## 1. Introduction

[2] Investigations of environmental variability (and in particular the natural variability of ground) are closely connected with a number of applications. The most important are for noncontact methods of soil physical properties studies and land mine detection research. The natural ground variability has a strong effect on the land mine detection. Understanding the nature of acoustic false alarms and its connection with the environmental phenomenology will directly contribute to both increased probability of detection of a mine and/or a decreased rate of false-target alarms. These tasks require thorough investigations of ground variability.

[3] Analysis of outdoor sound interaction with the ground has received considerable attention in the work of *Embleton et al.* [1977], *Attenborough et al.* [1986], *Bass and Bolen* [1980], *Embleton et al.* [1983], and *Champoux and Stinson* [1992], where a poroelastic model of the ground was explored. These studies have concentrated on sound propagation over the ground. Several authors have observed that an airborne acoustic wave incident on a ground surface could couple energy into the ground. These observations were based on measurements using geophones. Since the geophones are sensors that respond to the velocity of the medium, this signal was interpreted as being associated with the compressional wave in the elastic media [*Sabatier et al.*, 1986; *Albert and Orcutt*, 1989, 1990]. A poroelastic model of the ground that considers propagation in the media of two types of compressional waves: “fast” and “slow” waves, and shear wave is the most adequate theory for the description of the ground. This poroelastic model is a complicated model with 14 input parameters for each poroelastic layer. Some of the parameters are well-known physical parameters, but others, such as air permeability, air porosity, and pore tortuosity, are difficult to measure in situ [*Attenborough et al.*, 1986; *Sabatier et al.*, 1996]. Because of this, it is difficult to properly apply and test this model.

[4] It is possible to consider acoustic-to-seismic coupling within the framework of a viscoelastic model because ground motion in the poroelastic model of the ground is associated with the “fast” compressional wave. There are investigations which show that the acoustic-to-seismic transfer functions (TF) predicted in the framework of the elastic and the poroelastic models are very close to each other [*Harrop*, 1999] when attenuation in the viscoelastic model is only slightly less than the attenuation in the poroelastic model. In comparison with the poroelastic model, the viscoelastic model needs six parameters for each viscoelastic layer. Therefore it is reasonable to use the simpler viscoelastic model in the initial effort to understand the effect of natural variability of the ground on the acoustic-to-seismic transfer function over a broad range of frequencies and sound angles of incident.

[5] The mathematical model for reflection of plane waves from the multilayered elastic media was developed in a number of papers [*Gilbert and Backus*, 1966; *Thomson*, 1950; *Haskell*, 1953; *Frasier*, 1970; *Claerbout*, 1968; *Brekhovskikh and Godin*, 1999; *Fokina and Fokin*, 2000] and was successfully used for predicting sound propagation and reflection. Specific computational schemes were discussed by *Dunkin* [1965], *Thrower* [1965], *Schmidt and Jensen* [1985], and *Ivansson* [1999]. While the equations for wave propagation in inhomogeneous elastic media have been considered by a number of authors [*Ursin*, 1983; *Robins*, 1994, 1998; *Gupta*, 1966], there are no investigations of the influence of different parameters of the ground on the acoustic-to-seismic transfer function in wide frequency and angular ranges.

[6] Another important problem is obtaining the properties of layered elastic media by acoustical means. Today, many methods for determining the properties of elastic media have been developed [*Diachok et al.*, 1995; *Dosso and Wilmut*, 2000; *Tolstoy and Chapman*, 1998; *Gerstoft*, 1994]. These methods differ in procedure and the characteristics of acoustic signals used for obtaining the properties of the elastic media. Pulse methods based on measuring the characteristics of acoustic pulses and matched-field procedures are used most often [*Fokin and Fokina*, 2003].

[7] The determination of media parameters with the use of broadband acoustic signals also offers a possibility to improve the reliability of the results. Different characteristics of acoustic signals used for determining the properties of the media have different sensitivities to the parameters of the media. In this context, there is a need to investigate the characteristics that are sensitive to small variations of the media parameters. The resonance peaks of the acoustic-to-seismic transfer function can be qualified as such characteristics. Recently, the resonance approach has gained prominence, which is related to the development of computer methods and the progress in the theory of sound propagation [*Breit and Wigner*, 1936; *Fiorito et al.*, 1981, 1979; *Fokina and Fokin*, 2001; *Fokin and Fokina*, 2001, 2003; *Nagl et al.*, 1982].

[8] This paper deals with the effect of parameters in the layered viscoelastic ground on the acoustic-to-seismic transfer function. A matrix technique is used to describe sound interaction with the layered viscoelastic ground. The resonance approach is used for the estimation of a set of parameters for the ground model from experimental data. Analysis of experimental data obtained at an Army test site in Virginia allows one to determine the correlation between moisture content at the surface of the ground and to suggest a simple model explaining this phenomenon.