-waveform inversion, Impedance INTRODUCTION Two of the main goals of exploration geophysics consist in obtaining a structural image of the Earth’s interior and esti- mating some elastic parameters (velocities) and rock physics and fluid content parameters (porosity, saturation etc). The structural image corresponds to a map of acoustic impedance contrasts and the rock physics and fluid content are often derived from acoustic or elastic impedance maps. One ex- tracts this impedance information from primary reflections recorded in seismic data after having removed the wave prop- agation effects through migration. One can use different work- flows to create these impedance maps (van Riel 2000). The workflow generally includes tomography that gives the acous- tic background velocity, migration to obtain the reflectivity map, which more or less corresponds to the impedance con- ∗ E-mail: ReneEdouard.Plessix@shell.com trasts convolved by a wavelet, a well calibration to correct for the depth uncertainties but more importantly to fill the wavenumber gap between background velocity and reflectiv- ity and impedance inversion with reflectivity, background ve- locity and a velocity-density relation as inputs (Russell and Hampson 1991; Latimer, Davison and van Riel 2000). The wavenumber gap, i.e., the lack of intermediate wavenumber information in the seismic data, is a consequence of the lack of low frequencies in the seismic data (Claerbout 1983; Jannane et al. 1989; Ghosh 2000; Latimer et al. 2000). Indeed, tomog- raphy based on the moveout of the reflection curve versus offset provides a background velocity with frequency content up to about 1 Hz while the minimum frequency in the data spectrum is generally around 6 Hz (Latimer et al. 2000). The frequency content of the velocity background after tomogra- phy depends on the pick density. When possible, dense picking may lead to higher frequency content. However, classically, a frequency gap exists with real data. The lack of interme- diate wavenumbers dramatically hampers our ability to use C  2013 Shell Global Solutions International, The Netherlands 701