Gravimeter Research Articles

Measuring gravity change caused by water storage variations: Performance assessment under controlled conditions

Summary Subsurface water content is an important state variable in hydrological systems. Established methods to measure subsurface water content have a small support scale which causes scaling problems in many applications. Time-lapse relative gravimetry can give an integrated measure of soil water storage changes over tens to hundreds of cubic meters. The use of time-lapse gravimetry in hydrology has until recent years been limited by the large efforts required to obtain precise and accurate gravity data at the 1 μGal (10 −8 ms −2 ) scale. A typical modern relative gravimeter, the Scintrex CG-5, has a sensitivity of 1 μGal, corresponding to a layer of 0.024 m of water in an infinitely extended horizontal sheet. For gravity surveys using relative gravity meters, the precision is highly dependent on the methods used to operate the gravimeter in the field. Systematic errors, which are difficult to detect, can lead to a loss of accuracy. As a performance test of a CG-5 for applications of time-lapse gravity in hydrology, we have measured the change in water storage in an indoor basin. The experiment was designed to resemble a field application, e.g. a pumping test, a forced infiltration experiment or alluvial aquifer storage change along intermittent rivers, so that the results can be applied to field experiments. The use of a 20 m by 30 m rectangular basin with a known water volume resulted in complete control over the instrument accuracy. Precisions of 3 μGal and accuracies of

Subsurface Structure Derived from Detailed Gravity and Magnetic Investigations along the Pala-Maneri Traverse of the Main Central Thrust, NW Himalaya

Abstract A detailed, integrated gravity and magnetic study across the Main Central Thrust (MCT) along the Pala- Maneri traverse in Uttaranchal, NW Himalaya was carried out. The gravity data was acquired using a CG-3 gravity meter with an accuracy of 0.005 mGal, while magnetic data was acquired using a proton precession magnetometer with a station interval of 20 m. Data was collected along a 11.7 km, NE–SW traverse from Pala to Maneri along the proposed route of a hydroelectric headrace tunnel. The measured variation in the gravity field was approximately 70 mGal, with two prominent highs recorded at distances of 0.5 km, 7.5 km and lows at 3.0 km, 10.5 km from Maneri. The gravity highs can be attributed to presence of high-density rocks along the thrust planes. The sharp gravity low recorded at 10.5 km distance possibly indicates a sympathetic fault of the MCT that is highly saturated with fluids (water). The broad gravity low between 2.5 km and 4.0 km distance is likely to represent the gravity signature of the MCT itself. The measured variation in the magnetic field was approximately 285 nT. The associated gravity and magnetic signatures located several faults along the traverse including presence of the MCT at Kumaltigad.

Efficiency of Controlled‐Release Urea for a Potato Production System in Quebec, Canada

Proper N management is essential to prevent N losses and ensure high potato (Solanum tuberosum L.) yield and quality. Controlled‐release urea (CRU) could increase nitrogen use efficiency (NUE) by matching the release of N with potato N uptake. This 3‐yr study conducted in Quebec, Canada, compared four treatments, namely an unfertilized control (0 N), calcium ammonium nitrate (CAN) application at rates of 150 kg N ha−1 (150CAN) and 200 kg N ha−1 (200CAN), and CRU application at a rate of 150 kg N ha−1 (150CRU). The effects of treatments were assessed on potato yield, specific gravity, NUE, and chlorophyll meter readings (CMR) for two cultivars (Goldrush and Chieftain) along with soil nitrate adsorbed on anion exchange membranes (NO3AEMs). Marketable yield and the yield of jumbo and medium size classes significantly increased with N fertilization up to 150 kg N ha−1, and yields were higher with 150CRU than with 150CAN or 200CAN. Nitrogen fertilization increased CMR, but had no effect on specific gravity. The CRU continually released more nitrate during the growing season as indicated by higher NO3AEMs values, and had higher NUE compared with other treatments. Relative yield (RY) was significantly related by linear‐plateau functions to CMR and NO3AEMs measured 40 to 50 d after planting, hence providing the critical values of 41.9 for CMR and 15 μg cm−2 d−1 for NO3AEMs, above which the probability of a yield response to additional N is low. Controlled‐release urea is a promising N source for increasing the yield and NUE of potatoes produced in eastern Canada.

