Teresa Teixidó

Seismic reflection constraints on the glacial dynamics of Johnsons Glacier, Antarctica

Abstract During two Antarctic summers (1996–1997 and 1997–1998), five seismic refraction and two reflection profiles were acquired on the Johnsons Glacier (Livingston Island, Antarctica) in order to obtain information about the structure of the ice, characteristics of the ice-bed contact and basement topography. An innovative technique has been used for the acquisition of reflection data to optimise the field survey schedule. Different shallow seismic sources were used during each field season: Seismic Impulse Source System (SISSY) for the first field survey and low-energy explosives (pyrotechnic noisemakers) during the second one. A comparison between these two shallow seismic sources has been performed, showing that the use of the explosives is a better seismic source in this ice environment. This is one of the first studies where this type of source has been used. The analysis of seismic data corresponding to one of the reflection profiles (L3) allows us to delineate sectors with different glacier structure (accumulation and ablation zones) without using glaciological data. Moreover, vertical discontinuities were detected by the presence of back-scattered energy and the abrupt change in frequency content of first arrivals shown in shot records. After the raw data analysis, standard processing led us to a clear seismic image of the underlying bed topography, which can be correlated with the ice flow velocity anomalies. The information obtained from seismic data on the internal structure of the glacier, location of fracture zones and the topography of the ice-bed interface constrains the glacial dynamics of Johnsons Glacier.

Application of the CMP refraction method to an archaeological study (Los Millares, Almerı́a, Spain)

Obtaining information at an archaeological site by means of geophysical methods can reduce the need for intensive excavation. This paper addresses the use of seismic methods to reveal details in a non-destructive manner at the archaeological site of Los Millares (Almeria, Spain). The seismic refraction method provides information on the low frequency component of the model for the shallowest layers. In this way, it is possible to fix the thickness of the surface layer, as well as to determine a velocity model. Use of the refraction method in Los Millares has resulted in the determination of the depth of the calcaric surface upon which the foundations were built. The application of a recently developed method, common-midpoint (CMP) refraction, allows the detection of local heterogeneities in the near subsurface. This method uses the amplitude, phase and frequency information of the first arrivals. The results highlight the location of anomalous zones characterized by early first arrivals. According to a priori geological and archaeological information, these anomalies can be correlated with buried foundations providing the key information for planning future excavations.

Geophysical Survey of Johnsons Glacier (Antarctica): comparing seismic and RES data

Seismic and radio-echo sounding data have been acquired along coincident profiles at the temperate Johnsons Glacier (Livingston Island, South Shetland Islands, Antarctica). Seismic data have been processed and the results provide information about ice-thickness and bedrock topography. Preliminary results of the radio-echo sounding survey also show clear bedrock reflections, except in some highlycrevassed areas. Diffraction from englacial bodies and bottom irregularities are observed in many places. Further processing of seismic and radar data sets will allow us to compare the ice-thickness determined from both methods and to extract additional information about the internal structure and physical parameters of temperate ice.