Mark A. Zumberge

Constraints on the in situ density of CO2 within the Utsira formation from time-lapse seafloor gravity measurements

Constraints on the in situ density of CO2 within the Utsira formation from time-lapse seafloor gravity measurements

A new sea-floor gravimeter

A new reservoir management application uses precise time‐lapse gravity measurements on the sea floor to detect seawater infiltration in offshore natural gas fields during production. Reservoir models for the North Sea Troll field predict gravity changes as large as 0.060 mGal within a 3–5‐year period. We have constructed and deployed a new instrument—the ROVDOG (Remotely Operated Vehicle‐deployed Deep‐Ocean Gravimeter) system—for this application. Because the measurements must be relocated accurately (within 3 cm), we required a gravimeter which could be handled by an ROV and placed atop sea‐floor benchmarks. We have built an instrument based upon the Scintrex CG‐3M land gravimeter. Motorized gimbals level the gravimeter sensor within a watertight pressure case. Precision quartz pressure gauges provide depth information. A shipboard operator remotely controls the instrument and monitors the data. The system error budget considers both instrumental and field measurement uncertainties.The instrument prototy...

Resolving quadrature fringes in real time

In many interferometers, two fringe signals can be generated in quadrature. The relative phase of the two fringe signals depends on whether the optical path length is increasing or decreasing. A system is developed in which two quadrature fringe signals are digitized and analyzed in real time with a digital signal processor to yield a linear, high-resolution, wide-dynamic-range displacement transducer. The resolution in a simple Michelson interferometer with inexpensive components is 5 x 10(-13) m Hz(-1/2) at 2 Hz.

An optical fiber infrasound sensor: A new lower limit on atmospheric pressure noise between 1 and 10 Hz

A new distributed sensor for detecting pressure variations caused by distant sources has been developed. The instrument reduces noise due to air turbulence in the infrasound band by averaging pressure along a line by means of monitoring strain in a long tubular diaphragm with an optical fiber interferometer. Above 1 Hz, the optical fiber infrasound sensor (OFIS) is less noisy than sensors relying on mechanical filters. Records collected from an 89-m-long OFS indicate a new low noise limit in the band from 1 to 10 Hz. Because the OFIS integrates pressure variations at light-speed rather than the speed of sound, phase delays of the acoustical signals caused by the sensor are negligible. Very long fiber-optic sensors are feasible and hold the promise of better wind-noise reduction than can be achieved with acoustical-mechanical systems.

An ocean bottom gravity study of the southern Juan de Fuca Ridge

We use seafloor and sea surface gravity data to model density structure along the southern Juan de Fuca Ridge. We determine the average density of the shallow oceanic crust at the ridge using seafloor gravity measurements and then use these data in conjunction with sea surface gravity observations to construct density structure models. Of 63 seafloor gravity measurements obtained, 42 observations were distributed over the 3-km left-lateral overlapping rift zone (ORZ), located at latitude 45°03′N, and separating the Juan de Fuca ridge into the Cleft and Vance segments. A 21-measurement seafloor gravity transect was made perpendicular to the ridge strike at latitude 44°52′N, a region of linear ridge geometry on the Cleft segment. These seafloor gravity measurements, which are sensitive to shallow crustal density variations, were augmented by roughly 800 km of sea surface gravity measurements. Using a bathymetry-Bouguer anomaly analysis of the seafloor gravity data, we determined the average density of the shallow (roughly upper 2 km) oceanic crust at these two locations to be 2630 kg m−3±50 kg m−3. Within the uncertainties of the density determinations, there is no difference between the average shallow oceanic crustal density at the linear Cleft segment and at the Cleft-Vance ORZ. Using the seafloor measurements, we modeled the study areas underlying density to provide constraints on its fine-scale structure. Localized porosity of up to 17% within the upper 500 m of oceanic crust (layer 2A) provide a possible explanation for the observed gravity anomalies at the Cleft segment. Two-dimensional density models of the crust underlying the linear Cleft segment (44°52′N) show that no deep source is required to explain the topographical asymmetry observed between the east and west sides of the ridge axis. For the Cleft-Vance ORZ, three-dimensional modeling suggests low-density material between the rifts, in agreement with thickened layer 2A from seismic observations. In contrast, layer 2A variations do not explain the low density inferred at the northern end of the Cleft segment since seismic observations suggest a thin layer 2A. The magmatic activity recently observed in this region may have a distinct, deeper source.