Airborne Gravity & Magnetics – A fast track multi-system approach for petroleum exploration

Recent advances in mapping technologies coupled with the emerging needs of the exploration industry to precisely characterize the sub-surface systems, have led to the resurgence in interest for gravity and magnetic (so called potential field) methods in oil and natural gas exploration. The need for acquiring large potential field data sets at greater speed over highly prospective and frontier areas is increasing the demand for airborne measurement systems. In response to this need, UTSAeroquest offers the latest version of airborne gravity systems called TAGS (Turnkey Airborne Gravity System) on a fixed wing geophysical survey aircraft, which is already equipped with a magnetic acquisition system as well as radar and laser altimeters. Prior to offering the services of this new system, a pre-purchase test of the gravity meter was undertaken. The results provided a high level of confidence in offering this multisystem airborne geophysical approach to the petroleum industry.

소규모 이상체의 중력탐사를 위한 정밀지형보정

그 동안 많은 연구에서 지형보정을 위한 프로그램들이 개발되어왔으며, 매우 효과적으로 지형보정을 수행하는 것으로 인정받아왔다. 특히, 한반도 뿐만이 아니라 세계적으로도 수치 지형자료가 보편적으로 보급되어 지형보정을 위한 기반 자료는 충분히 확보되고 있다. 이번 연구는 일반적인 광역탐사가 아닌 소규모 광체나 지질구조에 대한 정밀한 중력탐사에 대한 자동지형보정을 수행할 수 있도록 하는 프로그램을 개발하였다. 소규모 지하자원이나 지하공동 등의 소규모 지질현상에 대한 중력탐사에는 기존의 광역탐사에 적합한 지형보정프로그램보다 더욱 정밀한 지형보정프로그램이 필요하다. 따라서 본 연구를 통해서 개발된 자동정밀지형보정프로그램으로 정밀중력탐사를 보다 효율적으로 수행할 수 있게되었다. 본 연구에서 개발된 정밀지형보정프로그램에서는 multiquadric equation을 이용한 지형구현을 통해 보다 정밀한 지형 생성이 가능하게 설계하였으며, 연구자의 설정에 따라 중력측정점 주변에 자세한 지형값을 넣어 정밀한 지형보정이 가능하도록 하였다. 또한 기존의 광역중력탐사에서 무시되던 지형과 중력계 사이의 거리 차에 따른 옵션을 설정하여 정밀한 지형보정 수치를 산출하도록 하였다. Many studies have successfully developed a number of terrain correction programs in gravity data. Furthermore, terrain data that is a basic data for terrain correction has widely been provided through internet. We have also developed our own precise gravity terrain correction program. The currently existing gravity terrain correction programs have been developed for regional scale gravity survey, thus a more precise gravity terrain correction program needs to be developed to correct terrain effect. This precise gravity terrain program can be applied on small size geologic targets, such as small scale underground resources or underground cavities. The multiquadric equation has been applied to create a mathematical terrain surface from basic terrain data. Users of this terrain correction program can put additional terrain data to make more precise terrain correction. In addition, height differences between terrain and base of gravity meter can be corrected in this program.

Gravity at sea -A memoir of a marine geophysicist-

A history of studies on the gravity measurements at sea in Japan is reviewed with an emphasis on the contribution of the author. The first successful measurements at sea were made in 1923 by Vening Meinesz in the Netherlands using the pendulum apparatus installed in a submarine. However, the gravity measurements using a submarine are not convenient because the access to a submarine is limited. Professor Chuji Tsuboi made a number of unsuccessful attempts at developing a gravity meter that can be operated on a normal surface ship by reducing the noise by minimizing the motion of the gravity meter through a mechanical design.I have chosen a new approach toward the measurements of gravity on a surface ship by simplifying the mechanical part using a string gravity meter that was installed directly on a vertical gyroscope in combination with the numerical and/or electronic reduction of noises. With this gravity meter TSSG (Tokyo Surface Ship Gravity Meter), we firstly succeeded in measuring gravity at sea onboard a surface ship in July 1961 and the measurements have been extended to the northwestern Pacific and beyond. The results reveal the fine structures of gravity field in and around trenches that provide important clues as to a number of geodynamic issues including the nature of the trench-trench interaction and the interaction of trenches with seamounts.

