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Numerical forward modeling, predicting an observable response given a mathematical representation of the Earth, is an important component of practical exploration work. In addition, derivatives which relate changes in response to changes in the Earth model are useful for experimental design and are a crucial element of linearized inversion techniques. Differentiation of kernels followed by numerical integration using a fast Hankel transform provides an efficient combination of forward and sensitivity modeling for frequency-domain horizontal electric dipole-dipole sounding over a layered seafloor. Our code is validated against an independent forward modeling technique using a mode analysis and against central difference derivatives. Efficiency is important in the application of regularized inversion to large data sets; we give an example from the East Pacific Rise, requiting 2000 elements in the Jacobian matrix. We illustrate the use of forward modeling and discrete analogs of the Fr•chet kernels to provide aid to physical intuition and experimental design in the context of the electrical conductivity of the oceanic lithosphere. By using the most favorable parts of range-frequency space, experiments using current technology should be capable of distinguishing a thicker, less resistive, from a thinner, more resistive "lithospheric resistor" layer. Introduction The electrical conductivity profile of normal oceanic lithosphere and uppermost asthenosphere still provides a subject of lively debate. In addition to the intrinsic geophysical interest in this problem, for example to establish limits on the oceanic lithosphere's water content and the submarine asthenosphere's melt content, this issue is of considerable technical importance for magnetotelluric data interpretation and possibly for the interpretation of motionally induced voltages in terms of large-scale ocean circulation. The seafloor horizontal electric dipole-dipole (HED) controlled source method has been instrumental in demonstrating the existence of highly resistive lithosphere at several locations under the ocean basins [Young and Cox, 1981; Cox et aI., 1986]. In this pair of papers, we use both sensitivity and experimental studies to address some further contributions of this method to the problem of the crust and uppermost mantle's conductivity profile. The present paper gives the necessary background on the experimental method and presents our computational method, an efficient combination of forward and derivative modeling for the HED experimental configuration over a one-dimensional seafloor. We test various aspects of our code, illustrate it in regularized inversion of a small seafloor data set, and demonstrate how insights obtained from forward and sensitivity mod-Paper number 95JB03739. 0148-0227/96/95JB.03739505.00. cling can be used in experimental design. In particular, we attempt to outline the limits of this method's deep sensitivity. Seafloor controlled-source measurements, as well as magnetotelluric studies using the TM (transverse magneti...

A reference model for the electrical conductivity structure beneath the deep seafloor is proposed and justified using a variety of geophysical evidence. The model consists of relatively conductive sediment and crustal layers of 6.5 km extent overlying a resistive (=10 -5 S/m) subcrustal channel of 30 km thickness and terminated in a deeper conductive layer and half-space. Its seafloor-to-seafloor response to a horizontal electric dipole source is explored as a function of frequency and range, showing that, compared with the response for a halfspace with the lowest conductivity in the reference model, significant enhancement of the field amplitude can occur at long ranges (>100 km) and low frequencies (<1 Hz). At the same time, marked attenuation relative to the half-space response is seen at higher frequencies. The field enhancement is due to trapping of electromagnetic energy in a leaky subcrustal waveguide, as demonstrated by computing the complex Poynting vector. The attenuation occurs in the relatively conductive sedimentary and crustal layers overlying the lithospheric waveguide when their electrical thickness exceeds a skin depth. The results indicate that attempts either to model controlled electromagnetic sources or to interpret controlled source data using half-space models for the Earth can be badly misleading. The practicality of lithospheric communications in the real Earth is also investigated. Using measured receiver noise figures and the reference model, the receiver bandwidth necessary to achieve a given signal-to-noise ratio as a function of range and frequency is estimated for a seafloor horizontal source of strength 105 A-m. The results indicate that significant (=100 km) ranges can be achieved only around 1 Hz with a bandwidth of =1 Hz at a SNR of 10, yielding a very low data rate of <3.5 bits/s. Longer ranges and higher frequencies are precluded by attenuation in the sediment and crustal layers and because the conductivity in the resistive channel is too large. INTRODUCTION Paper number 90RS00468. 0048-6604/90/90RS-00468508.00 communications and underwater detection. However, the attenuation (e-folding) scale of electromagnetic signals in seawater is quite small (270 m at 1 Hz) and decreases as the square root of the frequency. As a result, direct propagation through the sea has limited applications, and transmission mechanisms which increase the effective range become important. Recent theoretical investigations have focused on the "up-over-down" path through the atmosphere [e.g., Bubenik and Fraser-Smith, 1978; Fraser-Smith and Bubenik, 1979] and the "down-over-up" path along the relatively resistive seafloor [e.g., King et al., 1979; King and Brown, 1984; 825

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