## 1 Introduction: Acoustic Wave Transducers as Passive Cooperative Targets  
When using classical acoustic (seismics) and electromagnetic geophysical prospection techniques, measurements currently focus on monitoring sub-surface interfaces such as voids [1], pollution plumes [2], water table/permafrost depth [3; 4] or moisture concentration [5]. In the context of civil engineering and building integrity assessment, rebar or concrete homogeneity are classically investigated [6] with similar techniques, but operating at higher frequencies to improve spatial resolution at the expense of shallower depths. In this paper, we describe a cooperative target architecture enabling the use of existing hardware--GPR (Ground-Penetrating RADAR)--to complement sub-surface measurements with probing physical quantities including stress [7], temperature [8], pressure [9], torque [10] or chemical compound [11] concentration. As the sensor is embedded in concrete or buried in soil, no battery replacement can be considered after installation: the sensing element is passive, with no local battery source, and acts as a cooperative target to the RADAR wave, backscattering the information relative to the measured quantity. In our experiments, the information is conveyed through the reflected echo delays. Applications aim at assessing pavement, building or civil engineering aging conditions or integrity when subject to natural hazards such as earthquake [12], tsunami [13; 14] or typhoons [15] in the post-event inspection, beyond remote sensing techniques [16] or active sensor networks [17]. Well before the advent of RFID (RadioFrequency IDentification) tags for wireless radiofrequency communication between an active reader electronic system and a passive transducer, the idea of designing RADAR cooperative targets with varying properties was imagined from the experience acquired in monitoring reflected electromagnetic wave characteristics during the Second World War [18]. RFID devices are commercially successful even though they are based on a semiconductor system [19], which must be powered by a DC signal generated by the rectification of the incoming AC signal whose amplitude must exceed the diode threshold voltage. By contrast, cooperative targets following linear energy conversion processes [20] provide several benefits:  
* always returning a signal however minute the incoming power
* resistance to harsh environments
* simple design and manufacturing, often with a single lithography step, as well as operating principles. Dielectric cooperative targets are plagued by large dimensions: delaying the signal returned by the target beyond clutter requires a dielectric delay line at least as long as the longest travel path of the electromagnetic wave. Such a geometrical constraint is incompatible with compact sensors, with the option of virtually extending the dimensions of the transducer by multiplying the number of times the wave travels along the path (dielectric resonator approach). It is to be noted that the information qualified here as clutter, or noise and artifacts in the returned signal that prevent detecting the echoes, is the same information that would be called interface reflections in classical GPR investigations. Furthermore, the sensing principle is restricted to varying intrinsic properties of the dielectric material the target is made of--dielectric dependence with temperature or moisture--frustrating the surrounding electromagnetic field or feeding the delay line with a varying load acting as the sensing element itself [21; 22; 23]. Since the early days of analog radiofrequency signal processing, the conversion of electromagnetic waves to the \(10^{5}\) times slower acoustic wave was identified as a means of shrinking the transducer dimensions. At 300 MHz, a 1-m wavelength electromagnetic transducer is shrunk to a 10 \(\upmu\)m wavelength acoustic transducer, meeting the compact sensor design requirements.