Seismic-induced accelerations detected by two parallel gravity meters in continuous recording with a high sampling rate at Etna volcano

We analyse a microgravity data set acquired from two spring LaCoste & Romberg gravity meters operated in parallel at the same site on Etna volcano (Italy) for about two months (August – September 2005). The high sampling rate acquisition (2Hz) allowed the correlation of short-lasting gravity fluctuations with seismic events. After characterizing the oscillation behavior of the meters, through the study of spectral content and the background noise level of both sequences, we recognized fluctuations in the gravity data, spanning a range of periods from 1 second to about 30 seconds dominated by components with a period of about 15 ÷ 25 seconds, during time intervals encompassing both local seismic events and large worldwide earthquakes. The data analyses demonstrate that observed earthquake-induced gravity fluctuations have some differences due to diverse spectral content of the earthquakes. When local seismic events which present high frequency content excite the meters, the correlation between the two gravity signals is poor (factor < 0.3). Vice versa, when large worldwide earthquakes occur and low frequency seismic waves dominate the ensuing seismic wavefield, the resonance frequencies of the meters are excited and they react according to more common features. In the latter case, the signals from the two instruments are strongly correlated to each other (up to 0.9). In this paper the behaviors of spring gravimeters in the frequency range of the disturbances produced by local and large worldwide earthquakes are presented and discussed.

Deep Sea Gravity Measurements: GEOSTAR-2 Mission Results

A new concept gravity meter with sensitivity close to Hz ms / 10 2 8 ? ? in the range of 10 -5 ?1Hz intended for observation of the vertical component of the Earth gravity and teleseismic waves was implemented at the Istituto di Fisica dello Spazio Interplanetario (IFSI), CNR and successfully operated during the GEOSTAR-2 mission. The gravimeter has demonstrated a capability to operate for long time in an autonomous regime and a good reliability for operation in extreme environments; at the same time the experimental measurements gave the information for the further gravimeter’s implementation. Results of observation and data analysis included the registration of seismic waves excited by global earthquakes and the evaluation of the low frequency modes of free oscillations of the Earth are reported.

Influence of chelating agent and reaction time on the swelling process for preparation of porous TiO 2 particles

This study aimed to describe the preparation and characterization of porous titanium oxide particles of narrow particle size distribution by a single-step swelling process of polystyrene template microspheres. In this research, different specific surface areas, porous structures and densities of porous titanium oxide particles had been synthesized with various experimental parameters. Two main parameters were tested and discussed: (1) acetylacetone amount (from 0 to 1 ml) and (2) reaction time (from 2 to 32 h). Polystyrene template particles were polymerized by emulsifier free-emulsion polymerization (a kind of polymerization method). For the swelling process to be successful, a chelating agent (acetylacetone) was added to delay the aqueous hydrolysis of titanium (IV) isopropoxide in the water phase. The influences of various reaction parameters on size, morphology, composition, specific surface area, porous structure and density of particles were investigated. The properties of particles were analyzed by scanning electron microscope, Brunauer–Emmett–Teller, Fourier transform infrared analysis, thermogravimetric analysis, powder X-ray diffraction, and specific gravity meter.

Trial of Multidisciplinary Observation at an Expandable Sub-Marine Cabled Station “Off-Hatsushima Island Observatory” in Sagami Bay, Japan

Sagami Bay is an active tectonic area in Japan. In 1993, a real-time deep sea floor observatory was deployed at 1,175 m depth about 7 km off Hatsushima Island, Sagami Bay to monitor seismic activities and other geophysical phenomena. Video cameras monitored biological activities associated with tectonic activities. The observation system was renovated completely in 2000. An ocean bottom electromagnetic meter (OBEM), an ocean bottom differential pressure gauge (DPG) system, and an ocean bottom gravity meter (OBG) were installed January 2005; operations began in February of that year. An earthquake (M5.4) in April 2006, generated a submarine landslide that reached the Hatsushima Observatory, moving some sensors. The video camera took movies of mudflows; OBEM and other sensors detected distinctive changes occurring with the mudflow. Although the DPG and OBG were recovered in January 2008, the OBEM continues to obtain data.

