Stripping very low frequency communication signals with minimum shift keying encoding from streamed time-domain electromagnetic data

The objective of this research is to eliminate very low frequency (VLF) communication signals with frequencies in the range of 15-25 kHz from streamed time-domain electromagnetic data. The frequency-domain approach of notch filtering or bandwidth limitation is unsatisfactory when early delay-time data are required in a time-domain system. United States military VLF transmitters presently communicate 200 bits/s using minimum shift keying modulation, and it is possible to derive the encrypted bitstream from the data sampled at typical geophysical streaming rates. The method involves convolving the data with waveforms of frequency different by one-quarter of the bit rate above and below the carrier frequency and using the difference between the convolutions to predict the transmitted bits. The transmitted signal is then exactly recreated from the decoded bits, and the predicted signal at the receiver is subtracted from the data stream. It is possible to predict bit rates and encoding methods from other military transmitters through data analysis and again subtract the predicted signals from the streamed data. This procedure reduces the variance of data, implying that unwanted VLF signals have been reduced by a factor of three to nine in stations thousands of kilometers from a VLF transmitter. Much larger signal/noise improvements are predicted for stations within a few hundred kilometers of the VLF source. Lower degrees of improvement are noted from Indian and Chinese transmitters, which appear to have different encoding and modulation methods

The bubble snails (Gastropoda, Heterobranchia) of Mozambique: an overlooked biodiversity hotspot

This first account, dedicated to the shallow water marine heterobranch gastropods of Mozambique is presented with a focus on the clades Acteonoidea and Cephalaspidea. Specimens were obtained as a result of sporadic sampling and two dedicated field campaigns between the years of 2012 and 2015, conducted along the northern and southern coasts of Mozambique. Specimens were collected by hand in the intertidal and subtidal reefs by snorkelling or SCUBA diving down to a depth of 33 m. Thirty-two species were found, of which 22 are new records to Mozambique and five are new for the Western Indian Ocean. This account raises the total number of shallow water Acteonoidea and Cephalaspidea known in Mozambique to 39 species, which represents approximately 50 % of the Indian Ocean diversity and 83 % of the diversity of these molluscs found in the Red Sea. A gap in sampling was identified in the central swamp/mangrove bio-region of Mozambique, and therefore, we suggest that future research efforts concentrate on or at least consider this region.publishedVersio

A first application of a marine inductive source electromagnetic configuration with remote electric dipole receivers: Palinuro Seamount, Tyrrhenian Sea

We study a new marine electromagnetic configuration which consists of a ship‐towed inductive source transmitter and a series of remote electric dipole receivers placed on the seafloor. The approach was tested at the Palinuro Seamount in the southern Tyrrhenian Sea, at a site where massive sulfide mineralization has been previously identified by shallow drilling. A 3D model of the Palinuro study area was created using bathymetry data, and forward modeling of the electric field diffusion was carried out using a finite volume method. These numerical results suggest that the remote receivers can theoretically detect a block of shallowly‐buried conductive material at up to ∼100 m away when the transmitter is located directly above the target. We also compared the sensitivity of the method using either a horizontal loop transmitter or a vertical loop transmitter and found that when either transmitter is located directly above the mineralized zone, the vertical loop transmitter has sensitivity to the target at a farther distance than the horizontal loop transmitter in the broadside direction by a few 10s of meters. Furthermore, the vertical loop transmitter is more effective at distinguishing the seafloor conductivity structure when the vertical separation between transmitter and receiver is large due to the bathymetry. As a horizontal transmitter is logistically easier to deploy, we conducted a first test of the method with a horizontal transmitter. Apparent conductivities are calculated from the electric field transients recorded at the remote receivers. The analysis indicates higher apparent seafloor conductivities when the transmitter is located near the mineralized zone. Forward modeling suggests that the best match to the apparent conductivity data is obtained when the mineralized zone is extended southward by 40 m beyond the zone of previous drilling. Our results demonstrate that the method adds value to the exploration and characterization of seafloor massive sulfide deposits

Stable Isotope Evidence for Dietary Overlap between Alien and Native Gastropods in Coastal Lakes of Northern KwaZulu-Natal, South Africa