Precision of seafloor gravity and pressure measurements for reservoir monitoring

Changeswithgravityovertimehaveproventobevaluable forinferringsubsurfacedensitychangesassociatedwithproduction from oil and natural gas reservoirs. Such inferences allowthemonitoringofmovingfluidfrontsinareservoirand provideanopportunitytooptimizeproductionoverthelifeof the reservoir. Our group began making time-lapse seafloor gravityandpressuremeasurementsin1998.Todate,wehave surveyed sixfields offshore Norway; we have made three repeatsurveysatonefieldandonerepeatsurveyatanother.We incorporated a land-gravity sensor into a remotely operated seafloor housing. Three such relative gravity sensors mountedinasingleframearecarriedbyaremotelyoperatedvehicle ROV to concrete benchmarks permanently placed on the seafloor. Reference benchmarks sited outside the reservoir boundaries are assumed to provide stable fiducial points. Typicalsurveyslastfromafewdaystoafewweeksandcover from8to80benchmarks,withmultipleobservationsofeach. In our earliest surveys, we obtained an intrasurvey repeatability of approximately 20 Gal, but recently we have been achieving 3-Gal repeatability in gravity and approximately 5 mminbenchmarkdepthdeducedfromsimultaneouslyrecorded ambient seawater pressure. We attribute the improved precision to several operational factors, including the use of multiple gravity sensors, frequent benchmark reoccupation, precise relocation and orientation of the sensors, repeated calibrations on land, and minimization of vibrational and thermal perturbations to the sensors. We believe that high-precision time-lapse gravity monitoring can be used to trackchangesintheheightofagas-watercontactinaflooded reservoir,withaprecisionofafewmeters.

The crossover stress, anisotropy and the ice flow law at Siple Dome, West Antarctica

We used observations and modeling of Siple Dome, West Antarctica, a ridge ice divide, to infer the importance of linear deformation mechanisms in ice-sheet flow. We determined the crossover stress (a threshold value of the effective deviatoric stress below which linear flow mechanisms dominate over nonlinear flow mechanisms) by combining measurements of ice properties with in situ deformation rate measurements and a finite-element ice flow model that accounts for the effects of viscous anisotropy induced by preferred crystal-orientation fabric. We found that a crossover stress of 0.18 bar produces the best match between predicted and observed deformation rates. For Siple Dome, this means that including a linear term in the flow law is necessary, but generally the flow is still dominated by the nonlinear (Glen; n =3 ) term. The pattern of flow near the divide at Siple Dome is also strongly affected by crystal fabric. Measurements of sonic velocity, which is a proxy for vertically oriented crystal fabric, suggest that a bed-parallel shear band exists several hundred meters above the bed within the Ice Age ice.

Optical fiber infrasound sensor

Infrasound signals in the band 0.02 to 4 Hz are sensed in the presence of ambient noise generated chiefly by wind as integrated pressure variations, which induce detectable changes in the optical path length, along optic fibers, typically extending 100 m. to 1000 m. and more, arrayed at arbitrary geometries. Two fibers connected as a Michelson, Mach-Zehnder or equivalent interferometer where (i) one fiber is coupled to atmosphere while (ii) the other is not for being hermetically sealed in a tube, permit common mode rejection of noise from (i) temperature changes and (ii) strain, including ground vibration. Because the optic fiber infrasound sensors are longer than the distance over which wind-induced pressure changes are coherent, the effects of wind noise on the sensing of infrasound is reduced, and signal-to-noise ratio is increased over a wide bandwidth.

High-precision relative depth and subsidence mapping from seafloor water-pressure measurements

High-precision relative depth and subsidence mapping from seafloor water-pressure measurements

Measurement of vertical strain and velocity at Siple Dome, Antarctica, with optical sensors

As part of a larger program to measure and model vertical strain around Siple Dome on the West Antarctic ice sheet, we developed a new sensor to accurately and stably record displacements. The sensors consist of optical fibers, encased in thin-wall stainless-steel tubes, frozen into holes drilled with hot water, and stretched from the surface to various depths (up to 985 m) in the ice sheet. An optical system, connected annually to the fibers, reads out their absolute lengths with a precision of about 2 mm. Two sets of five sensors were installed in the 1997/98 field season: one set is near the Siple Dome core hole (an ice divide), and a second set is on the flank 7 km to the north (the ice thickness at both sites is approximately 1000 m). The optical-fiber length observations taken in four field seasons spanning a 3 year interval reveal vertical strain rates ranging from -229 ± 4 ppm a -1 to - 7 ± 9 ppm a. In addition to confirming a non-linear constitutive relationship for deep ice, our analysis of the strain rates indicates the ice sheet is thinning at the flank and is in steady state at the divide.