THE CAPACITY OF THE SCINTREX AUTOGRAV CG-3M NO. 4492 GRAVIMETER FOR "ABSOLUTE-SCALE" SURVEYS

In autumn 2001, the Institut für Erdmessung (IfE), received the new Scintrex Autograv CG-3M spring gravimeter no. 4492 (SC-4492). Compared with LaCoste-Romberg gravimeters, the Scintrex instrument offers the advantage of a more simple calibration function. Due to the straightforward sensor design (no micrometer screws and gearboxes, no astatisation), a modelling of periodical calibration terms and of a higher order polynomial calibration function is not required. The costly calibration measurements can be reduced to some relative observations between two reliable absolute gravity stations. No complex calibration system should be necessary. A reliable evaluation of the SC-4492 was done with respect to the stability of the calibration, measurement accuracy and precision, drift behaviour, and gravity range dependency of the calibration factor. Measurements were performed on the Hannover vertical calibration line (200 μm/s2 (20 mGal) range, 20-storied building, 10 μm/s2 interval), on the Cuxhaven-Harz north-south line (3000 μm/s2 range, 34 stations, 90 μm/s2 interval) of the gravimeter calibration system Hannover, and in the absolute gravimetry net of the Fennoscandian land uplift area (6200 μm/s2 range). A total gravity range of almost 0.015 m/s2 has been covered. In addition, the gravimeter was employed in the determination of vertical gravity gradients and centring to safety points for absolute gravimetry, and the observation of the horizontal gradient field at the IfE absolute gravity reference station in Clausthal. This paper presents the results as obtained in regional, local and microgravimetric surveys. The achieved accuracies are in the order of ±10 to 100 nm/s2 (±1 to 10 μGal). No instability of the calibration and no gravity range dependency could be proven within the order of 1·10-4. Overall, the gravity meter SC-4492 meets fully the expectations of IfE.

Preview Number 139

Scintrex is in the final stages of the development of a borehole gravity meter, for mining and geotechnical applications, designed to log inside NQ drill rods to 2000 m depth, using standard 4 conductor cable, with a sensitivity of better than 5 μGal, and operable in boreholes inclined from 30° to vertical. École Polytechnique of Montreal has developed forward modelling software, as part of this project.The first field test of the prototype probe was successfully conducted in December 2008 for Vale Inco in a deep borehole located in Norman township near Sudbury, Ontario. The results of this test show a large amplitude bipolar residual gravity anomaly, with the crossover at the location where the borehole intersected sulphides. Further analysis of the data is underway. A repeat log of the hole indicates that the Gravilog system has achieved operational specifications very close to its targets.Field tests for the other sponsors are planned during the first half of 2009, with production surveys to follow during the second half of the year.Gravity measurements inside boreholes provide evidence of density variations both in the immediate vicinity and at a distance from the hole. Scintrex’s development of a new borehole gravimeter will, for the first time, allow the application of gravity logging in typical mining and geotechnical boreholes.Primary applications of the Gravilog system in mining include the sensing and mass-estimates of massive sulphide bodies, either intersected by or remote from the hole; and accurate bulk density measurements of formations intersected by the hole.

Groundwater exploration and evaluation by using geophysical interpretation (case study: Al Qantara East, North Western Sinai, Egypt)

Different geophysical tools such as geoelectric, gravity, and magnetic have been applied to detect groundwater potentiality and structural elements, which controlled a geometry of the groundwater aquifers in the study area. Nineteen vertical electrical soundings measured using ABEM SAS 4000 equipment through Schlumberger configuration of AB/2 ranged from 1.5 to 1,000 m; the quantitative interpretation was carried out using manual and analytical techniques. The results of quantitative interpretation used to construct six geoelectrical cross-sections indicate that the subsurface sequence of the study area consists of seven geoelectrical units. These units are Quaternary sand sheet and sand dunes, Quaternary aquifer, marly limestone, clay, sandy clay, clay with sandstone intercalation, and deep Nubian sandstone aquifer. The isopach map of the Quaternary aquifer exhibits thickness of the Quaternary aquifer that increased at the northern and southern part (50 m) and decreased at the eastern and western part (5 m), and the depth of the aquifer increased at the northern part (40 m) and decreased at the central part to 6 m. The isoresistivity map of the aquifer shows a high resistivity at the northern part but the southern part reveals low resistivity according to the lithology. The water salinity increases in the direction of groundwater flow from 500 to 10,500 mg/l. The low water salinity is due to direct recharge from El-Sheikh Zayed Canal, which supplied fresh water to this area. Sixty-five gravity stations were measured using Auto-Grav gravity meter; different gravity corrections were applied on raw data. The corrected gravity values were plotted to represent a Bouguer anomaly map; the residual gravity anomaly map was used for delineation of the fault elements. The area was dissected by different fault elements of trends NW–SE, NE–SW, and E–W. In addition, 65 ground magnetic stations were measured at the same sites of gravity stations. The results of magnetic interpretation indicate that the depth of the basement is shallow at the western and southern parts of the area (4,500 m), but the central part exhibits greater depth of 7,900 m.

