Abstract
The success of a long term transient electromagnetic survey (TEM) rigorously calls for appropriate system calibration, in addition to advanced processing and inversion of the measured data. In fact, acquisition of TEM data can be affected by a variety of noise sources from both inside and outside the system, making it difficult, for example, to define an absolute turn off time and/or to synchronize transmitter and receiver. For these reasons, a reference site plays an important role. As first step, we performed the calibration of a Geonics 47 at the Lyngby reference site in Denmark. We then set up a new reference site using the same calibrated TEM instrument. The reference site was established in the San Rossore park area (Pisa), where we identified an area that matches the required conditions. Subsequently, a series of TEM measurements were collected in the selected area using two pre-calibrated TEM instruments: the Geonics 47 and the WalkTEM respectively. The reference responses were therefore jointly inverted, obtaining a 5 layers model that was appointed to be the TEM reference model for the site. Afterwards, based on that reference model, we calibrated the Geonics 47 and 57 instruments for a 100 x 100 m central loop configuration. A unique time-shift and a data level shift factor was calculated and applied to the TEM system as result of the calibration procedure. The San Rossore TEM reference site is now available for anyone interested in calibrating TEM systems.
Highlights
Near surface resolution of transient electromagnetic (TEM) is highly dependent on the specific instrument’s capability to measure the early time portion of the very shallow portion of the transient ground response
It is important to check the repeatability of the recorded data at the selected area which can be done by measuring the TEM data at the same site in different period of the year
This paper provides a detailed description of the reference and calibration site established in Italy which we propose as support for the calibration of any ground and airborne TEM systems that will operate in Italy, following the experience maturated in Denmark
Summary
Near surface resolution of transient electromagnetic (TEM) is highly dependent on the specific instrument’s capability to measure the early time portion of the very shallow portion (i.e. tens of meters) of the transient ground response. It is duly demonstrated that TEM data can suffer from issues due to the inherent difficulty in the description of the key-parameters of the instrumentation like an absolute turn off time and/or the transmitter and receiver synchronization [Christiansen et al.2011]. All these system-specific inaccuracies directly turn out to inappropriate 1D modeling of the subsoil, both in the very shallow as well as in the deeper part of the transient response. Each TEM system, both ground and airborne, adopts a specific waveform at one or more repetition frequencies in order to fulfill precise objectives and strategies [Liu 1998]
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