Tarebia granifera (Lamarck, 1822) is originally from South-East Asia, but has been introduced and become invasive in many tropical and subtropical parts of the world. In South Africa, T. granifera is rapidly invading an increasing number of coastal lakes and estuaries, often reaching very high population densities and dominating shallow water benthic invertebrate assemblages. An assessment of the feeding dynamics of T. granifera has raised questions about potential ecological impacts, specifically in terms of its dietary overlap with native gastropods.A stable isotope mixing model was used together with gut content analysis to estimate the diet of T. granifera and native gastropod populations in three different coastal lakes. Population density, available biomass of food and salinity were measured along transects placed over T. granifera patches. An index of isotopic (stable isotopes) dietary overlap (IDO, %) aided in interpreting interactions between gastropods. The diet of T. granifera was variable, including contributions from microphytobenthos, filamentous algae (Cladophora sp.), detritus and sedimentary organic matter. IDO was significant (>60%) between T. granifera and each of the following gastropods: Haminoea natalensis (Krauss, 1848), Bulinus natalensis (Küster, 1841) and Melanoides tuberculata (Müller, 1774). However, food did not appear to be limiting. Salinity influenced gastropod spatial overlap. Tarebia granifera may only displace native gastropods, such as Assiminea cf. ovata (Krauss, 1848), under salinity conditions below 20. Ecosystem-level impacts are also discussed.The generalist diet of T. granifera may certainly contribute to its successful establishment. However, although competition for resources may take place under certain salinity conditions and if food is limiting, there appear to be other mechanisms at work, through which T. granifera displaces native gastropods. Complementary stable isotope and gut content analysis can provide helpful ecological insights, contributing to monitoring efforts and guiding further invasive species research

Superparamagnetism in ground and airborne electromagnetics: Geometrical and physical controls

Superparamagnetic (SPM) effects are not uncommon in ground and airborne electromagnetic (AEM) geophysical data, with sources in the regolith typically being maghemite grains. Using laboratory parameters for maghemite and the Néel equation, a typical geophysical electromagnetic (EM) system is only sensitive to maghemites with dimensions in the 9–12 nm range. The power-law decay observed in a [Formula: see text] system depends on the magnetic nanoparticle volume distribution: Log normally distributed volumes centered on the detection window produce approximately [Formula: see text] decays after the electromagnetic responses have vanished. If most of the nanoparticles are smaller than the center of the system sensitivity, the power-law decays faster than the [Formula: see text] result, if most nanoparticles are larger, a response slower than [Formula: see text] results. Geologic origins of regolith maghemite favor a thin-layer geometry in the near surface. Shape-demagnetization effects imply that SPM responses observed will only arise from magnetization in the horizontal plane. For a ground transmitter, this is the very small area directly under the transmitter loop wire, causing only locally detectable effects. For an airborne transmitter, an extensive radial ring in which the magnetic field is subhorizontal is not demagnetized, and this results in detectable SPM airborne anomalies. For SPM sources in hard rock, a magnetization model consisting of spherical particles enclosed in a finite volume is adapted for SPM. This model shows that the falloff in SPM amplitude with airborne system altitude is rapid for surficial and finite-bedrock sources. As a result, fixed-wing surveys with a transmitter at a 90 m or higher altitude is much less likely to show SPM effects in the EM data. Because the secondary magnetic field of spherical particles is parallel to the transmitter dipole source direction at a colocated receiver, the model explains the empirical observation that concentric loop AEM systems do not detect any [Formula: see text]-component of an SPM response. </jats:p

3D-spectral CDIs: A fast alternative to 3D inversion?

Virtually all airborne electromagnetic (AEM) data is interpreted using stitched 1D conductivity sections, derived from constrained inversion or fast but fairly accurate approximations. A small subset of this AEM data recently has been inverted using either block 3D models or thin plates, which processes have limitations in terms of cost and accuracy, and the results are in general strongly biased by the choice of starting models. Recent developments in spectral modelling have allowed fast 3D approximations of the EM response of both vortex induction and current gathering for simple geological target geometries. Fitting these spectral responses to AEM data should be sufficient to accurately locate current systems within the ground, and the behaviour of these local current systems can in theory approximately define a conductivity structure in 3D. This paper describes the results of initial testing of the algorithm in fitting vortex induction in a small target at the Forrestania test range, Western Australia, using results from a versatile time-domain electromagnetic (VTEM)-Max survey