Results from the first field trial of a borehole gravity meter for mining applications

Scintrex is in the final stages of the development of a borehole gravity meter, for mining and geotechnical applications, designed to log inside NQ drill rods to 2,000 m depth, using standard 4 conductor cable, with a sensitivity of better than 5 µgal, and operable in boreholes inclined from 300 to vertical. Ecole Polytechnique of Montreal has developed forward modelling software, as part of this project. The first field test of the prototype probe was successfully conducted in December 2008 for Vale Inco in a deep borehole located in Norman township near Sudbury, Ontario. The results of this test show a large amplitude bipolar residual gravity anomaly, with the crossover at the location where the borehole intersected sulphides. Further analysis of the data is underway. A repeat log of the hole indicates that the Gravilog system has achieved operational specifications very close to its targets. Field tests for the other sponsors are planned during the first half of 2009, with production surveys to follow during the second half of the year. Gravity measurements inside boreholes provide evidence of density variations both in the immediate vicinity and at a distance from the hole. Scintrex?s development of a new borehole gravimeter will, for the first time, allow the application of gravity logging in typical mining and geotechnical boreholes. Primary applications of the Gravilog system in mining include the sensing and mass-estimates of massive sulphide bodies, either intersected by or remote from the hole; and accurate bulk density measurements of formations intersected by the hole.

The 4D microgravity method for waterflood surveillance: Part 3 — 4D absolute microgravity surveys at Prudhoe Bay, Alaska

The 4D microgravity method is becoming a mature technology. A project to develop practical measurement and interpretation techniques was conducted at Prudhoe Bay, Alaska, from 1994 through 2002. Beginning in 2003 these techniques have been systematically applied to monitor a waterflood in the gas cap of the Prudhoe Bay reservoir. Approximately 300 stations in a [Formula: see text] area are reoccupied in each survey year with sub-[Formula: see text] precision absolute gravity and centimeter precision Global Positioning System (GPS) geodetic measurements. The 4D gravity measured over epochs 2005–2003, 2006–2003, and 2007–2003 has been successfully modeled to track the mass of water injected since late in 2002. A new and improved version of the A-10 field-portable absolute gravity meter was developed in conjunction with this project and has proven to be a key element in the success of the 4D methodology. The use of an absolute gravity meter in a field survey of this magnitude is unprecedented. There are substantial differences between a 4D absolute microgravity survey and a conventional gravity survey in terms of station occupation procedures, GPS techniques, and the 4D elevation correction. We estimate that the overall precision of the 4D gravity signal in each epoch is less than [Formula: see text].

New Developments in Gravity Applications and Instruments

Gravity meters are commonly used to map spatial changes in the earth?s gravity, achieving resolution of parts per billion on microgravity surveys. The application of gravity surveys to monitor change and recent improvements in gravity instrumentation will be summarized. The use of gravity to track the progress of a waterflood in Prudhoe Bay, Alaska, provides a 4D Gravity case history. The surface gravity change caused by the injection of water into the gas cap is measured annually and compared to the expected change calculated from the reservoir model. 4D Gravity provides an effective ``early warning system' for injection and sequestration projects. A recent borehole gravity survey in Hanford, Washington, demonstrates the ability to measure bulk densities using a borehole gravimeter. The applications include geotechnical studies at waste disposal sites, bridges and structures, locating and monitoring thief zones in reservoirs, and grade control in iron mines. A new borehole gravity meter will be introduced during 2008 that can be used in smaller, inclined boreholes. Earth tides and other long period movements of the earth can be recorded by suitably configured gravity meters. The ground shaking in Luxembourg before and after the January 13, 2007, earthquake in Japan was recorded on a long period seismometer, a superconducting gravity meter and a new portable gPhone gravity meter. The match between the seismometer data and the gPhone data during the earthquake is excellent. The relatively inexpensive portable gPhone gravity meter provides the means to record earth movements along active fault zones and other critical locations. gPhone data recorded in Denver, Colorado on August 15, 2007, indicates F40pre-shock and post-shock ground motion as well as the 8.0 earthquake in Peru.

Commercial Gravity Operations – Modern GPS surveying and digital gravity meters

Gravity surveying has been available since the early 1900’s in the form of analogue gravity meters, barometric levelling and conventional optical line of sight surveying methods. The gravity method was more generally used for applications in the oil industry.With the advent of GPS technology in the form of centimetre accuracy from GPS surveying methods, commercial gravity operations developed. Line of sight surveying methods were no longer required and gravity meters became digital, easier and more reliable to use.Commercial gravity surveys are now undertaken on a routine basis from projects spanning hundreds of kilometers for mapping large sedimentary basins to very small localised engineering projects covering merely hundreds of meters.This paper outlines the development of the GPS surveying technology and the digital gravity meter. The integration of the two technologies are discussed with respect to the development of the commercial gravity survey method. Several applications and case studies are described with specific examples given from various international gravity contractors.

Gravity Surveying in Early Geophysics. II. From Mountains to Salt Domes

Progress in measurement of the force of gravity and its contribution to the understanding of geology, and to exploration for oil and mineral deposits, from the eighteenth to the early twentieth century is reviewed. Initially, work focused on determination of the mean density of the Earth. Pendulum observations during the trigonometric survey of India (1805-43) revealed a low-density zone beneath the Himalayas and led to development of the Pratt and Airy compensation models in 1855, followed by Dutton's concept of isostatic compensation in 1889. Use of the Eötvös torsion balance (1889) to map the gravity field over an oil-bearing structure in 1915-16 heralded economic applications. By the 1920s, it was being widely used to search for oil-bearing salt domes, coal and mineral deposits. With the introduction of the gravity meter in the 1930s, the torsion balance fell into disuse and the modern era of gravity surveying and prospecting began. With the development of progressively more sensitive instruments, the gravity meter has retained its place, becoming an essential companion to 3-D seismic surveys and, with new instrumentation, gradiometry has seen a revival as an extremely powerful exploration tool.

Porosity of the upper edifice of Axial Seamount

Seamounts are not solid basalt structures, but have relatively high porosities in their upper crustal sections. At Axial Seamount, off the coast of Oregon, United States, we used on-bottom gravity measurements with a Bell gravity meter within deep-sea submersible Alvin to determine a porosity of 31% for the uppermost ∼100 m of the edifice. The southwestern caldera wall has a porosity of 22% and the caldera floor has a slightly higher porosity of 33%. Seafloor observations and models indicate that these high porosities result from large-scale structural features such as lava tubes and cracks, large lava drain backs, or regions of open pillow basalts in the near subsurface. These high-porosity zones can affect the subsurface permeability, and models of hydrothermal upflow zones explain observed localized gravity anomalies. The variety of hydrothermal alteration, hydrothermally active areas, and open porous features appears to be related to the high porosity that is inferred from geophysical measurements on this active seafloor volcano.

Comparative Analysis of the Free Oscillations Generated by the Sumatra– Andaman Islands 2004 and the Chile 1960 Earthquakes

Abstract The Chile 1960 and Sumatra–Andaman Islands 2004 earthquakes are among the three greatest events that have occurred since the instrumental recording of seismic waves. The two events have similarities in hypocentral depth and focal mechanism but are very different when considering the extension of the fault plane. The recent event has close to 50% greater length than the Chile event, and although the moment magnitude of the Chile event is 0.2 to 0.5 points greater, the magnitude determination of the Sumatra–Andaman depends on the frequency range considered in the calculations (free oscillations with periods &amp;gt; 1000 sec or seismic waves, with periods of 300–500 sec). For the Chile event only the seismic moment for periods of T &amp;lt; 1000 sec is presently available, although the estimate of seismic moment at larger periods is crucial for a comparison of the two events. Our study makes a direct comparison of the free oscillation amplitudes of the two events by analyzing the records of the Grotta Gigante long-base tiltmeters, which have been recording tilt continuously since 1960. The particular mountings and physical dimension of the instruments make them particularly apt to record the torsional free oscillation waves, scarcely observed by standard long-period seismographs and not directly observable by the gravity meters. We determine the singlet frequencies for some of the lower spheroidal and torsional modes and a broad spectrum of multiplet frequencies for the higher modes. After correcting for the decay of the modes, we determine the amplitude ratios of the activated modes for the two events. The amplitude ratios vary between 1.5 and 3. Our results can be used directly for determining the ratios of the seismic moments of the two events over the frequency range 0.3–3 mHz, the only necessary correction being that of the site dependence of the free mode amplitudes.